Book - Comparative Embryology of the Vertebrates 1
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Nelsen OE. Comparative embryology of the vertebrates (1953) Mcgraw-Hill Book Company, New York.
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Part I The Period of Preparation
Part I - The Period of Preparation: 1. The Testis and Its Relation to Reproduction | 2. The Vertebrate Ovary and Its Relation to Reproduction | 3. The Development of the Gametes or Sex Cells
The events which precede the initiation of the new individual's development are:
- The preparation of the male and female parents and their reproductive structures for the act of reprcxluction (Chaps. 1 and 2).
- The preparation of the gametes (Chap. 3).
The anterior lobe of the pituitary gland, because of its secretion of the gonadotrophic (gonad-stimulating) hormones, is the pivotal structure in the reproductive mechanism.
The gonadotrophic hormones are:
- Follicle-stimulating hormone, FSH;
- Luteinizing hormone, LH (ICSH), and
- Luteotrophin, LTH.
The Testis and Its Relation to Reproduction
A. Introduction
1. General description of the male reproductive system
2. Importance of the testis
B. Anatomical features of the male reproductive system
1. Anatomical location of the testis
2. Possible factors involved in testis descent
3. General structure of the scrotum and the testis in mammals
a. Structure of the scrotum
b. General structure of the testis
4. Specific structures of the mammalian testis which produce the reproductive cells and the male sex hormone
a. Seminiferous tubules
b. Interstitial tissue
5. The testis of vertebrates in general
6. Accessory reproductive structures of the male
a. The reproductive duct in forms utilizing external fertilization
b. The reproductive duct in species practicing internal fertilization
C. Specific activities of the various parts of the male reproductive system
1. Introduction
a. Three general functions of the male reproductive system
b. Some definitions
2. Activities of the testis
a. Seasonal and non-seasonal types of testicular activity
b. Testicular tissue concerned with male sex-hormone production
c. Testicular control of body structure and function by the male sex hormone
1) Sources of the male sex hormone
2) Biological effects of the male sex hormone
a) Effects upon the accessory reproductive structures
b) Effects upon secondary sex characteristics and behavior of the individual
c) Effects upon the seminiferous tubules
d. Seminiferous-tubule activity and formation of sperm
e. The seminiferous tubule as a sperm-storing structure
3. Role of the reproductive duct in sperm formation
a. Vertebrates without a highly tortuous epididymal portion of the reproductive duct
b. The epididymis as a sperm-ripening structure
c. The epididymis and vas deferens as sperm-storage organs
d. Two types of vertebrate testes relative to sperm formation
4. Function of the seminal vesicles (vesicular glands)
5. Function of the prostate gland
6. Bulbourethral (Cowper’s) glands
7. Functions of seminal fluid
a. Amount of seminal fluid discharged and its general functions
b. Coagulation of the semen
c. Hyaluronidasc
d. Accessory sperm
e. Fructose
f. Enzyme-protecting substances
D. Internal and external factors influencing activities of the testis
1. Internal factors
a. Temperature and anatomical position of the testis
b. Body nourishment in relation to testicular function
c. The hypophysis and its relation to testicular function
2. External environmental factors and testis function
a. Light as a factor
b. Temperature influences
E. Internal factors which may control seasonal and continuous types of testicular function
F. Characteristics of the male reproductive cycle and its relation to reproductive conditions in the female
B. Anatomical Features of the Male Reproductive System
Before endeavoring to understand the general functions of the testis in relation to reproduction, it is best to review some of the structural relationships of the testis in the vertebrate group.
Fig. 1. Reinier de Graaf. Born in Holland, 1641; died in Delft, Holland, 1673. Author of important works on the generative organs of the female. Described the Graafian follicle in the ovary of mammals but erroneously believed it to be the mammalian egg. (From Corner, '43.)
1. Anatomical Location of the Testis
In most vertebrates other than mammals, the testes are suspended well forward within the peritoix eal^cayity. In the Mammalia, however, the condition is variable. In the monotrematous mammals. Echidna and Ornithorynchus, the testes are located within the peritoneal cavity near the kidneys. In the elephant the testes also are located in this area. Schulte (’37) describes the position of the testes in an Indian elephant (Elephas indicus), 20 years old, as being “retroperitoneal lying on each side medial to the lower pole of the kidney.†(The kidneys were found to lie retroperitoneally on either side of the lower thoracic and lumbar vertebrae, and each measured about 275 mm. in length.) However, in the majority of mammals the testes descend posteriad from the original embryonic site, the extent varying with the species. In some there is a slight posterior migration, and the testes of the adult are situated well forward in the pelvic region. Examples of this condition are found in conies, whales, sea cows, African jumping shrews, and in armadillos. In sloths and American anteaters, the testes may descend into the pelvic cavity and lie in the area between the urinary bladder and the posterior body wall. However, in most of the eutherian and marsupial mammals, a dual outpushing of the postero-ventral body wall occurs into which the testes come to lie either permanently, or, in some forms, temporarily during the breeding season. This outward extension of the body-wall tissues is known as the scrotum; it involves not only the skin, muscle and connective tissues of the body wall but the peritoneal lining as well (fig. 2). (The interested student may consult Weber (’28) and Wislocki (’33) for data concerning the extent of testis descent in mammals.)
The peritoneal evaginations into the scrotal sac are two in number, one for each testis; each evagination is known as a processus vaginalis (figs. 3E, F; 4A, B). In many mammals this evagination becomes separated entirely froln the peritoneal cavity, and the testis, together with a portion of the sperm-conveying duct, lies suspended permanently in a small antechamber known as the inguinal bursa or serous cavity of the scrotum (fig. 4B). (See Mitchell, ’39.) This condition is found in the horse, man, opossum, bull, ram, dog, cat, etc. In certain other mammals, such as the rat, guinea pig, and ground hog, the inguinal bursa does not become separated from the main peritoneal cavity, and a persistent inguinal canal remains to connect the inguinal bursa with the peritoneal cavity (fig. 4C). In some rodents the testes pass through this persisting inguinal canal into the scrotum as the breeding season approaches, to be withdrawn again after the breeding period is terminated. The ground squirrel, Citellus tridecemlineatus (Wells, ’35) and the ground hog, Marrnota monax (Rasmussen, ’17) are examples of mammals which experience a seasonal descent of the testis.
In the majority of those mammals possessing a scrotum, it is a permanent structure. In a few, however, it is a temporary affair associated with the breeding season, as in the bat, Myotis, where the testes pass into a temporary perineal pouch or outpushing of the posterior abdominal wall during the reproductive season, to be withdrawn again together with the scrotal wall when the breeding period is past (fig. 4D). A similar periodic behavior is true of many insectivores, such as the common shrews, the moles, and the European hedgehog (Marshall, ’ll).
Fig. 2. Sketch of male reproductive system in man.
The permanent scrotum is a pendent structure, in some species more so than others. In the bull and ram, it extends from the body for a considerable distance, whereas in the cat, hippopotamus, tapir, guinea pig, etc., it is closely applied to the integumentary wall. In primates, including man, in most carnivores, and many marsupials, the pendency of the scrotum is intermediate between the extremes mentioned above.
An exceptional anatomical position of the testes in the lower vertebrates is found in the flatfishes, such as the sole and flounder, where they lie in a caudal outpouching of the peritoneal cavity (fig. 5). The testis on either side may even lie within a special compartment in the tail. (The ovaries assume the latter position in the female.)
Fig. 3. Diagrammatic representations of the urogenital structures in the developing male pig, with special emphasis upon testicular descent. (A) Early relationship of the genital fold (genital ridge), mesonephric kidney and its duct, together with the metanephric kidney and the ureter in 20-mm. pig embryo. The relationship of the mesonephric and metanephric ducts to the urogenital sinus is shown. The Miillcrian duct is omitted. (B) Male pig embryo about 45-mm., crown-rump length, showing relationship of gonad and metanephric kidney. I he metanephric kidney is shown below (dorsal to) the mesonephric kidney. The gonad (testis) is now a well-defined unit. The portion of the genital fold tissue anterior to the testis becomes the anterior suspensory ligament of the testis, while the genital fold tissue caudal to the testis continues back to join the inguinal ligament of the mesonephros (the future gubernaculum). (C) About 80-mm., crown-rump, pig embryo. Observe that the metanephros is now the dominant urinary organ and has grown cephalad, displacing the mesonephric kidney which is regressing and moving caudally with the testis. The remains of the mesonephric kidney at this time are gradually being transformed into epididymal structures. (D) About 130~mm.. crown-rump, pig embryo. Observe that the testis is approaching the internal opening of the inguinal canal. The anterior suspensory ligament is now an elongated structure extending over the lateroventral aspect of the metanephric kidney; the gubernacular tissue is shown extending downward into the inguinal canal. (E) Later stage in testicular descent. The anterior suspensory ligament of the testis is a prominent structure, while the gubernaculum is compact and shortened. (F) The condition found in the full-term, fetal pig. The testis is situated in the scrotal swelling; the gubernaculum is much shortened, while the anterior suspensory ligament remains as a prominent structure, extending cephalad to the caudal portions of the metanephric kidney.
2. Possible Factors Involved in Testis Descent The descent of the testis within the peritoneal cavity and into the scrotum poses an interesting problem. In embryonic development extensive migration of cell substance, or of cells, tissues, and organ structures is one of many processes by which the embryonic body is formed. That is to say, the dynamic movement or displacement of developing body structures from their original position is a part of the pattern of development itself. The casual factors involved in such movements are still unknown, and the study of such behavior forms one of the many interesting aspects of embryological investigation awaiting solution.
Fig. 4. Diagrammatic drawings portraying the relationship of the testis to the processus vaginalis (peritoneal evagination) and the scrotum. The testis is at all times retroperitoneal, i.e., outside the peritoneal cavity and membrane. (A) Earlier stage of testicular descent at the time the testis is moving downward into the scrotum. (B) Position of the testis at the end of its scrotal journey in a form possessing permanent descent of the testis, e.g., man, dog, etc. (C) Testis-peritoneal relationship in a form which does not have a permanent descent of the testis — the testis is withdrawn into the peritoneal cavity at the termination of each breeding season. Shortly before the onset of the breeding period or “rut,†the testis once again descends into the scrotum, e.g., ground hog. (D) Position of testis in relation to body wall and peritoneum in the mole, shrev/, and hedgehog in which there is no true scrotum. The testis bulges outward, pushing the body wall before it during the breeding season. As the testis shrinks following the season of rut, the bulge in the body wall recedes. True also of bat, Myotis.
Fig. 5. Opened peritoneal cavity of a common flounder, Limanda ferruginea, showing the position occupied by the testes. Each testis is situated partly in a separate compartment on either side of the hemal processes of the tail vertebrae.
Various theoretical explanations have been proposed, however, to explain the movement of the testis posteriad from its original embryonic site. Classical theory mentions the mechanical pull or tightening stress of the gubernaculum, a structure which develops in relation to the primitive genital fold or genital ridge (figs. 3B, C; 351C-7).
The genital ridge extends along the mesial aspect of the early developing mesonephric kidney from a point just caudal to the heart to the posterior extremity of the mesonephric kidney near the developing cloacal structures (Hill, ’07). Anteriorly, the genital ridge (fold) merges with the diaphragmatic ligament of the mesonephros (fig. 3A). The gonad (testis or ovary) develops in a specialized region of the more cephalic portion of the genital ridge (Allen, ’04). (See fig. 3A.) The caudal end of the mesonephric kidney eventually becomes attached to the posterior ventral body wall by means of a secondary formation of another cord of tissue, the inguinal fold (fig. 3A). The latter is attached to the posterior ventral body wall near the area where the scrotal outpushing (evagination) later occurs. This inguinal fold later becomes continuous with the genital fold (fig. 3B). The inguinal fold thus becomes converted into a ligament, the inguinal ligament of the mesonephros, uniting the caudal portion of the mesonephric kidney and adjacent genital fold tissue with the area of scrotal evagination (fig. 3B). The gubernaculum represents a later musculo-connective tissue development of the inguinal ligament and the adjacent genital fold tissue. It contains smooth muscle fibers as well as connective tissue. As the scrotal evagination forms at the point where the gubernaculum attaches to the body wall, the gubernaculum from the beginning of its formation is connected with the developing scrotal sac.
As the testis migrates posteriad, the anterior suspensory ligament of the testis elongates and the gubernaculum shortens (fig. 3A~F). This decrease in length of the gubernaculum is both real and relative. It is real in that an actual shortening occurs; it is relative because the rapid enlargement of the developing pelvic cavity and its contained organs makes the length of the gubernaculum appear less extensive. This enlargement of the pelvic space and increase in size of its contained structures and a corresponding failure of the gubernaculum to elongate, certainly are factors in bringing about the intra-abdominal descent of the testis; that is, testis descent within the peritoneal cavity itself (Felix, '12).
Developmental preparations precede the extra-abdominal descent of the testes, for the scrotal chambers must be prepared in advance of the arrival of the testes. These developmental events are:
(1) two outpocketings of the abdominal wall which come to lie side by side below the skin to form the walls of the scrotal chamber, and
(2) an evagination of the peritoneum into each of the abdominal outpocketings which act as peritoneal linings for each pocket.
It is worthy of mention that the above outpushings of the abdominal wall and of the peritoneum precede the movement of the testes into the scrotum. They serve to illustrate the theory that a shortening of the gubernaculum is not sufficient to explain testis descent. Rather, that in this descent a whole series of developmental transformations are involved; the shortening of the gubernaculum and scrotal development merely represent isolated phases of the general pattern of movement and growth, associated with this descent.
More recent research emphasizes the importance of certain physiological factors relative to the descent problem. It has been determined, for example, that administration of the gonadotrophic hormone of pregnancy urine (chorionic gonadotrophin) or of the male sex hormone, testosterone, aid the process of extra-abdominal descent (i.c., descent from the inguinal ring area downward into the scrotum) . Hormone therapy, using chorionic gonadotrophin together with surgery, is used most often in human cryptorchid conditions. The androgen, testosterone, aids testicular descent mainly by stimulating the growth of the scrotal tissues and the vas deferens; however, it is not too successful in effecting the actual descent of the testis (Robson, '40; Wells, '3; Pincus and Thimann, '50).
The phenomenon of testicular migration thus is an unsolved problem. Many activities and factors probably play a part in ushering the testis along the pathway to its scrotal residence.
3. General Structure of the Scrotum and the Testis in Mammals
a. Structure of the Scrotum
The scrotal modification of the body wall generally occurs in the posteroventral area between the anus and the penial organ. However, in marsupials it is found some distance anterior to the latter.
Each scrotal evagination consists of three general parts: the skin with certain attendant muscles, the structures of the body wall below the skin, and the peritoneal evagination. The skin, with its underlying tunica daitos muscle tissue and superficial perineal fascia, forms the outer wall of the scrotum (fig. 6). Within this outer cutaneous covering lie the two body-wall and two peritoneal evaginations. The body-wall evaginations involve connective and muscle tissues of the external oblique, internal oblique, and trans versus muscles. The caudal part of each peritoneal outpocketing forms the serous cavity or inguinal bursa in which the testis is suspended after its descent, and its more anterior portion forms the inguinal canal (figs. 2, 4B, 6). The oblique and transversus layers of tissues thus are molded into a musculo-connective tissue compartment around each serous cavity. The median septum of the scrotum represents the area of partial fusion between the two musculo-connective tissue compartments, whereas the median raphe of the scrotum denotes the area of fusion of the two cutaneous coverings of the body-wall outpushings (fig. 6).
Consequently, passing inward from the superficial perineal fascia of the skin or outer wall, one finds the following tissue layers surrounding the testis:
- The external spermatic fascia represents the modified fascia of the external oblique muscle layer of the embryo.
- The middle spermatic fascia is a modification of the internal oblique muscular layer, whose tissue forms the cremaster muscle loops within the scrotum (fig. 6). (Some of the cremasteric musculature may be derived from the transversus layer.)
- The internal spermatic fascia or tunica vaginalis communis is derived from the transverse muscle layer of the embryo.
- Along the inner surface of the tunica vaginalis communis is the peritoneal membrane. The latter is reflected back over the surface of the suspended testis, and thus forms the visceral peritoneal covering of the testis. This lining tissue of the common vaginal tunic and the peritoneal membrane which covers the testis are derived from the original peritoneal evagination into the scrotal pocket; as such it forms the tunica vaginalis propria*
b. General Structure of the Testis
The testis is composed of the following structural parts:
(1) The inner layer of the tunica vaginalis propria, the tunica vaginalis internus, envelops the testis. The cavity between the outer and inner layers of the tunica vaginalis propria is the inguinal bursa. Obliteration by injury or infection of this inguinal bursa may cause degenerative changes in the testis. In other words, the testis normally must be free to move within its serous (peritoneal) cavity.
(2) Within the tunica vaginalis internus of the testis is a thick fibrous layer of connective tissue, the tunica albuginea (fig. 7). From this tunic, connective tissue partitions, the septula of the testis, extend inward and converge toward that testicular zone where supplying blood vessels enter and leave, including the lymphatics. The latter zone is known as the mediastinum testis and it represents a regional thickening of the tunica albuginea. Here the connective tissue fibers form a latticework which acts as a framework for the larger blood and lymph vessels and efferent ducts of the testis. The testis is attached to the scrotal wall in the mediastinal area.
(3) The spaces between the various septula partitions form the septula compartments. In the human testis there are about 250 septula compartments, each containing a lobule of the testis. The lobuli testis contain the convoluted portions of the seminiferous tubules. From one to three seminiferous tubules are found in each lobule; they may anastomose at their distal ends. The combined length of all the seminiferous tubules approaches 250 meters in the human. The convoluted portions of the seminiferous tubules empty into the straight tubules (tubuli recti) and these in turn unite with the rete tubules located within the substance of the mediastinum. Connecting with the rete tubules of the testis, there are, in man, from 12 to 14 ductuli efferentes (efferent ductules of the epididymis) of about 4 to 6 cm. in length which emerge from the mediastinum and pass outward to unite with the duct of the epididymis. The epididymal duct represents the proximal portion of the reproductive duct which conveys the male gametes to the exterior.
Fig. 6. Schematic drawing of the testis and its relationship within the scrotum. On the right side of the drawing the muscle and connective-tissue layers surrounding the inguinal bursa and testis are shown; on the left side may be seen the loops of the cremaster muscle surrounding the tunica vaginalis communis.
Fig. 7. Diagrammatic representation of the general structural relationship of the parts of the human testis. (Modified from Corner, 1943, after Spalteholz and Huber.)
4. Specific Structures of the Mammalian Testis Which Produce THE Reproductive Cells and the Male Sex Hormone
Two very essential processes involved in reproduction are the formation of the sex cells or gametes and the elaboration of certain humoral substances, known as sex hormones. Therefore, consideration will be given next to those portions of the testis which produce the sperm cells and the male sex hormone, namely, the seminiferous tubules and the interstitial tissue.
a. Seminiferous Tubules
The seminiferous tubules lie in the septula compartments (fig. 7). The word seminiferous is derived from two Latin words: semen, denoting seed, and ferre, which means to bear or to carry. The seminiferous tubule, therefore, is a male “seed-bearing†structure. Within this tubule the male gametes or sperm are formed, at least morphologically. However, the word semen has a broader implication in that it is used generally to denote the entire reproductive fluid or seminal fluid. The seminal fluid is a composite of substances contributed by the seminiferous tubules and various parts of the accessory reproductive tract.
The exact form and relationship of the various seminiferous tubules (tubuli seminiferi) which occupy each testicular compartment have been the object of much study. It is a generally accepted belief at present that the tubules within each testicular lobule are attached at their distal ends; that is, that they anastomose (fig. 7). Some investigators also believe that there may be other anastomoses along the lengths of these very much contorted and twisted structures. Moreover, it appears that the septula or testicular compartmental partitions are not always complete; the seminiferous tubules of one lobule thus have the opportunity to communicate with those of adjacent lobules. The seminiferous tubules of any one lobule join at their proximal ends and empty into a single straight seminiferous tubule. The straight tubules or tubuli recti pass into the mediastinum and join the anastomosing rete tubules of the rete testis.
The convoluted portions of the seminiferous tubules produce the sperm (spermia; spermatozoa). In the human testis, the length of one of these tubules is about 30 to 70 cm. and approximately 150 /a to 250 ix in diameter. Each tubule is circumscribed by a basement membrane of connective tissue and contains two cell types:
( 1 ) supporting or Sertoli cells, and
(2) spermatogenic cells or spermatogonia (see fig. 8 and Chap. 3).
The cells of Sertoli are relatively long, slender elements placed perpendicularly to the basement membrane to which they firmly adhere. These cells may undergo considerable change in shape, and some observers believe that they may form a syncytium, known as the “Sertolian syncytium.†Others believe them to be distinct elements. It is said that Sertoli cells may round up and form phagocytes which become free from the basement membrane and move, ameba-like, in the lumen of the seminiferous tubule, phagocytizing degenerating sperm cells. However, their main function appears to be associated with the development of sperm during the period when the latter undergo their transformation from the spermatid condition into the adult
Fig. 8. Semidiagram matic representation of section of cat testis, showing seminiferous tubules and interstitial tissue, particularly the cells of Leydig.
Sperm form. The Sertolian cells thus may act as nursing elements during sperm metamorphosis.
The spermatogenic cells or spermatogonia (germinal epithelium of the tubule) lie toward the outer portion of the seminiferous tubule between the various Sertoli elements. As a rule spermatogonia lie apposed against the basement membrane of the tubule (see fig. 8 and Chap. 3).
b. Interstitial Tissue
The interstitial tissue of the testis is situated between the seminiferous tubules (fig. 8). It consists of a layer of connective tissue applied to the basement membrane of the seminiferous tubule and of many other structures, such as small blood and lymph vessels, connective tissue fibers, connective tissue cells, mast cells, fixed macrophages, etc. The conspicuous elements of this tissue are the so-called interstitial cells or cells of Leydig (fig. 8). In man, cat, dog, etc., the cells of Leydig are relatively large, polyhedral elements, possessing a granular cytoplasm and a large nucleus.
5. The Testis of Vertebrates in General
In the vertebrate group, the testis shows marked variations in shape and size. In many fishes, the testes are irregular, lobular structures, but in other fishes, amphibia, reptiles, birds, and mammals, they assume an ovoid shape. The size of the testis is extremely variable, even in the same species. The testis of the human adult approximates 4 to 5 cm. in length by 3 cm. wide and weighs about 14 to 19 Gm. The testis of the horse averages 11 cm. long by 7 cm. wide with a weight of 30 to 35 Gm., while that of the cat is 1.6 cm. long and 1.1 cm. wide with a weight of 1.5 Gm. In the mud puppy, Necturus, the testis is approximately 3.5 cm. long and 0.8 cm. wide with a weight of 0.3 Gm. The testis of the large bullfrog is 1 .2 cm. by 0.5 cm. with a weight of 0.8 Gm. In comparison to the foregoing, Schulte (’37) gives the weight of each testis of an Indian elephant as two kilograms!
Regardless of size or shape, the presence of seminiferous tubules and interstitial tissue may be observed in all vertebrate testes. In some species the seminiferous tubule is long; in others it is a short, blunt affair. The interstitial cells may be similar to those described above, or they may be small, inconspicuous oval elements.
6. Accessory Reproductive Structures of the Male a. The Reproductive Duct in Forms Utilizing External Fertilization
The accessory reproductive organs of the vertebrate male are extremely variable in the group as a whole. A relatively simple reproductive duct (or in some no duct at all) is the rule for those forms where fertilization is effected in the external medium. In cyclostome fishes, for example, the reproductive cells are shed into the peritoneal cavity and pass posteriad to emerge externally by means of two abdominal pores. Each pore empties into the urogenital sinus. In teleost fishes (perch, flounder, etc.) the conveying reproductive duct is a short, simple tube continuous with the testis at its caudal end and passing posteriorly to the urogenital sinus (fig. 9A). In frogs and toads, as well as in certain other fishes, such as Amia and Polypterus, the male reproductive duct is a simple, elongated tube associated with the testis by means of the efferent ductules of the latter, coursing posteriad to open into the cloaca (frogs and toads) or to the urogenital sinus (Amia; Polypterus) (fig. 9B, C). Simplicity of sperm duct development and external union of the gametes are associated reproductive phenomena in the vertebrate group.
b. The Reproductive Duct in Species Practicing Internal Fertilization
An entirely different, more complex male reproductive duct is found (with some exceptions) in those vertebrates where gametic union occurs within the protective structures of the maternal body. Under these circumstances there may be a tendency for one male to serve several females. Enlargement of the duct with the elaboration of glandular appendages, and structures or areas for sperm storage is the rule under these conditions (fig. 9D--F). This form of the male genital tract is found not only in those species where an intromittent organ deposits the sperm within the female tract, but also where the sperm are deposited externally in the form of spermatophores (fig. 10).
In many species, the reproductive duct is greatly lengthened and becomes a tortuous affair, especially at its anterior or testicular end. In fact, the cephalic end of the duct may be twisted and increased to a length many times longer than the male body itself. This coiled, cephalic portion is called the duct of the epididymis (epididymides, plural). (See figs. 7, 9E.) The word epididymis is derived from two Greek words: epi = upon, and didymis = testicle. The epididymis, therefore, is the body composed of the tortuous epididymal duct and the efferent ducts of the testis which lie upon or are closely associated with the testis. The complex type of reproductive duct is composed thus of two main portions, an anterior, contorted or twisted portion, the epididymal duct, and a less contorted posterior part, the vas deferens or sperm duct proper (fig. 9D, E).
In some vertebrates, in addition to the above complications, the caudal end of the reproductive duct has a pronounced swelling or diverticulum, the seminal vesicle (e.g., certain sharks and certain birds). The latter structures are true seminal vesicles in that they store sperm during the reproductive period.
The epididymal duct in man is a complex, coiled canal composed of a head (caput), a body (corpus), and a tail (cauda). (See fig. 7.) It is C-shaped with its concavity fitting around the dorsal border of the testis, the head portion being located at the anterior end of the latter. The total length of the epididymal duct in man is said to be about 4 to 7 m. In other mammals the epididymal duct may be much longer. For example, in the ram, from 40 to 60 m.; in the boar, 62 to 64 m.; in the stallion, 72 to 86 m. (Asdell, ’46). At its caudal end it becomes much less tortuous and gradually passes into the vas deferens (ductus deferens).
Fig. 9. Various vertebrate testes and reproductive ducts, emphasizing the relative simplicity of the duct where external fertilization is the rule while complexity of the duct is present when internal fertilization is utilized. There are exceptions to this rule, however. (A) Flounder (Limanda ferruginea). (B) Frog (Rana catesbiana). (C) Urodele (Cryptobranchus alleganiensis). (D) Dog shark (Squalus acanthias), (E) Urodele (Necturus rnaculosus). (F) Rooster (G alius domestic us).
The ductus deferens has a length of about 30 to 35 cm. in man. Leaving the scrotum it passes anteriad together with accompanying nerves and blood vessels in the subcutaneous tissue over the front of the pelvic bone into the peritoneal cavity through the inguinal ring (fig. 2). Here it separates from the other constituents of the spermatic cord (i.e., it separates from the nerves and blood vessels) and passes close to the dorsal aspect of the bladder and dorsally to the ureter. It then turns posteriad along the dorsal aspect of the neck of the bladder and the medial region of the ureter, and accompanied by its fellow duct from the other side, it travels toward the prostate gland and the urethra. Just before it enters into the substance of the prostate, it receives the duct of the seminal vesicle. The segment of the vas deferens from the ureter to the seminal vesicle is considerably enlarged and is called the ampulla. After receiving the duct of the seminal vesicle, the vas deferens becomes straightened and highly muscularized — as such it is known as the ejaculatory duct. The latter pierces the prostate gland located at the caudal end of the bladder and enters the prostatic portion of the urethra; from this point the urethra conveys the genital products.
The auxiliary glands associated with the genital ducts of the human male consist of the seminal vesicles, the prostate gland, Cowper’s glands, and the glands of Littre.
The seminal vesicles are hollow, somewhat tortuous bodies (fig. 2). Each vesicle arises in the embryo as an outpushing (evagination) of the vas deferens. The prostate gland has numerous excretory ducts which empty into the urethra. It represents a modification of the lining tissue of the urethra near the urinary bladder together with surrounding muscle and connective tissues. Cowper’s (bulbourethral) glands are small pea-shaped structures placed at the base of the penial organ; their ducts empty into the urethra. The glands of Littre are small, glandular outgrowths along the urethra and are closely associated with it.
To summarize the matter relative to the structural conditions of the reproductive duct in the male of those species which practice internal fertilization:
- A lengthening and twisting of the duct occurs.
- A sperm-storage structure is present, either as a specialized portion of the duct or as a sac-like extension.
- Certain auxiliary glands may be present. These glands are sometimes large and vesicular structures, such as the seminal vesicles of the human duct, or they may be small glands distributed along the wall of the duct, such as the glands of Littre.
C. Specific Activities of the Various Parts of the Male Reproductive System
1. Introduction
a. Three General Functions of the Male Reproductive System
The activities of the testes and the accessory parts of the male reproductive system result in the performance of three general functions as follows:
- formation of the semen,
- delivery of the semen to the proper place where the sperm may be utilized in the process of fertilization, and
- elaboration of the male sex hormone.
b. Some Definitions
Semen or seminal fluid is the all-important substance which the male contributes during the reproductive event. It is the product of the entire reproductive system, including special glands of the accessory reproductive structures. The semen is composed of two parts:
(1) The sperm (spermatozoa, spermia) are the formed elements which take part in the actual process of fertilization.
(2) The seminal plasma, a fluid part, is a lymph-like substance containing various substances dissolved or mixed in it. These contained substances are important as a protection for the sperm and as an aid to the process of fertilization.
With regard to the second function of the male genital system, namely, the delivery of sperm to the site of fertilization, it should be observed that
Fig. 10. Spermatophores of common urodeles. (Redrawn from Noble: Biology of the Amphibia, New York, McGraw-Hill.) (A) Triturus viridescens. (After Smith.) (B) Desmognathus fuscus, (After Noble and Weber.) (C) Eiirycea hislineata.
in some vertebrates this is a more simple problem than in others. In those
forms which practice external fertilization, the male system simply discharges
the seminal fluid into the surrounding external medium. However, in those
vertebrates where internal fertilization is the rule, the female system assumes
some of the burden in the transport of the semen to the region where fertilization is consummated, thus complicating the procedure. In these instances,
the male genital tract is called upon to produce added substances to the seminal
fluid which aid in protecting the sperm en route to the fertilization site.
The elaboration of the androgenic or male sex hormone is a most important function. Androgenic or male sex hormone substances are those organic compounds which induce maleness, for they aid the development of the male secondary sex characteristics, enhance the growth and functional development of the male accessory reproductive structures, and stimulate certain aspects of spermatogenesis.' Like the estrogens, androgens are not confined to a particular sex; they have been extracted from the urine of women and other female animals. The androgens derived from urinary concentrates are androsterone and dehydroisoandrosterone. These two androgens are not as powerful as that prepared from testicular tissue. Testicular androgen was first isolated from testicular tissue in 1935 and was given the name testosterone. It also has been synthesized from cholesterol. It is the most powerful of the androgens and probably similar, if not identical, with the substance produced in the testis (Koch, ’42).
2. Activities of the Testis
a. Seasonal and N on-seasonal Types of Testicular Activity
The testis has two main functions: the production of sperm and formation of the male sex hormone. In many vertebrates these two activities represent a continuous procedure during the reproductive life of the male animal. This
Fig. II. Seasonal spermatogenesis and accessory gland development in the ground squirrel, Citellus tridecemlineatus. Stippling below base line shows period of hibernation, whereas crosshatching reveals the reproductive period. (From Turner: General Endocrinology, Philadelphia, Saunders, after L, J. Wells.)
condition is found in certain tropical fish, in the common fowl and various wild tropical birds, and in many mammals, such as man, the dog, bull, stallion, cat, etc. On the other hand, in the majority of vertebrates these activities of the testis are a seasonal affair. This condition is found in most fish, practically all amphibia, all temperate -zone-inhabiting reptiles, most birds, and many mammals. Among the latter, for example, are the ferret, deer, elk, fox, wolf, and many rodents, such as the midwestern ground squirrel. Seasonal periodicity is true also of the common goose and turkey.
Sperm-producing periodicity is not correlated with any particular season, nor is spermatogenesis always synchronized with the mating urge, which in turn is dependent upon the male sex hormone. In some forms, these two testicular functions may actually occur at different seasons of the year, as for example, in the three-spined stickleback, Gasterosteus aculeatus (fig. 15). (See Craig-Bennett, ’31.) In general, it may be stated that sperm are produced during the weeks or months which precede the development of the mating instinct. Many species follow this rule. For example, in the bat of the genus Myotis, sperm are produced during the late spring and summer months, while mating or copulation takes place during the fall or possibly early the next spring (Guthrie, ’33). In the common newt, Triturus viridescens, spermatogenesis comes to pass during the warm months of the summer, and sperm are discharged from the testis into the reproductive ducts during the late fall and early spring, while copulation is accomplished in the early spring. The testes in this species are quiescent during the cool winter months. In the midwestern ground squirrel, Citellus tridecemlineatus, spermatogenesis begins in November and is marked during February and March (fig. 11). The animal hibernates away the winter months and emerges the first part of April in a breeding condition. Mating occurs in the early spring (Wells, ’35). In the garter snake, Tharnnophis radix, sperm are produced in the testes in the summer months, stored in the epididymides during the hibernation period in the fall and winter, and used for copulation purposes in the spring (Cieslak, ’45). Again, in the Virginia deer, Odocoileus virginianus borealis, studied by Wislocki (’43), active spermatogenesis is realized during the summer and early autumn months, while the mating season or “rut†which results from the driving power of the male sex hormone, is at its peak in October and November (fig. 12). In the fox. Bishop (’42) observed spermatogenesis to begin in the late fall months, while mating is an event of the late winter and early spring. In April and May the seminiferous tubules again assume an inactive state (fig. 13). In the common frog, Rana pipiens, spermatogenesis is present in the summer months and morphogenesis of spermatids into sperm happens in large numbers during September, October, and November. Sperm are stored in the testis over the winter, and the mating instinct is awakened in the early spring (Glass and Rugh, ’44). Following the mating season in spring and early summer the testis of the teleost, Fundulus heteroclitus, is depleted of sperm until the next winter and spring (Matthews, ’38).
As the seasonal type of testicular activity is present in a large number of vertebrate species, it seems probable that it represents the more primitive or fundamental type of testicular functioning.
b. Testicular Tissue Concerned with Male Sex-hormone Production
While one cannot rule out the indirect effects which activities of the seminiferous tubules may have upon the functioning of the testis as a whole, including the interstitial tissue, direct experimental evidence and other observations suggest that the interstitial tissue holds the main responsibility for the secretion of the male sex hormone, testosterone, or a substance very closely allied to it. For example, if a testis from an animal possessing a permanent scrotum is removed from the inguinal bursa and placed within the peritoneal cavity, the seminiferous tubules tend to degenerate, but the interstitial tissue remains. The sex hormone, under these circumstances, continues to be produced. Again, males having cryptorchid testis (i.e., testes which have failed in their passage to the scrotum) possess the secondary sex characteristics of normal males but fail to produce sperm cells. Also, it has been demonstrated that the mammalian fetal testis contains the male sex hormone. However, in this fetal condition, the seminiferous tubules are present only in an undeveloped state, whereas interstitial tissue is well differentiated. It is probable in this case that the interstitial tissue of the fetal testis responds to the luteinizing hormone in the maternal blood.
Fig. 12. Sections of the testis of the deer, Odocoileus virginianus borealis. (After Wislocki.) (A) Seminiferous tubules of deer in June. Observe repressed state of tubules and absence of sperm. (B) Epididymal duct of same deer. Observe absence of sperm and smaller diameter of duct compared with (D). (C) Seminiferous tubules of October deer; spermatogenic activity is marked. (D) Epididymal duct, showing well-developed epididymal tube and presence of many sperm.
In hypophysectomized male rats injected with dosages of pure folliclestimulating hormone (FSH) or with small doses of pure luteinizing hormone (LH; ICSH), the seminiferous tubules of the testis respond and spermatogenesis occurs. However, the interstitial tissue remains relatively unstimulated and the accessory structures continue in the atrophic state. If larger doses of the luteinizing factor are given, the interstitial tissue responds and the secondary sexual characters are developed, showing a relationship between interstitial activity and sex-hormone production. (Consult Evans and Simpson in Pincus and Thimann, ’50, pp. 355, 356.)
From certain species whose reproductive activities are confined to a particular season of the year, there also comes evidence that the interstitial tissue is the site of sex-hormone production. In the behavior of testicular tissue in the stickleback, Gasterosteus, as shown by van Oordt (’23) and Craig-Bennett (’31) sperm are produced actively in the seminiferous tubules during one period of the year when the interstitial tissue is in an undeveloped condition. The secondary sex characters also are in abeyance at this season of the year. However, during the months immediately following sperm production, sperm are stored within the seminiferous tubules and active spermatogenesis is absent. When the seminiferous tubules thus have completed their spermatogenic activity, the interstitial tissue begins to increase, followed by a development of secondary sex characteristics (figs. 14, 15). A similar difference in the rhythm of development of these two testicular tissues can be shown for many other vertebrates. All of these suggestive facts thus serve to place the responsibility for male sex-hormone production upon the interstitial tissue, probably the cells of Leydig.
Fig. 13. Sections of seminiferous tubules of silver fox. (After Bishop.) (A) Regressed state of tubules following breeding season. (B) Tubule from fox during the breeding season, characterized by active spermatogenesis.
Fig. 14. Sections of the testis of the stickleback (Gasterosteus pungitius). (Modified from Moore, ’39, after Van Oordt.) Cf. fig. 13, (A) Spermatogenic activity with many
formed sperm in seminiferous tubules before the mating season, interstitial tissue in abeyance. (B) At mating period. Interstitial tissue well developed, spermatozoa stored in the tubules with spermatogenic activity absent.
Fig. 15. Seasonal reproductive cycle in the stickleback {Gasterosteus aculeatus). Cf. fig. 14. Breeding season is indicated by crosshatching below base line. Observe that spermatogenic activity follows rise of temperature, whereas interstitial-tissue and sexcharacter development occur during ascending period of light. (Redrawn from Turner: General Endocrinology, Philadelphia, Saunders, modified from Craig-Bennett, 1931.)
c. Testicular Control of Body Structure and Function by the Male
Sex Hormone
1) Sources of the Male Sex Hormone. Testosterone is prepared from testicular extracts. It is the most potent of the androgens and is believed to be
the hormone produced by the testis. The chemical formula of testosterone is:
on
ciu
o
CHs
J\J\J
The testis, however, is not the only site of androgen formation. As mentioned above, androgens are found in the urine of female animals, castrates, etc. It seems probable that the suprarenal (adrenal) cortex may secrete a certain androgenic substance, possibly adrenosterone, a weak androgen. Many androgens have been synthesized also in the laboratory (Schwenk, ’44).
2) Biological Effects of the Male Sex Hormone. The presence of the male sex hormone in the male arouses the functional development of the accessory reproductive structures, the secondary sexual characters, and also stimulates the development of the seminiferous tubules.
a) Effects upon the Accessory Reproductive Structures. Castration or removal of the testes from an animal possessing a continuous type of testicular activity produces shrinkage, and a general tendency toward atrophy, of the entire accessory reproductive structures. Injection of testosterone or other androgens under such conditions occasions a resurgence of functional development and enlargement of the accessory structures (fig. 16). Moreover, continued injections of the androgen will maintain the accessories in this functional state (Moore, ’42; Dorfman in Pincus and Thimann, ’50). Similarly, under normal conditions in those vertebrates which possess the seasonal type of testicular function, the accessory reproductive organs shrink in size with a loss of functional activity when the testis undergoes regression during the period immediately following the active season. An enlargement and acquisition of a normal functional condition of the accessories follows testicular development as the breeding season again approaches (Bishop, ’42; Wislocki, ’43; Matthews, ’38; Turner, C. L., ’19). (Compare figs. 12A-D.)
b) Effects upon Secondary Sex Characteristics and Behavior of THE Individual. In addition to the primary effects upon the reproductive system itself, the androgens induce many other secondary structures and alterations of the physiology and behavior of the individual. The influence of the testicular hormone has been demonstrated in all of the vertebrate groups from fishes to mammals (Dorfman in Pincus and Thimann, ’50). Examples of testosterone stimulation are: the singing and plumage of the male bird; hair development of certain mammals; the crowing and fighting, together with spur, comb, and wattle growth in the rooster. The disagreeable belligerency and positive energy drive of the bull, stallion, or human male may be attributed, largely, to the action of testicular hormone. However, lest we disparage this aggressive demeanor unduly, it should be recognized that upon such explosive force rests the preservation of species and races in some instances. As an example, witness that hairy dynamo of the barren northern tundras, the bull muskox, whose fiery pugnaciousness when the need arises undoubtedly has been a strong factor in the preservation of this species.
An excellent example of the effect of testosterone is shown in the development of antlers and change in behavior of the Virginia deer, Odocoileus virginianus borealis (Wislocki, ’43). In the northern climate, the testes and male accessory organs reach a profound condition of regression in April and May. Growth of the new antlers starts at this time, and during the late summer the antlers grow rapidly and begin to calcify. During the summer, also, the testes develop rapidly, and spermatogenesis results. Loss of the “velvet†covering of the antlers is experienced during September, and mating is the rule in October and November. The antlers are shed in midwinter. If the testes are removed after the naked antler condition is reached, the antlers are shed rapidly. Testosterone administered to does or to young males which have been castrated induces the development of antlers. The general scheme of antler development suggests, possibly, that the testicular hormone acts upon an anterior pituitary factor, and this activated factor in turn initiates antler growth. Hardening of the antlers and loss of velvet results from testosterone stimulation. Loss of the antler is synchronized with a decrease in the amount of testosterone in the blood stream, accompanied by the acquisition of a docile, non-belligerent, more timid behavior.
c) Effects upon the Seminiferous Tubules. Testosterone has a stimulating effect upon the seminiferous tubule and sperm formation. This matter is discussed in Chap. 3.
d. Seminiferous-tubule Activity and Formation of Sperm See Chap. 3.
e. The Seminiferous Tubule as a Sperm-storing Structure
See p. 3 1 .
3. Role of the Reproductive Duct in Sperm Formation
a. Vertebrates Without a Highly Tortuous Epididyrnal Portion of the Reproductive Duct
In a large number of vertebrates, morphologically developed sperm pass from the testis through the efferent ductules of the epididymis (vasa efferentia) to the epididyrnal duct where they remain for varying periods. However, in many vertebrates the anterior (proximal) portion of the sperm duct does not form a tortuous epididyrnal structure similar to that found in other vertebrates. This condition is present in the common frog, Rana; in the hellbender, Cryptobranchus; in the bowfin, Amia; etc. Because of this fact, the sperm pass directly into the vas deferens or sperm duct (Wolffian duct) without undergoing a sojourn through a convoluted epididyrnal portion of the duct.
Correlated with the type of testis and sperm-duct relationship in the frog, is the fact that one may obtain viable, fertilizing sperm directly from the testis. For example, if one removes the testis from a living frog and macerates it in pond water or in an appropriate saline solution, active sperm are obtained which are capable of fertilizing eggs in a normal manner. That is, the frog testis matures sperm morphologically and physiologically. This type of testicular maturation is characteristic of many of the lower vertebrates possessing simple reproductive ducts.
Fig. 16. Effects of the male sex hormone upon the functional development of the accessory reproductive structures of the male rat. (After Turner: General Endocrinology , Philadelphia, Saunders, p. 324.) (A) Normal male rat condition produced by injection of crystalline male sex hormone for 20 days into castrate before autopsy. (B) Castrated male litter mate of (A) receiving no replacement therapy.
Fig. 17. Diagrammatic drawings of the two types of testicular-reproductive relationships occurring in the vertebrate group. (A) Simplified type of reproductive duct connected with the testis by means of efferent ductules. The duct-testis relationship of many
telepst fishes is similar to this but does not possess the efferent ductules, the sinus-like
reproductive duct being attached directly to the testis. Sperm cells (spermatozoa) are
matured and stored within the testis. This type of relationship generally is found where
fertilization is external or where sperm are discharged all at once during a short reproductive period. (B) More complicated variety of reproductive duct, connected with the
testis by means of efferent ducts, but possessing an anterior twisted portion, the epididymal
duct in which the sperm are stored and physiologically matured. This type of duct
generally is found in those vertebrates which utilize internal fertilization and where
sperm are discharged over a short or extended reproductive period.
b. The Epididymis as a Sperm-ripening Structure
On the other hand, in those forms which possess an anterior convoluted epididymal portion of the reproductive duct, the journey of the sperm through this portion of the duct appears to be necessary in order that fertilizable sperm may be produced. In mammals it has been shown that the epididymal journey somehow conditions the physiological ripening of the sperm. Sperm taken from the mammalian testis will not fertilize; those from the caudal portion of the epididymis will, provided they have been in the epididymis long enough. Under normal conditions sperm pass through the epididymis slowly, and retain their viability after many days’ residence in this structure. Sperm prove to be fertile in the rabbit epididymis up to about the thirty-eighth day; if kept somewhat longer than this, they become senile and lose the ability to fertilize, although morphologically they may seem to be normal (Hammond and Asdell, ’26). In the rat, they may live up to 20 to 30 days in the epididymis and still be capable of fertilization (Moore, ’28). It has been estimated that the epididymal journey in the guinea pig consumes about two weeks, although they may live and retain their fertilizing power as long as 30 days in epididymides which have been isolated by constriction (Moore and McGee, ’28; Young, ’31; Young, ’31b). It is said that in the bull, sperm within the epididymis may live and be motile for two months. As a result of these facts, it may be concluded that the epididymal journey normally is a slow process, and that it is beneficial for the development of sperm “ripeness†or ability to fertilize.
c. The Epididymis and Vas Deferens as Sperm-storage Organs
Along with the maturing faculty, the epididymal duct and vas deferens also act as sperm-storage organs. As observed on p. 23, in the bat, Myotis, sperm are formed in great numbers in the seminiferous tubules and pass to the epididymal duct where they are stored during the fall, winter, and early spring months; the epididymal journey thus is greatly prolonged in this species. In the ovoviviparous garter snake, Thamnophis radix, sperm are produced during the summer months; they pass into the epididymides during early autumn and are stored there during the fall and winter. In the mammal, sperm are stored in the epididymal duct.
Aside from its main purpose of transporting sperm to the exterior (see sperm transport, p. 177), the caudal portion of the sperm duct or vas deferens also is capable of storing sperm for considerable periods of time. In the common perch, Perea ftavescens, sperm are developed in the testes in the autumn, pass gradually into the accessory reproductive ducts, and are stored there for five or six months until the breeding season the following spring (Turner, C. L., ’19). Again, in mammals, the ampullary region of the vas deferens appears to be a site for sperm storage. For example, the ampulla of the bull sometimes is massaged through the rectal wall to obtain sperm for artificial insemination. In this form sperm may be stored in the ampulla and still be viable, for as long as three days. Similarly, in lower vertebrates large numbers of sperm may be found in the posterior extremities of the vas deferens during the breeding season. Thus, the reproductive duct (and its epididymal portion when present) is instrumental in many vertebrate species as a temporary storage place for the sperm.
d. Two Types of Vertebrate Testes Relative to Sperm Formation
The importance of the epididymal duct in many vertebrates and its relative absence in others, focuses attention upon the fact that in many vertebrate species sperm are produced, stored, and physiologically matured entirely within the confines of the testis (frog, bowfin, stickleback, etc.). The reproductive duct under these circumstances is used mainly for sperm transport. In many other vertebrate species sperm are morphologically formed in the testis and then are passed on into the accessory structures for storage and physiological maturation. Functionally, therefore, two types of testes and two types of accessory reproductive ducts are found among the vertebrate group of animals (fig. 17). It naturally follows that the testis which produces, stores, and physiologically matures sperm is best adapted for seasonal activity, particularly where one female is served during the reproductive activities. That is, it functions as an “all at one time†spawning mechanism. On the other hand, that testis which produces sperm morphologically and passes them on to a tortuous epididymal duct for storage and physiological maturing is best adapted for the continuous type of sperm production or for the service of several females during a single seasonal period. The sperm, under these conditions, pass slowly through the epididymal duct, and, therefore, may be discharged intermittently.
4. Function of the Seminal Vesicles (Vesicular Glands)
The seminal vesicles show much diversity in their distribution among various mammals. Forms like the cat, dog, opossum, rabbit, sloth, armadillo, whale, do not possess them, while in man, rat, elephant, mouse, they are welldeveloped structures. It was formerly thought that the seminal vesicles in mammals acted as a storehouse for the sperm, hence the name. In reality they are glandular structures which add their contents to the seminal fluid during the sexual act.
5. Function of the Prostate Gland
The prostate gland also is a variable structure and is found entirely in the marsupial and eutherian mammals. In marsupials it is confined to the prostatic portion of the urethral wall; in man it is a rounded, bulbous structure which surrounds the urethra close to the urinary bladder. In many other mammals it is a much smaller and less conspicuous structure. It discharges its contents into the seminal fluid during the orgasm. It is probable that the prostatic and vesicular fluids form the so-called “vaginal plug’’ in the vagina of the rat, mouse, etc.
6. Bulbourethral (Cowper’s) Glands
The bulbourethral glands are absent in the dog but present in most other mammals. In marsupials and monotremes these structures are exceptionally well formed. In the opossum there are three pairs of bulbourethral glands. The mucous contents of these and other small urethral glands are discharged at the beginning of the sexual climax and, as such, become part of the seminal fluid.
7. Functions of Seminal Fluid
a. Amount of Seminal Fluid Discharged and Its General Functions
As stated previously, the semen or seminal fluid is composed of two parts, the sperm cells (spermia; spermatozoa) and the seminal plasma. The presence of the sperm cells represents the most constant feature, although they may vary considerably from species to species in size, shape, structure, and number present. The seminal plasma varies greatly as to composition and amount discharged.
The quantity of seminal fluid discharged per ejaculate and the relative numbers of sperm present in man and a few other vertebrate species associated with him are as follows:*
Species
Volume of Single Ejaculate, Most Common Value, in CC.
Sperm Density in Semen, Average Value, per CC.
Boar
250
CC.
100,000,000
per
CC.
Bull
4-5
CC.
1,000,000,000
per
CC.
Cock
0.8
CC.
3,500,000,000
per
CC.
Dog
6
CC.
200,000,000
per
CC.
Man
3.5
CC.
100,000,000
per
CC.
Rabbit
1
CC.
700,000,000
per
CC.
Ram
1
CC.
3,000,000,000
per
CC.
Stallion
70
CC.
120,000,000
per
CC.
Turkey
0.3
CC.
7,000,000,000
per
CC.
- Modified from Mann (’50).
Two important branches of study involving the semen pertain to:
(1) the chemical and physiological nature and numerical presence of the sperm, and
(2) the physiology and biochemistry of the seminal plasma.
(See Mann, ’50, for discussion and bibliography.) As a result of the studies thus far, a considerable body of information has been accumulated.
The main function of the semen, including the plasma and accessory sperm, appears to be to assist the sperm cell whose chance fortune it is to make contact with the egg. Once this association is accomplished, the egg seemingly takes over the problem of fertilization. The seminal plasma and the accessory numbers of sperm appear to act as an important protective bodyguard and also as an aid for this event. Modern research emphasizes, therefore, that the work of the male reproductive system is not complete until this contact is made.
b. Coagulation of the Semen
In many mammalian species, the semen tends to coagulate after its discharge from the male system. In the mouse, rat, guinea pig, opossum, rhesus monkey, etc., the semen coagulates into a solid mass, the vaginal plug, once it reaches the vagina of the female. The probable function of the vaginal plug is to prevent the semen from seeping out of the vagina. The formation of this plug may be due to a protein present in the contents of the seminal vesicle which comes in contact with the enzyme, vesiculase. In the rat and guinea pig this catalyst probably is produced by the “coagulating gland,†a specialized structure associated with the seminal vesicles in these forms. Some of it also may come from the prostate.
Coagulation of the seminal fluid also occurs in man, stallion, and boar but it is entirely absent in the dog, bull, and many other animals. Human semen coagulates immediately after discharge but liquefies a short time afterward. This liquefaction may be due to the presence of two enzymes, fibrinogenase and fibrinolysin, found in human semen and both derived from the prostate. These enzymes are found also in dog semen. In the latter their property of inhibiting blood coagulation may be of use where considerable amounts of blood may be present in the female genital tract at the onset of full estrous conditions. Another important contribution of the prostate gland is citric acid. Its role is not clear but it may enter into the above coagulation-liquefaction process (Mann, ’50, p. 348).
c. Hyaluronidase
Various enzymes have been demonstrated to be present in the semen of certain invertebrates and vertebrates. One such enzyme is hyaluronidase which appears to be produced in the testes of the rat, rabbit, boar, bull, and man. It is not found in the testes of vertebrates below the mammals. Its specific function is associated with the dispersal of the follicle cells surrounding the egg; in so doing it may aid the process of fertilization in mammals.
d. Accessory Sperm
One sperm normally effects a union with the egg in fertilization. Accessory sperm may enter large-yolked eggs, but only one is intimately involved in the union with the egg pronucleus. However, what is meant by accessory sperm here is the large number of sperm which normally clusters around the egg during the fertilization process in many animal species. A suggestion of a function for these accessory sperm follows from the fact that hyaluronidase may be extracted from the semen, presumably from the sperm themselves. Rowlands (’44) and also Leonard and Kurzrok (’46) have shown that a seminal fluid deficient in sperm numbers may fertilize if hyaluronidase extracted from sperm (?) is added to such a weakened sperm suspension. The implication is that the accessory sperm thus may act as “cupbearers†for the one successful sperm in that they carry hyaluronidase which aids in liquefying the follicle cells and other gelatinous coating material around the egg.
e. Fructose
An older concept in embryology maintained that sperm were unable to obtain or utilize nourishment after they departed from the testis. More recent investigation has shown, however, that sperm do utilize certain sugar materials, and that their survival depends upon the presence of a simple sugar in the medium in which they are kept. (See Mann, ’50.)
The sugar that is found normally in semen is fructose. It varies in quantity from species to species, being small in amount in the semen of the boar or stallion but considerably larger in quantity in the seminal fluid of the bull, man, and rabbit. The seat of origin of this sugar appears to be the seminal vesicle, at least in man, although the prostate may also be involved, particularly in the rabbit and also in the dog. The dog, however, has but a small amount of fructose in the seminal discharge. The real function of seminal fructose “might be as a readily utilizable store of energy for the survival of motile spermatozoa†(Mann, ’50, p. 360).
f. Enzyme-protecting Substances
Runnstrom (personal communication) and his co-workers have demonstrated that the fertilizing life of sea-urchin sperm is increased by certain substances found in the jelly coat of the sea-urchin egg. Presumably these substances are protein in nature, and, according to Runnstrom, they may act to preserve the enzyme system of the sperm. Similarly, the seminal fluid may act to preserve the enzyme system of the sperm, while en route to the egg, especially within the female genital tract.
D. Internal and External Factors Influencing Activities of the Testis
Conditions which influence testicular activity are many. Many of the factors are unknown. Nevertheless, a few conditions which govern testis function have been determined, especially in certain mammalian species. The general results of experimental determination of some of the agents which affect testicular function are briefly outlined below.
1. Internal Factors
a. Temperature and Anatomical Position of the Testis
It is well known that in those mammals which have a permanent scrotal residence of the testes failure of the testis or testes to descend properly into the scrotum results in a corresponding failure of the seminiferous tubules to produce sperm. In these instances the testis may appear shriveled and shrunken (fig. 18). However, such cryptorchid (ectopic) conditions in most cases retain the ability to produce the sex hormone at least to some degree. A question therefore arises relative to the factors which inhibit seminiferous tubule activity within the cryptorchid testis.
The failure of cryptorchid testes to produce viable sperm has been of interest for a long time. Observations have demonstrated that the more hidden
Fig. 18. Experimental unilateral cryptorchidism in adult rat. The animal's left testis was confined within the abdominal cavity for six months, whereas the right testis was pernfitted to reside in the normal scrotal position. Observe the shrunken condition of the cryptorchid member. (After Turner: General Endocrinology, Philadelphia, Saunders.)
the testis (i.e., the nearer the peritoneal cavity) the less likely are mature
sperm to be formed. A testis, in the lower inguinal canal or upper scrotal
area is more normal in sperm production than one located in the upper
inguinal canal or inside the inguinal ring. Studies made upon peritoneal and
scrotal temperatures of rats, rabbits, guinea pigs, etc., demonstrate a temperature in the scrotum several degrees lower than that which obtains in
the abdomen. These observations suggest that the higher temperature of the
non-scrotal areas is a definite factor in bringing about seminiferous tubule
injury and failure to produce sperm.
With this temperature factor in mind, Dr. Carl R. Moore (in Allen, Danforth, and Doisy, ’39) and others performed experiments designed to test its validity as a controlling influence. They found that confinement alone of an adult guinea pig testicle in the abdomen led to marked disorganization of all seminiferous tubules in seven days. After several months of such confinement the seminiferous tubules experience marked degenerative changes and only Sertoli cells remain (fig. 19A, B). The interstitial tissue, however, is not greatly impaired. If such a testis is kept not too long within the abnormal position and once again is returned to the scrotum, spermatogenesis is rejuvenated (fig. 20A, B). In a second experiment, the scrotum of a ram was encased loosely with insulating material; a rapid degeneration of the seminiferous tubules followed. Young (’27, ’29) in a third type of experiment found that water 6 to 7° warmer than the body temperature applied to the external aspect of the guinea-pig testis for a 15-minute period evoked degenerative
Fig. 19. Sections of experimental, cryptorchid, guinea-pig, seminiferous tubules and interstitial tissue. (Modified from C. R. Moore in Sex & Internal Secretions, Williams & Wilkins, Baltimore, 1939.) (A) Testis confined to abdomen for three months. (B)
Testis confined to abdomen for six months. Observe degenerate state of seminiferous tubule after six months’ confinement. Interstitial tissue not greatly affected by confinement.
changes with temporary sterility (fig. 21). Recovery, however, is the rule in
the latter instance. Summarizing the effects of such experiments involving
temperature, Moore (in Allen, Danforth, and Doisy, ’39, p. 371) concludes:
“The injury developing from applied heat, although more rapidly effective,
is entirely similar to that induced by the normal body temperature when the
testicle is removed from the scrotum to the abdomen.â€
The position of the scrotum and its anatomical structure is such as to enhance its purpose as a regulator of testicular temperature (figs. 2, 6). When the surrounding temperature is cold, the contraction of the dartos muscle tissue of the scrotal skin contracts the scrotum as a whole, while the contraction of the cremaster muscle loops pulls the testes and the scrotum closer to the body, thus conserving the contained heat. When the surrounding temperature is warm, these muscles relax, producing a more pendulous condition to permit heat loss from the scrotal wall.
In accordance with the foregoing description of the scrotum as a necessary thermoregulator for the testis, it has been further shown for those mammals which possess a scrotum that testis grafts fare much better when transplanted to the scrotal wall or into the anterior chamber of the eye (Turner, C. D., '48). The anterior chamber of the eyeball possesses a temperature much cooler than the internal parts of the body.
Fig. 20. Sections of testis during and after abdominal confinement. (Modified from
C. R. Moore in Sex & Internal Secretions, Williams & Wilkins, Baltimore, 1939.) (A)
Section of left testis to show degenerate state of seminiferous tubules after 24 days of abdominal confinement. (B) Section of right testis 74 days after replacement in scrotum. Observe spermatogenic activity in tubules.
Fig. 21. Effect of higher temperature applied to external surface of guinea-pig testis. Water, 47®, was applied to surface of scrotum for period of 10 minutes. Testis was removed from animal 12 days after treatment. Seminiferous tubules are degenerate. (Modified from Moore, ’39; see also Young, ’27, J. Exp. Zool., 49.)
Two types of seminiferous tubules are thus found in mammals. In a few mammalian species (see p. 6) the temperature of the peritoneal cavity is favorable to the well-being of the seminiferous tubule; in most mammalian species, however, a lower temperature is required. On the other hand, the activities of the interstitial tissue of the testis appear to be much less sensitive to the surrounding temperature conditions, and the male sex hormone may be produced when the testes are removed from the scrotum and placed within the peritoneal cavity.
With regard to the functioning of the testis within the peritoneal cavity of birds it has been suggested that the air sacs may function to lower the temperature around the testis (Cowles and Nordstrom, ’46). In the sparrow, Riley (’37) found that mitotic activity in the testis is greatest during the early morning hours when the bird is resting and the body temperature is lower, by 3 or 4° C.
b. Body Nourishment in Relation to Testicular Function
The testis is a part of, and therefore dependent upon, the well-being of the body as a whole. However, as observed in the preceding pages the interstitial cells and their activities in the production of the male sex hormone are less sensitive to the internal environment of the body than are the seminiferous tubules.
The separation of these two phases of testicular function is well demonstrated during starvation and general inanition of the body as a whole. A falling off of sperm production is a definite result of starvation diets, although the germinative cells do not readily lose their ability to proliferate even after prolonged periods of starvation. But the interstitial cells and the cells of Sertoli are not as readily affected by inadequate diets or moderate starvation periods. Sex drive may be maintained in a starving animal, while his ability to produce mature, healthy sperm is lost. On the other hand, long periods of inanition also affect sex hormone production and the sexual interests of the animal.
Aside from the abundance of food in a well-rounded dietary regime, adequate supplies of various vitamins have been shown to be essential. Vitamin Bi is essential to the maintenance of the seminiferous tubules in pigeons. Pronounced degenerative changes in the seminiferous tubules of rats and other mammals occur in the absence of vitamins A and E (Mason, ’39). Prolonged absence of vitamin E produces an irreparable injury to the testis of rats; injury produced by vitamin A deficiency is reparable. The B-complex of vitamins seems to be especially important for the maintenance of the accessory reproductive structures, such as the prostate, seminal vesicles, etc. The absence of vitamin C has a general body effect, but does not influence the testis directly. Spme of these effects may be mediated through the pituitary gland. As vitamin D is intimately associated with the mineral metabolism of the body, it is not easy to demonstrate its direct importance.
c. The Hypophysis and Its Relation to Testicular Function
The word “hypophysis†literally means a process extending out below. The early anatomists regarded the hypophysis cerebri as a process of the brain more or less vestigial in character. It was long regarded as a structure through which waste materials from the brain filtered out through supposed openings into the nasal cavity. These wastes were in the form of mucus or phlegm, hence the name “pituitary,†derived from a Latin word meaning “mucus.†The word pituitary is often used synonymously with the word hypophysis.
The hypophysis is made up of the pars anterior or anterior lobe, pars intermedia or intermediate lobe, and a processus infundibuli or posterior lobe. The anterior lobe is a structure of great importance to the reproductive system; its removal (ablation) results in profound atrophic changes throughout the entire reproductive tract.
The importance of the pituitary gland in controlling reproductive phenomena was aroused by the work of Crowe, Cushing, and Homans (TO) and by Aschner (’12) who successfully removed the hypophysis of young dogs. One of the first fruits of this work was a demonstration of the lack of genital development when this organ was removed. Since that time many
the other cohabitants of man — rats, mice, cats, rabbits, etc. — have been hypophysectomized, and in all cases a rapid involution and atrophy of the genital structures results from pituitary removal. The testis undergoes profound shrinkage and regression following hypophysectomy, the degree of change* varying with the species. In the rooster and monkey, for example, regressive changes are more marked than in the rat. (Consult Smith, ’39, for data and references.)
A striking demonstration of the influence of the hypophysis upon the genital tract is the result of its removal from a seasonal-breeding species, such as the ferret. Ablation of the pituitary in this species during the nonbreeding season causes slight if any change in the testis and accessory reproductive organs. However, when it is removed during the breeding season, a marked regression to a condition similar to that present during the nonbreeding season occurs (Hill and Parkes, ’33).
The experimental result of hypophysectomy on many animal species thus points directly to this structure as the site of hormonal secretion, particularly to the anterior lobe (Smith, ’39). The initial work on the relation of pituitary hormones and the gonad was done upon the female animal. The results of these studies aroused the question whether one or two hormones were responsible. The latter alternative was suggested by the work of Aschheim and Zondek (’27) and Zondek (’30) who concluded that two separate substances appeared to be concerned with the control of ovarian changes.
Nevertheless, for a time the concept of only one gonad -con trolling (gonadotrophic) hormone was produced by the pituitary, continued to gain attention, and some workers suggested that the two ovarian elfects of follicular growth and luteinization of the follicle were due to the length of time of administration of one hormone and not to two separate substances. However, this position soon was made untenable by research upon the gonadotrophic substances derived from the pituitary gland. Studies along this line by Fevold, Hisaw, and Leonard (’31) and Fevold and Hisaw (’34) reported the fractionation, from pituitary gland sources, of two gonadotrophic substances, a follicle-stimulating factor or FSH and a luteinization factor or LH. This work has been extensively confirmed. It should be observed in passing that the male pituitary gland contains large amounts of FSH, although, as mentioned below, the function of the testis and the male reproductive system relies to a great extent upon the luteinizing factor. Some investigators refer to the LH factor as the interstitial-cell-stimulating hormone, ICSH. (See Evans, ’47; and also Evans and Simpson in Pincus and Thimann, ’50.)
The action of these two hormones upon testicular tissue, according to present information, is somewhat as follows: If pure follicle-stimulating hormone, FSH, which produces only FSH effects in the female, is injected in low doses into hypophysectomized male rats, the seminiferous tubules are stimulated and spermatogenesis occurs. Under these conditions, the interstitial tissue remains unstimulated and the accessories continue in an atrophic state. It has further been demonstrated that slight amounts of the luteinizing gonadotrophic hormone, LH (ICSH), added to the above injections of FSH, effects a much better stimulation of the spermatogonial tissue, and the interstitial tissue also develops well.
On the other hand, when pure LH (ICSH) is given alone in small doses, spermatogenesis is stimulated with slight or no effect upon the male accessory structures. However, when larger doses of the LH (ICSH) factor alone are injected, the interstitial tissue is greatly stimulated, and the testicular weight increases much more than when FSH alone is given. Furthermore, the accessory reproductive structures are stimulated and become well developed, suggesting the elaboration of the male sex hormone. In agreement with these results, the administration alone of testosterone, the male sex hormone, increases the weight and development of the accessory structures in hypophysectomized animals and it also maintains spermatogenesis. It appears, therefore, that the effects of the LH substance upon the seminiferous tubules and the accessory organs occur by means of its ability to arouse the formation of the male sex hormone.
A summary of the actions of the pituitary gonadotrophic hormones upon testicular tissue may be stated as follows:
( 1 ) Pure FSH in small doses stimulates the seminiferous tubules and spermatogenesis with little or no effect upon the interstitial tissue or the accessory reproductive structures, such as the seminal vesicles or prostate gland;
(2) Small doses of pure LH also stimulate spermatogenesis with little or no stimulation of the accessory structures;
(3) Pure LH (ICSH) in larger doses stimulates the development of the interstitial tissue with the subsequent secretion of the male sex hormone and hypertrophy of the accessory reproductive organs;
(4) The male sex hormone in some way aids or stimulates the process of spermatogenesis, suggesting that the action of LH occurs through the medium of the sex hormone (fig. 22).
(Consult Evans and Simpson in Pincus and Thimann, ’50, for data and references; also Turner, C. D., ’48.)
The foregoing results of the action of the FSH and LH upon testicular function might suggest that the LH substance alone is essential in the male animal. However, it should be observed that without the presence of FSH, LH is not able to maintain the tubules in a strictly normal manner, the tubules showing a diminution of size. Also, in extreme atrophic conditions of the tubules, pure FSH stimulates spermatogenesis better than similar quantities of LH. It is probable that FSH and LH (ICSH) work together to effect complete normality in the male. This combined effect is known as a synergistic effect. It also is of interest that the injection of small doses of testosterone propionate into the normal male, with the pituitary gland intact, results in inhibition of the seminiferous tubules, probably due to the suppression of pituitary secretion by the increased atnount of the male sex hormone in the blood. However, high doses, while they likewise inhibit the pituitary, result in a level of androgen which stimulates the seminiferous tubules directly (Ludwig, ’50).
Aside from the above actions upon testicular tissue by the luteinizing hormone (LH;ICSH) certain other functions of this substance should be mentioned (see fig. 22). One of these is the apparent dependence of the Sertoli cells upon the presence of the interstitial cells (Williams, ’50). Interstitial tissue behavior and development in turn relies mainly upon LH (ICSH) (Fevold, ’39; Evans and Simpson in Pincus and Thimann, ’50). As the sperm are intimately associated with the Sertoli elements during the latter phases of spermatogenesis in which they transform from the spermatid into the form of the adult sperm, a very close association and reliance upon the presence of the luteinizing hormone thus appears to be established in sperm development.
A further study of the LH factor is associated with the maintenance of the seminiferous tubules themselves. In aged males, the interstitial tissue and the seminiferous tubules normally involute and regress with accumulation of large amounts of connective tissue material. In testicular grafts made into the rabbit’s ear, Williams (’50) found, when interstitial tissue was present in the grafts, the seminiferous tubules were more nearly normal; when absent, the tubules underwent fibrosis.
Another function of the LH substance apparently is concerned with release of the sperm from the Sertoli cells. De Robertis, et al. (’46), showed that anterior pituitary hormones possibly cause release of sperm from the Sertoli cells in the toad by the production of vacuoles and apical destruction of the cytoplasm of the Sertoli elements. In testicular grafts Williams (’50) accumulated evidence which suggests that vacuoles and secretion droplets in the Sertoli cells occurred as a result of LH administration. The combined results of these investigators suggest that sperm release from the Sertoli cell is dependent, in some way, upon LH (ICSH) activity.
A final function is concerned with the physiological maturing of sperm in the reproductive duct, at least in many vertebrate species. The well-being of the epididymis and vas deferens is dependent upon the presence of the male sex hormone (Creep, Fevold, and Hisaw, ’36). As the male sex hormone results from stimulation of the interstitial cells by the interstitial-cellstimulating substance, LH (ICSH), the connection between this substance and the physiological maturation of the sperm cell is obvious.
2. External Environmental Factors and Testis Function
As we have seen above, the anterior lobe of the hypophysis acts as the main internal environmental factor controlling the testes and, through them, the reproductive ducts. It has been observed also that food, vitamins, and anatomical position of the testis are important influences in regulating testicular function. Furthermore, general physiological conditions such as health or disease have an important bearing upon the gonads (Mills, ’19). All of the above conditions are contained within the body of the organism, and as such represent organismal conditions.
Fio. 22. Chart showing the effects of the hypophyseal anterior lobe upon the developing gametes. It also suggests the various factors influencing pituitary secretion of the
gonadotrophic hormones, FSH and LH. Observe that the primitive gamete in the cortex
of the ovary is subjected to the cortical environment and develops into an oocyte, whereas
in the medullary or testicular environment it develops into a spermatocyte. Experiments
upon sex reversal have demonstrated that the medullary and cortical portions of the
gonad determine the fate of the germ cell. In the male area or medulla, the germ cell
differentiates in the male direction, while in the cortex, the differentiation is in the
direction of the female gamete or oocyte, regardless of the innate sex-chromosome constitution of the primitive germ cell. The fate of the germ cell thus is influenced by four
main sets of factors: (1) Internal and external environmental factors, controlling the
secretions of the pituitary body, (2) Fnvironment of the testicular tissue (medulla) and
possible humoral substances produced in this tissue, (3) Environment of the ovarian
tissue (cortex) and possible humoral substances elaborated there, and (4) Secretions of
the anterior lobe of the pituitary body.
The following question naturally arises: Do factors or conditions external
to the body impinge themselves in such a way as to control pituitary and
gonadal function?
a. Light as a Factor
Aside from the supply of nutritive substances or the collision of the many nervous stimuli with the individual which may arouse or depress the sexual activities, two of the most important obvious external factors are temperature and light. Research on the reproductive behavior of many animal species, during the past twenty years, has shown that both of these factors have great significance on the reproductive activities of many vertebrate species. Bissonnette (’30, ’32, ’35, a and b) has accumulated evidence which demonstrates that light is a potent factor in controlling the reproductive behavior of the European starling (Sturnus vulgaris) and also of the ferret (Putorius vulgaris). In the starling, for example, the evidence shows that green wave lengths of the spectrum inhibit testicular activity, while red rays and white light arouse the reproductive function (fig. 23). The addition of electric lighting to each day’s duration produced a total testis size in midwinter which surpassed the normal condition in the spring. In the ferret artificially increased day length beginning at the first part of October brings the testis to maximum size and activity coupled with a normal mating impulse as early as November and December (fig. 24). Under normal conditions the male ferret is able to breed only during February and early March,
These findings relative to the influence of light on the reproductive periodicity of animals confirm a fact which has been known for a long time, namely, that seasonal breeders brought from the northern hemisphere to the southern hemisphere reverse their breeding season. For example, ferrets which normally breed from spring to summer in the northern hemisphere shift their breeding habits to the September-February period when moved to the southern hemisphere. Inasmuch as the hypophysis is instrumental in bringing about secretion of the gonadotrophic hormones responsible for the testicular activity, it is highly probable that light coming through the eyes (see Hill and Parkes, ’33) influences the nervous system in some way arousing the hypophysis and stimulating it to secrete these substances in greater quantity. However, one must keep in mind the caution given by Bissonnette, that light is not the only factor conditioning the sexual cycles of ferrets and starlings.
While numerous animals, such as the migratory birds, ferret, mare, many fish, frogs, etc., normally are brought into a breeding condition during the period of light ascendency, a large number of animals experience a sexual resurgence only during the time of year when the light of day is regressing in span. This condition is found in some sheep, goats, buffalo in nature.
Fig. 23. Sections of testis of the starling (Sturnus vulgaris), showing the effect of electric lighting added to the bird’s normal daily duration of light during the autumn. (After Bissonnette, Physiol. Zool., 4.) (A) Inside young control bird — no light added
— kept inside as control for (B) from November 9 to December 13. (B) Inside young
experimental bird, receiving additional light from “25 watt†bulb from November 9 to December 13. Total treatment, 34 davs.
Fig. 24. Sections of testis and epididymis, showing modification of sexual cycle in the
ferret, Putorius vulgaris, by exposure to increasing periods of light. (After Bissonnette,
’35b.) (A) Seminiferous tubules from normal male over 1 year old, made on October
3, no lighting. (B) Epididymis of normal male on October 3, no lighting. (C) Seminiferous tubules of experimental male on November 7, 36 days of added lighting. (D) Epididymis of experimental males on Nov Tiber 7, 36 days of added lighting.
deer, some fish, etc. Bissonnette (’41) working with goats found that: “Increasing daily light periods from January 25 to April 5 — followed by diminishing periods until July 5, while temperatures remained normal for the seasons,
with four Toggenburg female goats and one male Toggenburg and one Nubian
female — led to cessation of breeding cycles in February instead of March,
followed by initiation of breeding cycles in May and June instead of September.†In the ewe, Yeates (’47) also found that a change from increasing
daylight to decreasing length of day induced reproductive activity. In a similar
manner. Hoover and Hubbard (’37) were able to modify the sexual cycle
in a variety of brook trout which normally breeds in December to a breeding
season in August.
b. Temperature Influences
In the case of the animals mentioned above, temperature does not appear to be a major factor in inducing reproductive activity. However, in many animals temperature is vitally influential in this respect. For example, in the thirteen-lined spermophile (ground squirrel) Wells (’35) observed that breeding males kept at 40° F. continued in a breeding condition throughout the year. Under normal conditions this rodent hibernates during the winter months and comes forth in the spring ready to breed; sperm proliferation and general reproductive development take place during the period of hibernation. As the temperature rises during the spring and summer, testicular atrophy ensues, followed by a period of spermatogenesis and reproductive activity when the lowered temperatures of autumn and winter come again. Light, seemingly, is not a factor in this sexual cycle. Another instance of temperature control occurs in the sexual phase of the common red newt, Triturus viriciescens. Here it is the rising temperature of the summer which acts as the inducing agent, and sperm thus produced are discharged into the accessory ducts during the fall and winter to be used when copulation occurs in early spring. However, if this species is kept at a relatively low temperature of 8 to 12° C. during the summer months, spermatogenesis is inhibited and the testis regresses. In the stickleback, Gasterosteus aculeatus, as reported by Craig-Bennett (’31), spermatogenesis occurs during July to early September and appears to be conditioned by a rising temperature, whereas the interstitial tissue and the appearance of secondary sexual features reach their greatest development under increased light conditions and slowly rising temperatures (fig. 15). Bissonnette, in his work on ferrets, also observed a difference in the behavior of these two testicular components; the interstitial tissue responds to large increases of daily light periods, whereas the seminiferous tubules are stimulated by small, gradually increasing periods of light.
The above examples emphasize the importance of a single environmental factor on the pituitary-gonadal relationship. However, in the hedgehog, Allanson and Deansley (’34) emphasize temperature, lighting, and hormone injections as factors modifying the sexual cycles, while Baker and Ransom (’32, ’33, a and b) show that light, food, temperature, and locality affect the sexual cycles and breeding habits of the field mouse. In some vertebrates, therefore, a single factor may be the dominant one, whereas in others, numerous factors control the action of the pituitary and reproductive system.
E. Internal Factors Which May Control Seasonal and Continuous Types of Testicular Function
In endeavoring to explain the differences in response to external environmental factors on the part of seasonal and continuous breeders, one must keep in mind the following possibilities:
(1) The anterior lobe of the hypophysis in some forms (e.g., ferret) cannot be maintained in a secretory condition after it has reached its climax; that is, it apparently becomes insensitive to the light factor. As a result, regression of the pituitary and testis occurs (Bissonnette, ’35b).
(2) In the starling, the anterior hypophysis may be maintained by the lighting, but the testis itself does not respond to the presence of the hypophyseal hormones in the blood (Bissonnette, ’35b). The possibility in this instance may be that testicular function wanes because the body rapidly eliminates the hormone in some way (see Bachman, Collip, and Selye, ’34).
(3) Consideration also must be given to the suggestion that the activities of the sex gland by the secretion of the sex hormone may suppress anterior lobe activity (Moore and Price, ’32).
We may consider two further possibilities relative to continuous testicular function :
(4) If the “brake actions†mentioned above are not present or present only in a slight degree, a degree not sufficient to interrupt the activities of the anterior lobe or of the sex gland, a more or less continuous function of the testis may be maintained.
(5) When several or many environmental factors are concerned in producing testicular activity, a slight altering of one factor, such as light, may prove insufficient to interrupt the pituitary-germ-gland relationship, and a continuous breeding state is effected in spite of seasonal changes.
Underlying the above possibilities which may control testicular function is the inherent tendency or hereditary constitution of the animal. In the final analysis, it is this constitution which responds to environmental stimuli, and moreover, controls the entire metabolism of the body. In other words, the above-mentioned possibilities tend to oversimplify the problem. The organism as a whole must be considered; reproduction is not merely an environmentalpituitary-sex gland relationship.
F. Characteristics of the Male Reproductive Cycle and Its Relation to Reproductive Conditions in the Female
As indicated above, reproduction in the male vertebrate is either a continuous process throughout the reproductive life of the individual or it is a discontinuous, periodic affair. In the continuous form of reproduction the activities of the seminiferous tubules and the interstitial or hormone-producing tissues of the testis function side by side in a continuous fashion. In the discontinuous, periodic type of testicular function, the activities of the seminiferous tubules and of the interstitial tissue do not always coincide. The activities of the seminiferous tubules, resulting in the production of sperm for a particular reproductive cycle, tend to precede, in some species by many months, the activities of the sex-hormone-producing tissue. Evidently, the output of the FSH and LH substances from the pituitary gland are spread out over different periods of the year to harmonize with this activity of the testicular components.
It will be seen in the next chapter that a continuous breeding faculty is not present in the female comparable to that of the male. All females are discontinuous breeders. In some species, the cycles follow each other with little rest between each cycle unless the female becomes pregnant or “broody.†Some have a series of cycles over one part of the year but experience sexual quiescence over the remaining portion of the year. However, in most female vertebrates there is but one reproductive cycle per year.
In harmony with the above conditions, the continuous variety of testicular function is always associated with the condition in the female where more than one reproductive cycle occurs per year. Continuous reproductive conditions in the male, therefore, are adapted to serve one female two or more times per year or several different females at intervals through the year. Furthermore, the complicated, highly glandular, greatly extended type of male-reproductive-duct system is adapted to conditions of (1) continuous breeding, or (2) service to more than one female during one breeding season of the year, whereas the simple type of reproductive duct is adapted to the type of service where all or most of the genital products are discharged during one brief period. In other words, the entire male reproductive system and reproductive habits are adapted to the behavior of female reproductive activities.
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The Development of the Gametes or Sex Cells
A. General Considerations
In the two preceding chapters the conditions which prepare the male and female parents for their reproductive responsibilities are considered. This chapter is devoted to changes which the male and female germ cells must experience to enable them to take part in the processes involved in the reproduction of a new individual.
The gamete is a highly specialized sex cell or protoplasmic entity so differentiated that it is capable of union (fertilization; syngamy) with a sex cell 6T the opposite sex to form the zygote from which the new individual arises. TE^ process of differentiation whereby the primitive germ cell is converted into the mature gamete is called the maturation of the germ cell.
The main events which culminate in the fully-developed germ cell are possible only after the primitive or undifferentiated germ cell has reached a certain condition known as the definitive state. When this stage is reached, the germ cell has acquired the requisite qualities which make it possible for it to differentiate into a mature gamete. Before the definitive state is reached, germ cells pass through an eventful history which involves:
- their so-called "origin" or first detectable appearance among the othercells of the developing body, and
- their migration to the site of the future ovary or testis.
After entering the developing substance of the sex gland, the primitive germ cells experience a period of multiplication. If the sex gland is that of the male, these undifferentiated sex cells are called spermatogonia; if female, they are known as oogonia.
B. Controversy Regarding Germ-cell Origin
The problems of germ-cell origin in the individual organism and of the continuity of the germ plasm from one generation to the next have long been matters of controversy. Great interest in these problems was aroused by the ideas set forth by Waldeyer, Nussbaum, and Weismann during the latter part of the nineteenth century. Waldeyer, 1870, as a result of his studies on the chick, presented the “germinal epithelium†hypothesis, which maintains that the germ cells arise from the coelomic epithelium covering the gonad. Nussbaum, 1880, championed the concept of the extra-gonadal origin of the germ cells. According to this view, derived from his studies on frog and trout development, the germ cells arise at an early period of embryonic development outside the germ-gland area and migrate to the site and into the substance of the germ gland.
At about this time the speculative writings of August Weismann aroused
great interest. In 1885 and 1892 Weismann rejected the popular Darwinian
theory of pangenesis, which held that representative heredity particles or
“gemmules†passed from the body cells (i.e., soma cells) to the germ cells
and were there stored in the germ cells to develop in the next generation
(Weismann, 1893). In contrast to this hypothesis he emphasized a complete
independence of the germ plasm from the somatoplasm. He further suggested
that the soma did not produce the germ plasm as implied in the pangenesis
theory, but, on the contrary, the soma resulted from a differentiation of the
germ plasm.
According to the Weismannian view, the germ plasm is localized in the
chromosomal material of the nucleus. During development this germ plasm
is segregated qualitatively during successive cell divisions with the result that
the cells of different organs possess different determiners. However, the nuclear
germ plasm (Keimplasma) is not so dispersed or segregated in those cells
which are to become the primitive sex cells; they receive the full complement
of the hereditary determiners for the various cells and organs characteristic
of the species. Thus, it did not matter whether the germ cells were segregated
early in development or later, so long as the nucleus containing all of the
determinants for the species was kept intact. In this manner the germ plasm,
an immortal substance, passed from one generation to the next via the nuclear
germ plasm of the sex or germ cells. This continuity of the nuclear germ
plasm from the egg to the adult individual and from thence through the
germ cells to the fertilized egg of the next generation, constituted the Weismann
“Keimbahn†or germ-track theory. The soma or body of any particular generation is thus the “trustee†for the germ plasm of future generations.
The Weismannian idea, relative to the qualitative segregation of the chromatin materials, is not tenable for experimental and cytological evidence suggests that all cells of the body contain the same chromosomal materials. However, it should be pointed out that Weismann was one of the first to suggest that the chromosome complex of the nucleus acts as a repository for all of the hereditary characteristics of the species. This suggestion relative to the role of the nucleus has proved to be one of the main contributions to biological theory in modern times.
Fig. 60. Representation of the concept of the early embryonic origin of the primordial germ cells and their migration into the site of the developing germ gland. (A-C are adapted from the work of Allen, Anat. Anz. 29, on germ cell origin in Chrysemys; D-F are diagrams based on the works of Dustin, Swift, and Dantschakoff, etc., referred to in the table of germ-cell origins included in the text.) (A~C) Germ cells arising within the primitive entoderm and migrating through the dorsal mesentery to the site of the primitive gonad, shown in (D), where they become associated in or near the germinal epithelium overlying the internal mesenchyme of the gonad. (E, F) Increase of the primitive gonia within the developing germ gland, with a subsequent migration into the substance of the germ gland of many germ cells during the differentiation of sex.
Fig. 61 Diagrammatic representation of the process of chromatin diminution in the nematode worm, Ascaris cquorum (A. megalocephala), and of the “Keimbahn†(in black, E). One daughter cell shown by the four black dots of each division of the germcell line (i.e., the stem-cell line) is destined to undergo chromatin diminution up to the 16-cell stage. At the 16-cell stage, the germ-cell line ceases to be a stem cell (e.g., P 4 ), and in the future gives origin only to sperm cells (E). (A-D, copied from King and Beams (’38); E, greatly modified from Durkin (’32).)
Animal pole of the cleaving egg (A) is toward the top of the page. (B) Metaphase conditions of the second cleavage. Observe the differences in the cleavage planes of the prosomatic cell, Si, and that of the stem cell, P,. (C) Anaphase of the second cleavage of Si. Observe that the ends of the chromosomes in this cleaving cell are left behind on the spindle. (D) It is to be noted that the ends of the chromosomes are not included in the reforming nuclei of the two daughter cells of S,. thus effecting a diminution of the chromatin substance. In P„ P., and E.M. ST. of (D), the chromosomes are intact. E.M. ST. = second prosomatic cell. MST = mesoderm-stomodaeal cell.
A second contributory concept to the germ-cell (germ-plasm) theory was
made by Nussbaum, 1880; Boveri, 1892, ’10, a and b, and others. These
investigators emphasized the possibility that a germinal cytoplasm also is
important in establishing the germ plasm of the individual. A considerable
body of observational and experimental evidence derived from embryological
studies substantiates this suggestion. Consequently, the modern view of the
germ cell (germ plasm) embodies the concept that the germ cell is composed of the nucleus as a carrier of the hereditary substances or genes and a peculiar,
specialized, germinal cytoplasm. The character of the cytoplasm of the germ
cell is the main factor distinguishing a germ cell from other soma cells.
The matter of a germinal cytoplasm suggests the necessity for a segregation of the germinal plasm in the form of specific germ cells during the early development of the new individual. As a result, great interest, as well as controversy, has accumulated concerning this aspect of the germ-cell problem: namely, is there a separate germinal plasm set apart in the early embryo which later gives origin to the primordial germ cells, and the latter, after migration (fig. 60), to the definitive gonia; or according to an alternative view, do some or all of the definitive germ cells arise from differentiated or relatively undifferentiated soma cells? The phrase primary primordial germ cells often is used to refer to those germ cells which possibly segregate early in the embryo, and the term secondary primordial germ cells is employed occasionally to designate those which may arise later in development.
The dispute regarding an early origin or segregation of the germinal plasm
in the vertebrates also occurs relative to their origin in certain invertebrate
groups, particularly in the Coelenterata and the Annelida (Berrill and Liu,
’48). In other fnvertebrata, such as the dipterous insects and in the ascarid
worms, the case for an early segregation is beyond argument. An actual demonstration of the continuity of the Keimbahn from generation to generation
is found in Ascaris megalocephala described by Boveri in 1887. (See Hegner,
’14, Chap. 6.) In this form the chromatin of the somatic cells of the body
undergoes a diminution and fragmentation, whereas the stem cells, from which
the germ cells are ultimately segregated at the 16-cell to 32-cell stage, retain
the full complement of chromatin material (fig. 61). Thus, one cell of the
16-cell stage retains the intact chromosomes and becomes the progenitor of
the germ cells. The other 15 cells will develop the somatic tissues of the body.
The diminution of the chromatin material in this particular species has been
shown to be dependent upon a certain cytoplasmic substance (King and
Beams, ’38).
In some insects the Keimbahn also can be demonstrated from the earliest stages of embryonic development. In these forms a peculiar polar plasm within the egg containing the so-called “Keimbahn determinants’’ (Hegner, ’14, Chap. 5) always passes into the primordial germ cells. That is, the ultimate formation and segregation of the primordial germ cells are the result of nuclear migration into this polar plasm and the later formation of definite cells from this plasm (fig. 62). The cells containing this polar plasm are destined thus to be germ cells, for they later migrate into the site of the developing germ glands and give origin to the definitive germ cells.
Many investigators of the problem of germ-cell origin in the vertebrate group of animals have, after careful histological observation, described the germ cells as taking their origin from among the early entodermal cells (see table, pp. 121-124). On the other hand, other students have described the origin of the germ cells from mesodermal tissue — some during the early period of embryonic development, while others suggest that the primordial germ cells arise from peritoneal (mesodermal) tissue at a much later time.
In more recent years much discussion has been aroused relative to the origin of the definitive germ cells in mammals, particularly in the female. According to one view the definitive germ cells which differentiate into the mature gametes of the ovary arise from the germinal epithelium (peritoneal covering) of the ovary during each estrous cycle (figs. 39A, 63, 64). For example, Evans and Swezy (’31) reached the conclusion that all germ cells in the ovaries of the cat and dog between the various reproductive periods degenerate excepting those which take part in the ovulatory phenomena. Accordingly, the new germ cells for each cycle arise from the germinal epithelium. A similar belief of a periodic proliferation of new germ cells by the germinal epithelium has been espoused by various authors. (See Moore and Wang, ’47; and Pincus, ’36, Chap. II.) More recent papers have presented views which are somewhat conflicting. Vincent and Dornfeld, ’48 (fig. 63) concluded that there is a proliferation of germ cells from the germinal epithelium of the young rat ovary, while Jones (’49), using carbon granules as a vital-marking technic, found no evidence of the production of ova from the germinal epithelium in rat ovaries from 23 days until puberty. In the adult rat, she concedes that a segregation of a moderate number of oogonia from the germinal epithelium is possible.
Fio. 62. Early development of the fly, Miflj/o/’. {A) Miastor metraloas. (B) Miastor americana. In (A) the division figures I and III (II not shown) are undergoing chromatin diminution, while nucleus IV divides as usual. In (B) one segregated germ is shown at the pole of the egg. This cell will give origin to the germ cells. Other division figures experiencing chromatin diminution.
Fig. 63. Cells proliferating inward from germinal epithelium of the ovary of a oneday-old rat. Observe cords of cells (Pfliiger’s cords) projecting into the ovarian substance. Within these cords of cells are young oogonia. (After Vincent and Dornfeld, ’48.)
Fig. 64. Cellular condition near the surface of the ovary of a young female opossum. This section of the ovary is near the hilar regions, i.e., near the mesovarium. Observe young oocytes and forming Graafian follicles. Primitive germ cells may be seen near the germinal epithelium.
Aside from the above studies of carefully-made, histological preparations relative to the time and place of origin of the primordial and definitive germ cells, many experimental attacks have been made upon the problem. Using an x-ray-sterilization approach, Parkes (’27); Brambell, Parkes and Fielding (’27, a and b), found that the oogonia and oocytes of x-rayed ovaries of the mouse were destroyed. In these cases new germ cells were not produced from the germinal epithelium. Brambell (’30) believed that the destruction of the primitive oogonia was responsible for the lack of oogenesis in these x-rayed ovaries. However, this evidence is not conclusive, for one does not know what injurious effects the x-rays may produce upon the ability of the various cells of the germinal epithelium to differentiate.
An experimental study of the early, developing, amphibian embryo relative
to the origin of the primordial germ cells also has been made by various investigators. Bounoure (’39) applied a vital-staining technic to certain anuran
embryos. The results indicate that the germinal plasm in these forms is associated with the early, entodermal, organ-forming area located at the vegetal
pole of the cleaving egg. This germinal plasm later becomes segregated into
definite cells which are associated with the primitive entoderm. At a later
period these cells migrate into the developing germ gland or gonad. On the
other hand, experimental studies of the urodele embryo indicate that the
early germinal plasm is associated with the mesoderm (Humphrey, ’25, ’27;
Nieuwkoop, ’49). Existence of an early germinal plasm associated with the
entoderm in the Anura and with the mesoderm in the Urodela thus appears
to be well established for the amphibia.
The evidence derived from amphibian studies together with the observations upon the fish group presented in the table (see pp. 121-124) strongly
suggests that an early segregation of a germinal plasm (germ cells) occurs in
these two major vertebrate groups. Also, in birds, the experimental evidence
presented by Benoit (’30), Goldsmith ('35), and Willier ('37) weighs the
balance toward the conclusion that there is an early segregation of germ cells
from the entoderm. Similar conditions presumably are present in reptiles. In
many vertebrates, therefore, an early segregation of primordial germ cells and
their ultimate migration by: (1) active ameboid movement, (2) by the
shifting of tissues, or (3) through the blood stream (see table, pp. 121-124)
to the site of the developing gonad appears to be well substantiated.
The question relative to the origin of the definitive ova in the mammalian ovary is still in a confused state as indicated by the evidence presented above and in the table on pp. 121-124. Much more evidence is needed before one can rule out the probability that the primordial germ cells are the progenitors of the definitive germ cells in the mammals. To admit the early origin of primordial germ cells on the one hand, and to maintain that they later disappear to be replaced by a secondary origin of primitive germ cells from the germinal epithelium has little merit unless one can disprove the following position, to wit: that, while some of the primordial germ cells undoubtedly do degenerate, others divide into smaller cells which become sequestered within or immediately below the germinal epithelium of the ovary and within the germinal epithelium of the seminiferous tubules of the testis, where they give origin by division to other gonial cells. Ultimately some of these primitive gonia pass on to become definitive germ cells.
However, aside from the controversy whether or not the primordial germ cells give origin to definitive germ cells, another aspect of the germ-cell problem emphasizing the importance of the primitive germ cells is posed by the following question: Will the gonad develop into a functional structure without the presence of the primordial germ cells? Experiments performed by Humphrey (’27) on Ambystoma, and the above-mentioned workers — Benoit (’30), Goldsmith (’35), and Willier (’37) — on the chick, suggest that only sterile gonads develop without the presence of the primordial germ cells.
Finally, another facet of the germ-cell problem is this; Are germ cells completely self differentiating? That is, do they have the capacity to develop by themselves; or, are the germ cells dependent upon surrounding gonadal tissues for the influences which bring about their differentiation? All of the data on sex reversal in animals, normal and experimental (Witschi, in Allen, Danforth, and Doisy, ’39), and of other experiments on the development of the early embryonic sex glands (Nieuwkoop, ’49) suggest that the germ cells are not self differentiating but are dependent upon the surrounding tissues for the specific influences which cause their development. Furthermore, the data on sex reversal shows plainly that the specific chromosome complex (i.e., male or female) within the germ cell does not determine the differentiation into the male gamete or the female gamete, but rather, that the influences of the cortex (in the female) and the medulla (in the male) determine the specific type of gametogenesis.
The table given on pp. 121-124 summarizes the conclusions which some authors have reached concerning germ-cell origin in many vertebrates. It is not complete; for more extensive reviews of the subject see Everett ('45), Heys ('31 ), and Nieuwkoop ('47, '49).
Species
Place of Origin, etc.
A uthor
Entospheniis wihieri (brook lamprey)
Germ cells segregate early in the embryo; definitive germ cells derived from "no other source"
Okkelberg. 1921.
J. Morphol. 35
Petromyzon marinus unicolor (lake lamprey)
Definitive germ cells derive from: a) early segregated cells, primordial germ cells, and b) later from coelomic epithelium. Suggests that primordial germ cells may induce germ-cell formation in peritoneal epithelium
Butcher. 1929.
Biol. Bull. 56
Squalus acanthias
Germ cells segregate from primitive entoderm; migrate via the mesoderm into site of the developing gonad
Woods. 1902.
Am. J. Anat. 1
Amia and
Lepidosteiis
Germ cells segregate early from entoderm; continue distinct and migrate into the developing gonad via the mesoderm (see fig. 60)
Allen. 1911.
J. Morphol. 22
Species
Place of Origin, etc.
Author
Lophius piscatorius
Germ cells segregate from primitive entoderm; migrate through mesoderm to site of gonad; migration part passive and part active
Dodds. 1910.
J. Morphol. 21
Fundulus
heteroclitus
Germ cells segregate from peripheral entoderm lateral to posterior half of body; migrate through entoderm and mesoderm to the site of the developing gonad
Richards and Thompson. 1921.
Biol. Bull. 40
Cottus bairdii
Primordial germ cells derive from giant cells, in the primitive entoderm; migrate through the lateral mesoderm into the site of the developing gonad
Hann. 1927.
J. Morphol. 43
Lebistes reticulatus (guppy)
Germ cells segregate early in development; first seen in the entodermmesoderm area; migrate into the sites of the developing ovary and testis, giving origin to the definitive germ cells
Goodrich, Dee, Flynn, and Mercer. 1939. Biol. Bull. 67
Rana tempo raria
Germ cells segregate from primitive entoderm; migrate into developing genital glands
Witschi. 1914.
Arch. f. mikr. Anat. 85
Rana temporaria
Primordial germ cells from entoderm discharged at first spawning. Later, the definitive germs cells of adults originate from peritoneal cells
Gatenby. 1916.
Quart. J. Micr. Sc. 61
Rana catesbiana
Primordial germ cells segregate from primitive entoderm; definitive germ cell derives from primordial cells according to author’s view but admits possibility of germinal epithelium origin
Swingle. 1921.
J. Exper. Zool. 32
Rana temporaria, Triton alpestris, Bufo vulgaris
Primordial germ cells segregate from entoderm
Bounoure. 1924. Compt. rend. Acad. d. Sc. 178, 179
Rana sylvatica
Primordial germ cells originate from entoderm and migrate into the developing gonads. They give origin to the definitive sex cells
Witschi. 1929.
J. Exper. Zool. 52
H emidactylium scutatum
Primordial germ cells arise in mesoderm between somite and lateral plate; move to site of gonad by shifting of tissues
Humphrey. 1925.
J. Morphol. 41
Species
Place of Origin, etc.
Author
Ambystorna
maculatiirn
Most germ cells somatic in origin from germinal epithelium, although a few may come from primordial germ cells of entodermal origin
McCosh. 1930.
J. Morphol. 50
Triton, and Ambystorna mexicanum
Germ cells differentiate from lateral plate mesoderm
Nieuwkoop. 1946.
Arch. Neerl. de zool. 7
Chrysemys
marginata
(turtle)
Primordial germ cells from entoderm; most of definitive germ cells arise from peritoneal cells
Dustin. 1910.
Arch, biol., Paris. 25
Sternotherus
odoratus (turtle)
Primordial cells segregate early from entoderm; later definitive cells derive from these and from peritoneal epithelium
Risley. 1934.
J. Morphol. 56
Callus ( domesticus) gallus (chick)
Germ cells arise from primitive cells in entoderm of proamnion area and migrate by means of the blood vessels to the site of the developing gonad. Definitive germ cells of sex cords and later seminiferous tubules derive from primordial germ cells
Swift. 1914, 1916.
Am. J. Anat. 15, 20
Callus (domesticus) gallus (chick)
Primordial germ cells arise from enlodermal cells
Dantschakoff. 1931. Zeit. f. Zellforsch., mikr. Anat. 15
Chick and albino rat
Early primordial cells degenerate; definitive cells from peritoneal epithelium
Firket. 1920.
Anat. Rec. 18
Didelpliys
virginiana
(opossum)
Germ cells arise from germinal epithelium
Nelsen and Swain. 1942.
J. Morphol. 71
Mils musculiis (mouse)
Oogonia derived from primordial germ cells; spermatogonia from epithelial cells of testis cords
Kirkham. 1916.
Anat. Rec. 10
Mus musculus (mouse)
Primordial germ cells of ovary arise from germinal epithelium during development of the gonads. These presumably give origin to the definitive sex cells
Brambell. 1927.
Proc. Roy. Soc. London, s.B. 101
Felis dornestica (cat)
Primordial cells segregate early but do not give origin to definitive germ cells which derive from germinal epithelium
de Winiwarter and Sainmont. 1909. Arch, biol., Paris. 24
Species
Place of Origin, etc.
A uthor
Felis dome Stic a (cat)
Definitive ova derived from germinal epithelium of the ovary at an early stage of gonad development
Kingsbury. 1938.
Am. J. Anat. 15
Cavia porcellus (guinea pig)
Primordial germ cells from entodermal origin degenerate; the primordial germ cells derived from the germinal epithelium give rise to the definitive germ cells in the testis
Bookkout. 1937.
Zeit. f. Zellforsch, mikr. Anat. 25
Homo sapiens (man)
Primordial germ cells found in entoderm of yolk sac; migrate by ameboid movement into developing gonad
Witschi. 1948. Carnegie Inst., Washington Publ. 575. Contrib. to Embryol.' 32
C. Maturation (Differentiation) of the Gametes
1. General Considerations
Regardless of their exact origin definitive germ cells as primitive oogonia or very young oocytes are to be found in or near the germinal epithelium in the ovaries of all vertebrates in the functional condition (figs. 39B, 64). In the testis, the primitive spermatogonia are located within the seminiferous tubules as the germinal epithelium, in intimate association with the basement membrane of the tubule (figs. 65, 66).
The period of coming into maturity (maturation) of the gametes is a complicated affair. It involves profound transformations of the cytoplasm, as well as the nucleus. Moreover, a process of ripening or physiological maturing is necessary, as well as a morphological transformation. The phrase “maturation of the germ cells†has been used extensively to denote nuclear changes. However, as the entire gamete undergoes morphological and physiological change, the terms nuclear maturation, cytosomal maturation, and physiological maturation are used in the following pages to designate the various aspects of gametic development.
One of the most characteristic changes which the germ cell experiences during its maturation into a mature gamete is a reduction of chromatin material. Because of this, the germ cell which begins the maturing process is called a meiocyte. This word literally means a cell undergoing diminution and it is applied to the germ cell during meiosis or the period in which a reduction in the number of chromosomes occurs. The word haplosis is a technical name designating this reduction process.
Fig. 65. Semidiagrammatic representation of a part of the seminiferous tubule of the cat testis.
The word meiocyte thus is a general term applicable to both the developing male and female germ cells. On the other hand, the word spermatocyte is given to the developing male gamete during the period of chromosome diminution, whereas the word oocyte is applied to the female gamete in the same period. When, however, the period of chromosome diminution is completed and the chromosome number is reduced to the haploid condition, the developing male gamete is called a spermatid while the female gamete is referred to as an ootid or an egg. {Note: the word egg is applied often to the female gamete during the various stages of the oocyte condition as well as after the maturation divisions have been accomplished.)
The reduction of chromatin material is not the only effect which the meiotic process has upon the chromatin material, or possibly upon the developing cytosomal structures as well. This fact will become evident during the descriptions below concerning the meiotic procedures.
Another prominent feature of the gametes during the meiocyte period is their growth or increase in size. This growth occurs during the first part of the meiotic process when the nucleus is in the prophase condition and it involves both nucleus and cytoplasm. The growth phenomena are much more pronounced in the oocyte than in the spermatocyte. Due to this feature of growth, the oocyte and spermatocyte also are regarded as auxocytes, that is growing cells, a name introduced by Lee, 1897. The words meiocyte and auxocyte thus refer to two different aspects of the development of the oocyte and the spermatocyte.
2. Basic Structure of the Definitive Sex Cell as It Starts to Mature or Differentiate into the Male Meiocyte (i.e., the Spermatocyte) or the Female Meiocyte (i.e., the Oocyte)
The definitive sex cells of both sexes have a similar cytological structure.
The component parts are (fig. 68):
- nucleus,
- investing cytoplasm,
- idiosome,
- Golgi substance, and
- chondriosomes.
The nucleus is vesicular and enlarged, and the nuclear network of chromatin may appear reticulated. A large nucleolus also may be visible. The
investing cytoplasm is clearer and less condensed in appearance than that
of ordinary cells. The idiosome (idiozome) is a rounded body of cytoplasm
which, in many animal species, takes the cytoplasmic stain more intensely
than the surrounding cytoplasm. Within the idiosome it is possible to demonstrate the centrioles as paired granules in some species. Surrounding the
idiosome are various elements of the Golgi substance, and near both the
idiosome and Golgi elements, is a mass of chondriosomes (mitochondria) of
various sizes and shapes. The idiosome and its relationship with the Golgi
material, the mitochondria, and the centrioles varies considerably in different
species of animals.
Much discussion has occurred concerning the exact nature of the idiosome.
Some investigators have been inclined to regard the surrounding Golgi substance as a part of the idiosome, although the central mass of cytoplasm containing the centrioles is the “idiosome proper†of many authors (Bowen, ’22).
Again, when the maturation divisions of the spermatocyte occur, the idiosome
and surrounding Golgi elements are broken up into small fragments. However, in the spermatids the Golgi pieces (dictyosomes) are brought together
once more to form a new idiosome-like structure, with the difference that the
latter “seems never to contain the centrioles†(Bowen, ’22). It is, therefore,
advisable to regard the idiosome as being separated into its various components during the maturation divisions of the spermatocyte and to view the
reassemblage of Golgi (dictyosomal) material in the spermatid as a different
structure entirely. This new structure of the spermatid is called the acroblast
(Bowen, ’22; Leuchtenberger and Schrader, ’50). (See fig. 68B.) A similar
breaking up of the idiosome occurs in oogenesis (fig. 68F, G). However, all
meiocytes do not possess a typical idiosome. This fact is demonstrated in
insect spermatocytes, where the idiosomal material is present as scattered
masses to each of which some Golgi substance is attached.
The various features which enter into the structure of the definitive germ cell do not behave in the same way in each sex during gametic differentiation. While the behaviors of the chromatin material in the male and female germ cells closely parallel each other (fig. 67), the other cytosomal features follow widely divergent pathways, resulting in two enormously different gametic entities (fig. 68A-H).
Fig. 66. Section of part of a seminiferous tubule of human testis. (Redrawn from Gatenby and Beams, ’35.)
Fig. 67. Diagrammatic representation of the nuclear changes occurring during meiosis
in spermatocyte and oocyte. Six chromosomes, representing three homologous pairs, are
used. Observe the effects of the crossing over of parts of chromatids. The diplotene condition of oocyte depicted by arrows and the enlarged nucleus. The haploid condition is
shown in each of the spermatids or in the egg and its three polocytes.
Fig. 68. Possible fate of the primitive meiocyte and its cytoplasmic inclusions when exposed to testicular or ovarian influences. Particular attention is given to the idiosome. Under male-forming influences the idiosome components are dispersed during the maturation divisions and are reassembled into three separate component structures, namely, (1) Acroblast of Golgi substance, (2) centriolar bodies, and (3) mitochondrial bodies (see B). Each of these structures, together with the post-nuclear granules of uncertain origin, play roles in spermatogenesis as shown. Under ovarian influences the idiosome is dispersed before the maturation divisions. The Golgi substance and mitochondria play (according to theory, see text) their roles in the formation of the deutoplasm.
3. Nuclear Maturation of the Gametes
Most of our information concerning the maturation of the nucleus pertains to certain aspects of chromosome behavior involved in meiosis, particularly the reduction of the chromosome number together with some activities of “crossing over†of materials from one chromosome to another. But our information is vague relative to other aspects of nuclear development. For example, we know little about the meaning of growth and enlargement of the nucleus as a whole during meiosis, an activity most pronounced in the oocyte. Nor do we know the significance of nuclear contraction or condensation in the male gamete after meiosis is completed. Therefore, when one considers the nuclear maturation of the gametes, it is necessary at this stage of our knowledge to be content mainly with observations of chromosomal behavior.
a. General Description of the Chromatin Behavior During Somatic and Meiotic Mitosis
As the maturation behavior of the chromatin components in the spermatocyte and oocyte are similar, a general description of these activities is given in the following paragraphs. Before considering the general features and details of the actions of the chromosomes during meiosis, it is best to recall some of the activities which these structures exhibit during ordinary somatic and gonial mitoses.
Cytological studies have shown that the chromosomes, in most instances, are present in the nucleus in pairs, each member of a pair being the homologue or mate of the other. Homologous chromosomes, therefore, are chromosomal pairs or mates. During the prophase condition in ordinary somatic and gonial mitoses, the various chromosomal mates do not show an attraction for each other. A second feature of the prophase stage of ordinary cell division is that each chromosome appears as two chromosomes. That is, each chromosome is divided longitudinally and equationally into two chromosomes. At the time when the metaphase condition is reached and the chromosomes become arranged upon the metaphase plate, the two halves or daughter chromosomes of each original chromosome are still loosely attached to each other. However, during anaphase, the two daughter chromosomes of each pair are separated and each of the two daughter nuclei receives one of the daughter chromosomes. Reproduction of the chromatin material and equational distribution of this material into the two daughter cells during anaphase is a fundamental feature of the ordinary type of somatic and gonial mitoses. The two daughter nuclei are thus equivalent to each other and to the parent nucleus. In this way, chromosomal equivalence is passed on ad infinitum through successive cell generations.
On the other hand, a different kind of chromosomal behavior is found during meiosis, which essentially is a specialized type of mitosis, known as a meiotic mitosis. In one sense it is two mitoses or mitotic divisions with only one prophase; that is, two metaphase-anaphase separations of chromosomes preceded by a single, peculiar prophase. The peculiarities of this meiotic prophase may be described as follows: As the prophase condition of the nucleus is initiated, an odd type of behavior of the chromosomes becomes evident — a behavior which is entirely absent from ordinary somatic mitosis: namely, the homologous pairs or mates begin to show an attraction for each other and they approach and form an intimate association. This association is called synapsis (figs. 67, 69, zygotene stage). As a result, the two homologous chromosomes appear as one structure. As the homologous chromosomes are now paired together and superficially appear as one chromosome, the number of “chromosomes†visible at this time is reduced to one-half of the ordinary somatic or diploid number. However, each “chromosome†is in reality two chromosomes and, therefore, is called a bivalent or twin chromosome.
Fig. 69. Steps in spermatogenesis in the grasshopper. In the center of the chart is represented a longitudinal section of one of the follicles of a grasshoper testis with its various regions of spermatogenic activity. In the upper right of the chart the apical-cell complex is depicted with its central apical cell, spermatogonia, and surrounding epithelial cells. The primary spermatogonia lie enmeshed between the extensions of the apical cell and the associations of these extensions with the surrounding epithelial elements of the complex. (Also see Wenrich, 1916, Bull. Mus. Comp. Zool. Harvard College, 60.)
While the homologous chromosomes are intimately associated, each mate
reproduces itself longitudinally just as it would during an ordinary mitosis
(fig. 67, pachytene stage). (The possibility remains that this reproduction of
chromatin material may have occurred even before the synaptic union.)
Hence, each bivalent chromosome becomes transformed into four potential
chromosomes, each one of which is called a chromatid. This group of chromatids is, collectively speaking, a tetrad chromosome. (As described below,
interchange of material or crossing over from one chromatid to another may
take place at this time.) As a result of these changes, the nucleus now contains the haploid number of chromosomes, (i.e., half of the normal, diploid
number) in the form of tetrads (fig. 67, pachytene stage). However, as each
tetrad represents four chromosomes, actually there is at this time twice the
normal number of chromosomes present in the nucleus (fig. 67; compare
leptotene, pachytene, diplotene and diakinesis).
The next step in meiosis brings about the separation of the tetrad chromosome into its respective chromatids and it involves two divisions of the cell.
These divisions are known as meiotic divisions. As the first of these two
divisions begins, the tetrad chromosomes become arranged in the mid- or
metaphase plane of the spindle. After this initial step, the first division of
the cell occurs, and half of each tetrad (i.e., a dyad) passes to each pole of
the mitotic spindle (fig. 67, first meiotic division). Each daughter cell (i.e.,
secondary spermatocyte or oocyte) resulting from the first maturation
(meiotic) division thus contains the haploid or reduced number of chromosomes in the dyad condition, each dyad being composed of two chromatids.
A resting or interphase nuclear condition occurs in most spermatocytes, following the first maturation division, but in the oocyte it usually does not
occur (fig. 69, interkinesis).
As the second maturation division is initiated, the dyads become arranged on the metaphase plate of the mitotic spindle. As division of the cell proceeds, half of each dyad (i.e., a monad) passes to the respective poles of the spindle (fig. 67, second meiotic division; fig. 69). As a result of these two divisions, each daughter cell thus contains the haploid or reduced number of chromosomes in the monad (monoploid) condition (fig. 67, spermatid or egg). Meiosis or chromatin diminution is now an accomplished fact.
It is to be observed, therefore, that the meiotic phenomena differ from those of ordinary mitosis by two fundamental features:
(1) In meiosis there is a conjugation (synapsis) of homologous chromosomes during the prophase stage, and while synapsed together each of the homologues divides equationally; and
(2) following this single prophase of peculiar character, two divisions follow each other, separating the associated chromatin threads.
While the meiotic prophase is described above as a single prophase preceding two metaphase-anaphase chromosome separations, it is essentially a double prophase in which the process of synapsis acts to suppress one of the equational divisions normally present in a mitotic division; a synapsed or double chromosome, therefore, is substituted for one of the longitudinal, equational divisions which normally appears during a somatic prophase. It is this substitution which forms the basis for the reduction process, for two mitotic divisions follow one after the other, preceded by but one equational splitting, whereas in ordinary mitosis, one equational splitting of the chromosomes always precedes each mitotic division.
b. Reductional and Equational Meiotic Divisions and the Phenomenon of
Crossing Over
In the first meiotic division (i.e., the first maturation division), if the two chromatids which are derived from one homologous mate of the tetrad are separated from the two chromatids derived from the other homologous mate the division is spoken of as reductional or disjunctional. In this case the two associated chromatids of each dyad represent the original chromosome which synapsed at the beginning of meiotic prophase (fig. 67, tetrads B and C, first meiotic division). If, however, the separation occurs not in the synaptic plane but in the equational plane, then the two associated chromatids of each dyad come, one from one synaptic mate and one from the other; such a division is spoken of as an equational division (fig. 67, tetrad A, first meiotic division). There appears to be no fixity of procedure relative to the separation of the tetrads, and great variability occurs. However this may be, one of the two meiotic divisions as far as any particular tetrad is concerned is disjunctional (reductional) and the other is equational, at least in the region of the kinetochore (see p. 135 and fig. 70). If the first division is reductional, the second is equational and vice versa. Disjunction in the first maturation division is often referred to as pre-reduction, while that in the second maturation division is called post-reduction.
Fig. 70. Some of the various possibilities which may occur as a result of the exchanges
of parts of chromatids during the crossing-over phenomena associated with meiosis. Two
chiasmata (singular, chiasma) are shown in (A), (C), (E). Observe that homologous
chromosome A has split equationally into chromatids A and A', while homologous
chromosome B has divided equationally into B and B'. The resulting interchanges between respective chromatids of the original homologous chromosomes are shown in (B),
(D), (F). The kinetochore (place of spindle-fiber attachment) is indicated by the oval
or circular area to the left of the chromatids. (Modified from White: Animal Cytology
and Evolution, London, Cambridge University Press, 1943.)
The foregoing statement regarding disjunctional and equational divisions
should be considered in the light of the phenomenon of crossing over. In
the latter process, a gene or groups of genes may pass from one chromatid
to the other and vice versa during their association at the four strand stage
(fig. 70). In the region of the centromere or kinetochore (i.e., the point)
of the achromatic, spindle-fiber attachment) and nearby regions, cross overs
are thought not to occur (fig. 70, kinetochore). Consequently, in the regions
of the kinetochore, the statements above regarding disjunctional and equational divisions of the chromosomes appear to be correct. However, the terms
disjunctional and equational may mean little in other regions of the chromosomes of a tetrad during the meiotic divisions. For example, let us assume as
in fig. 70 (see also fig. 67), that we have chromatids A and A', B and B',
A and B representing the original homologues or synaptic chromosomes which
have divided into these chromatids respectively. Then during the tetrad stage
of association or slightly before, let us assume that there has been a crossing
over of genes from chromatid A to chromatid B and from chromatid B to
chromatid A in a particular area (fig. 70A). (It is to be observed that chromatids A' and B' are not involved in this particular instance.) Further, let us
assume that AA' and BB' as a whole are separated at the first maturation
division, the kinetochore and immediate regions would represent a disjunctional division, but for the particular area where crossing over is accomplished,
the division would be equational (fig. 70A, B; central portions of chromatids
A and B in fig. 70B). Thus, it would be for other regions where cross overs
may have occurred. Other cross-over possibilities are shown in fig. 70C-F.
c. Stages of Chromatin Behavior During the Meiotic Prophase in Greater Detail
The following five stages of chromatin behavior within the prophase nucleus during meiosis are now in common usage. They are based on the stages originally described by H. von Winiwarter, ’00. The substantive form is presented in parentheses.
1) Leptotene (Leptonema) Stage. The leptotene stage (figs. 69, 71) represents the initial stage of the meiotic process and is seen especially well in the spermatocyte. At this time the nucleus of the differentiating germ cell begins to enlarge, and the diploid number of very long, slender chromatin threads make their appearance. (Compare “resting†and leptotene nuclei in figs. 69, 71.) The chromatin threads may lie at random in the nucleus or they may be directed toward one side, forming the so-called “bouquet†condition (fig. 69, leptotene stage) . The nucleolus is evident at this time (fig. 7 IB) .
2) Zygotene or Synaptene (Zygonema) Stage. The zygotene stage (figs. 69, 71, 85) is characterized by a synapsis of the chromatin threads. This synapsis or conjugation occurs between the homologous chromosomes, that is, the chromosomes which have a similar genic constitution. Synapsis appears to begin most often at the ends of the threads and progresses toward the middle (fig. 67, zygotene). At this stage the chromatin threads may show a strong tendency to collapse and shrink into a mass toward one end of the nucleus (fig. 85C, D). This collapsed condition, when present, is called synizesis. The zygotene stage gradually passes into the pachytene condition.
Fig. 71. Certain aspects of the oocyte nucleus during the meiotic prophase. (A-G)
Chromatin and nuclear changes in the oocyte of the cat up to the diplotene condition
when the germinal vesicle is fully developed. (After de Winiwarter and Sainmont, Arch,
biol., Paris, 24.) (H, I) Germinal vesicle in the dogfish, Scy Ilium canicula, and in Amphioxus. (After Marechal, La Cellule, 24.) Observe the typical “lamp-brush†chromosome conditions in the germinal vesicle of the shark oocyte. These lamp-brush chromosomes are developed during the diplotene stage of meiosis by great attenuation of the
chromosomes and the formation of lateral extensions or loops from the sides of the
chromosomes.
3) Pachytene (Pachynema) Stage. Gradually, the synapsis of the homologous chromosomes becomes more complete, and the threads appear shorter
and thicker. The contracted threads in this condition are referred to as
pachynema (figs. 69, 71, 85E). The nucleus in this manner comes to contain
a number of bivalent chromosomes, each of which is made up of two homologous mates arranged side by side in synaptic union, known technically as
parasynapsis. (Telosynapsis probably is not a normal condition.) Consequently, the number of chromosomes now appears to be haploid. Each pachytene chromosome (i.e., each of the pair of homologous chromosomes) gradually divides equationally into two daughter thread-like structures, generally
referred to as chromatids. The exact time at which division occurs during
meiosis is questionable. The entire group of four chromatids which arise from
the splitting of the synapsed homologues is called a tetrad.
4) Diplotene (Diplonema) Stage. In the diplotene stage (figs. 67, 69, 71, 85F, G), two of the chromatids tend to separate from the other two. (See fig. 70A, C, E.) The four chromatids in each tetrad may now be observed more readily, at least in some species, because the various chromatids of each tetrad show a repulsion for one another, and the chromatids move apart in certain areas along their length. This condition is shown in both the male and female meiocyte, but in the latter, the repulsion or moving apart is carried to a considerable degree and is associated with a great lengthening and attenuation of the chromatids. (See fig. 67.) In the female meiocyte at this stage, the chromosomes become very diffuse and are scattered throughout the nucleus, somewhat resembling the non-mitotic condition (figs. 71F-T, 72B-E). The peculiar behavior of the chromosomes and nucleus of the oocyte in the diplotene stage of meiosis is described more in detail on p. 141.
Although there is a tendency for the chromatids to widen out or separate from each other at this time, they do remain associated in one or more regions. In these regions of contact, the paired chromatids appear to exchange partners. This point of contact is called a chiasma (plural, chiasmata). Hence, a chiasma is the general region where the chromatids appear to have exchanged partners when the tetrad threads move apart in the diplotene state. (See fig. 70, chiasmata.)
5) Diakinesis. The diplotene stage gradually transforms into the diakinesis state (figs. 67, 69, 72F, 85H) by a process of marked chromosomal contraction. There also may be an opening up of the tetrads due to a separation of the homologous mates in the more central portions of the tetrad, with the result that only the terminal parts of the chromatids remain in contact. This latter process is called “terminalization.†Coincident with this partial separation, a further contraction of the tetrads may occur. As a result, at the end of diakinesis the tetrads may assume such curious shapes as loops, crosses, rings, etc., scattered within the nucleus of the female and male meiocyte (fig. 69, diakinesis). The nuclear membrane eventually undergoes dissolution, and
Fig. 72. Growth of the nucleus during meiosis in the amphibian egg, showing the enlarged germinal vesicle and diplotene lamp-brush chromosomes with lateral loops. (A) Early diplotene nucleus of the frog. (B, C, E) Different phases of the diplotene nucleus in this form. These figures are based upon data provided by Duryee (’50) and sections of the frog ovary. (D) Drawing of the unfixed germinal vesicle of Triturus. Some aspects of the attenuate chromatin threads with lateral loops are shown. The nucleoli are numerous and occupy the peripheral region of the germinal vesicle. (F) Semidiagrammatic drawing of the later phases of the developing frog egg. It shows the germinal vesicle assuming a polar condition, with the initial appearance of germinal vesicle shrinkage before the final dissolution of the nuclear membrane. Observe that the chromosomes are contracting and now occupy the center of the germinal vesicle.
Fig. 73. Various aspects of Sertoli-cell conditions in the fowl. (Redrawn from Zlotnick,
Quart. J. Micr. Sc., 88.) (A) Resting Sertoli cell, showing mitochondria. (B) Sertoli element at the beginning of cytoplasmic elongation. (C) Sertoli cell with associated
late spermatids.
Fig. 74. Types of chordate sperm. All the chordate sperm belong to the flagellate
variety. (A) Amphioxus (protochordate). (B) Salmo (teleost). (C) Perea (teleost).
(D) Petromyzon (cyclostome). (E) Raja (elasmobranch). (F) Bufo (anuran). (G)
Rana (anuran). (H) Salamandra (urodele). (I) Anguis (lizard). (J) Crex (bird).
(K) Fringilla (bird). (L) Turdus (bird). (M) Echidna (monotrematous mammal).
(N) Mus (eutherian mammal). (O, P) Man (full view and side view, respectively).
the tetrads become arranged on the metaphase plate of the first maturation division. (See figs. 69, first maturation division; 72F, 119A, B.) This division is described on pp. 132 and 133.
d. Peculiarities of Nuclear Behavior in the Oocyte During Meiosis; the Germinal Vesicle
Although the movements of the chromosomes during meiosis in the developing male and female gamete appear to follow the same general behavior
Fig. 75. Non-flagellate sperm. (A-C) Ameboid sperm of Polyphemus. (After Zacharias.) (D) Lobster, Homarus. (After Herrick.) (E) Decapod Crustacea, Galathea (Anomura). (After Koltzoff.) (F) Nematode woim, Ascaris.
Fig. 76. Conjugate sperm of grasshopper associated temporarily to form the “sperm boat.â€
pattern (fig. 67), some differences do occur. For example, in the female when
the diplotene stage is reached, the repulsion of the tetrad threads is greater
(figs. 67, $ and 9 \ 12). Furthermore, the chromatids elongate and become
very attenuate although they appear to retain their contacts or chiasmata
(fig. 72). Side loops and extensions from the chromatids also may occur,
especially in those vertebrates with large-yolked eggs (e.g., amphibia, fishes,
etc.). (See figs. 71H, 72B-D.) When these lateral extensions are present, the
chromosomes appear diffuse and fuzzy, taking on the characteristics which suggest their description as “lamp-brush†chromosomes. Another difference
of chromatic behavior is manifested by the fact that the chromosomes in the
developing female gamete during the diplotene stage are not easily stained
by the ordinary nuclear stains, whereas the chromosomes in the spermatocyte
stain readily.
Fig. 77. Spatula-type sperm of various mammals. (Compiled from Bowen; Gatenby
and Beams; Gatenby and Woodger; see references in bibliography.) Observe the vacuole
inside the head of the sperm. Gatenby and Beams found that this vacuole, in some
instances, stains similar to a nucleolus, but suggest it may be a hydrostatic organ, or
respiratory structure. (P. 20, Quart. J. Micr. Sc., 78.)
Aside from the differences in chromosomal behavior, great discrepancies
in the amount of growth of the nucleus occur in the two gametes during meiosis.
The nucleus of the oocyte greatly increases in size and a large quantity of
nuclear fluid or sap comes to surround the chromosomes (figs. 7 IF, G; 72C,
^ F). Correlated with this increase in nuclear size, the egg grows rapidly.
and deutoplasmic substance is deposited in the cytoplasm (fig. 68F-H). As differentiation of the oocyte advances, the enlarged nucleus or germinal vesicle assumes a polar position in the egg (figs. 68H, 70F). When the oocyte has finished its growth and approaches the end of its differentiation, the
Fig. 78. Different shapes and positions of the acrosome. (A) Type of acrosome
found in Mollusca, Echinodermata, and Annelida. (B) Reptilia, Aves, and Amphibia.
(C) Lepidoptera. (D) Mammalia. (E) Many Hemiptera and Coleoptera. (After
Bowen, Anat. Rec., 28.) (F) Sperm of certain birds, i.e., finches. (After Retzius, Biol.
Untersuchungen, New Series 17, Stockholm, Jena.) Observe the well-developed acrosome in the form of a perforatorium. The spiral twist of the acrosome shown in this drawing is characteristic of passerine birds.
Fig. 79. Sperm of urodele amphibia. (After Meves, 1897, Arch. f. mikr. Anat. u.
Entwichlingsgesch., 50; McGregor, 1899, J. Morphol., 15. (A~E) Stages in the morphogenesis of the sperm of Salamandra. (F) Diagram of head, middle piece, etc. of the
sperm of the urodele.
chromosomes within the germinal vesicle condense once again, decrease in length (fig. 72F), and assume conditions more typical of the diakinesis stage (figs. 67; 1 19A). The tetrad chromosomes now become visible. Following the latter chromosomal changes, the nuclear membrane breaks down (fig. 119A), and the chromatin elements pass onto the spindle of the first maturation division (fig. 119B). The nuclear sap, membrane, nucleolus, and general framework pass into the surrounding cytoplasmic substance (figs. 119A; 132A-C). This nuclear contribution to the cytoplasm appears to play an important part in fertilization and development, at least in some species (fig. 132C; the clear protoplasm is derived from the nuclear plasm).
e. Character of the Meiotic (Maturation) Divisions in the Spermatocyte Compared with Those of the Oocyte
1) Dependent Nature of the Maturation Divisions in the Female Meiocyte.
The maturation divisions in the developing male gamete occur spontaneously and in sequence in all known forms. But in most oocytes, either one or both of the maturation divisions are dependent upon sperm entrance. For example, in Ascaris, a nematode worm (fig. 133), and in Nereis, a marine annelid worm (fig. 130), both maturation divisions occur after the sperm has entered and are dependent upon factors associated with sperm entrance. A similar condition is found in the dog (van der Stricht, ’23; fig. 115) and in the fox (Asdell, ’46). In the urochordate, Styela, the germinal vesicle breaks down, the nuclear sap and nucleolus move into the surrounding protoplasm, and the first maturation spindle is formed as the egg is discharged into the sea water (fig. 116A, B). Further development of the egg, however, awaits the entrance of the sperm (fig. 116C--F). Somewhat similar conditions are found in other Urochordata. In the cephalochordate, Amphioxus, and in the vertebrate group as a whole (with certain exceptions) the first polar body is formed and the spindle for the second maturation division is elaborated before normal sperm entrance (figs. 117C, D; 119D). The second maturation division in the latter instances is dependent upon the activities aroused by sperm contact with the oocyte. In the sea urchin, sperm can penetrate the egg before the maturation divisions occur; but, under these conditions, normal development of the egg does not occur. Normally in this species both maturation divisions are effected before sperm entrance, while the egg is still in the ovary. When the egg is discharged into the sea water, the sperm enters the egg, and this event affords the necessary stimulus for further development (fig. 131).
2) Inequality of Cytoplasmic Division in the Oocyte. When the first maturation division occurs, the two resulting cells are called secondary spermatocytes in the male and secondary oocytes in the female (figs. 67, 69). The secondary spermatocytes are smaller both in nuclear and cytoplasmic volume. They also form a definite nuclear membrane. Each secondary spermatocyte then divides and forms two equal spermatids. In contrast to this condition of equality in the daughter cells of the developing male gamete during and following the maturation divisions, an entirely different condition is found in the developing female gamete. In the latter, one of the secondary oocytes is practically as large as the primary oocyte, while the other or first polar body (polocyte) is extremely small in cytoplasmic content although the nuclear material is the same (fig. 117D). During the next division the secondary oocyte behaves in a manner similar to that of the primary oocyte, and a small second polocyte is given off, while the egg remains large (fig. 117E, F). Unlike the secondary spermatocyte, the secondary oocyte does not form a nuclear membrane. The polar body first formed may undergo a division, resulting in a total of three polar bodies (polocytes) and one egg (ootid).
/. Resume of the Significance of the Meiotic Phenomena
In view of the foregoing data with regard to the behavior of the male and female gametes during meiosis, the significant results of this process may be summarized as follows:
(1) There is a mixing or scrambling of the chromatin material brought about by the crossing over of genic materials from one chromatid to another.
(2) Much chromatin material with various genic combinations is discarded during the maturation divisions in the oocyte. In the latter, two polar bodies are ejected with their chromatin material as described above. The egg thus retains one set of the four genic combinations which were present at the end of the primary oocyte stage; the others are lost. (A process of discarding of chromatin material occurs in the male line also. For although four spermatids and sperm normally develop from one primary spermatocyte, great quantities of sperm never reach an egg to fertilize it, and much of the chromatin material is lost by the wayside.)
(3) A reduction of the number of chromosomes from the diploid to the haploid number is a significant procedure of all true meiotic behavior.
(For more detailed discussions and descriptions of meiosis, see De Robertis, et al., ’48; Sharp, ’34, ’43; Snyder, ’45; White, ’45.)
4. Cytosomal (Cytoplasmic) Maturation of the Gametes a. General Aspects of Cytoplasmic Maturation of the Gametes
During the period when the meiotic prophase changes occur in the nucleus of the oocyte, the cytoplasm increases greatly and various aspects of cytoplasmic differentiation are effected. That is, differentiation of both nuclear and cytoplasmic materials tend to occur synchronously in the developing
Fig. 80. Morphogenesis of guinea-pig and human sperm. (A) Spermatocyte of guinea pig before first maturation division. The Golgi complex with included proacrosomic granules and centrioles is shown. (After Gatenby and Woodger, ’21.) (B) Young sister
spermatids of guinea pig. (C) Later spermatid of guinea pig showing acroblast with proacrosomic granules. (D) Young human spermatocyte, showing Golgi apparatus with proacrosomic granules similar to that shown in (A). (After Gatenby and Beams, ’35.) (E) Spermatid of guinea pig later than that shown in (C), showing acroblast with Golgi substance being discarded from around the acroblast. (F) Later human spermatid, showing Golgi substance surrounding acroblast with acrosome bead. (After Gatenby and Beams, ’35.) (G) Later human spermatid, showing acroblast, with acrosome bead
within, surrounded by a vacuole. (After Gatenby and Beams, ’35.) (H) Later spermatid
of guinea pig, showing outer and inner zones of the acrosome. The inner zone corresponds somewhat to the acrosome bead shown in (G) of the human spermatid. (After Gatenby and Wigoder, Proc. Roy. Soc., London, s.B., 104.)
female gamete. In the male gamete, on the other hand, the meiotic processes are completed before morphological differentiation of the cytoplasm is initiated.
Another distinguishing feature in the morphogenesis of the sperm relative to that of the egg is that the cytoplasmic differentiation of the sperm entails a discarding of cytoplasm and contained cytoplasmic structures, whereas the oocyte conserves and increases its cytoplasmic substance (fig. 68). In regard to the behavior of the cytoplasms of the two developing gametes, it is interesting to observe that the idiosome-Golgi-mitochondrial complex behaves very differently in the two gametes (fig. 68).
A third condition of egg and sperm differentiation involves the possible function of the “nurse cells.†In the vertebrate ovary the follicle cells which surround the egg have much to do with the conditions necessary for the differentiation of the oocyte. The latter cannot carry the processes of differentiation to completion without contact with the surrounding follicle cells. Spermiogenesis also depends upon the presence of a nurse cell. In the vertebrate seminiferous tubule, the Sertoli cell is intimately concerned with the transformation of the spermatid into the morphologically adult sperm, and a close contact exists between the developing sperm element and the Sertoli cell during this period (figs. 65, 66, 73). In the discharge of the formed sperm elements into the lumen of the tubule, the Sertoli cell also is concerned (Chap. 1).
b. Morphogenesis (Spermiogenesis; Spermioteleosis) of the Sperm
1) Types of Sperm. There are two main types of sperm to be found in animals, namely, flagellate and non-flagellate sperm (figs. 74, 75). Flagellate sperm possess a flagellum or tail-like organelle; non-flagellate sperm lack this structure. The flagellate type of sperm is found quite universally among animals; non-flagellate sperm occur in certain invertebrate groups, particularly in the nematode worms, such as Ascaris, and in various Crustacea, notably the lobster, crab, etc. (fig. 75). Flagellate sperm may be either uniflagellate or biflagellate. Single flagellate sperm occur in the majority of animals, while a biflagellate form is found in the platode, Procerodes. However, biflagellate sperm may be found as abnormal specimens among animals normally producing uniflagellate sperm.
Conjugate sperm are produced in certain animal species. For example, two sperm heads adhere closely together in the opossum (fig. 125), also in the beetle, Dytiscus, and in the gastropod, Turritella. Many sperm heads become intimately associated in the grasshopper to form the so-called “sperm boat†(fig. 76). However, all conjugate sperm normally separate from each other in the female genital tract.
2) Structure of a Flagellate Sperm. The flagellate sperm from different species of animals vary considerably in size, shape, and morphological details. Some possess long, spear-shaped heads, some have heads resembling a hatchet, in others the head appears more or less cigar-shaped, while still others possess a head which resembles a spatula (fig. 74). The spatula-shaped head is found in the sperm of the bull, opossum, man, etc. The description given below refers particularly to the spatula-shaped variety. Although all flagellate sperm resemble one another, diversity in various details is the rule,
Fig. 81. Later stages of human spermatogenesis. (Redrawn from Gatenby and Beams,
1935.)
Fig. 82. Stages of guinea-pig spermatogenesis. Observe dual nature of the acrosome; also, middle-piece bead (kinoplasmic droplet). (A-C redrawn from Gatenby and Beams, 1935; D redrawn from Gatenby and Woodger, ’21.)
and the description given below should be regarded as being true of one type of sperm only and should not be applied to all flagellate sperm.
A fully differentiated spatulate sperm of the mammals possesses the following structural parts (fig. 77).
a) Head. Around the head of the sperm there is a thin, enveloping layer of cytoplasm. This cytoplasmic layer continues posteriad into the neck, middle piece, and tail. Within the cytoplasm of the head is the oval-shaped nucleus. Over the anterior half of the nucleus the apical body or acrosome is to be found, forming, apparently, a cephalic covering and skeletal shield for the nucleus. The caudal half of the nucleus is covered by the post-nuclear cap. This also appears to be a skeletal structure supporting this area of the nucleus; moreover, it affords a place of attachment for the anterior centrosome and the anterior end of the axial filament.
In human and bull sperm the acrosome is a thin cap, but in some mammalian sperm it is developed more elaborately. In the guinea pig it assumes the shape of an elongated, shovel-shaped affair (fig. 82), while in the mouse and rat it is hatchet or lance shaped (fig. 74N). In passerine birds the acrosome is a pointed, spiral structure often called the perforatorium (fig. 78). On the other hand, in other birds, reptiles, and amphibia it may be a simple, pointed perforatorial structure (figs. 74, 78, 79). In certain invertebrate species, it is located at the caudal or lateral aspect of the nucleus (figs. 75, 78).
b) Neck. The neck is a constricted area immediately caudal to the posterior nuclear cap and between it and the middle piece. Within it are found the anterior centriole and the anterior end of the axial filament. In this particular region may also be found the so-called neck granule.
c) Connecting Body or Middle Piece. This region is an important portion of the sperm. One of its conspicuous structures is the central core composed of the axial filament and its surrounding cytoplasmic sheath. At the distal end of the middle piece, the central core is circumscribed by the distal, or ring centriole. Investing the central core of the middle piece is the mitochondrial' sheath. The enveloping cytoplasm is thicker to some degree in this area of the sperm than that surrounding the head.
d) Flagellum. The flagellum forms the tail or swimming organ of the sperm. It is composed of two general regions, an anterior principal or chief piece and a posterior end piece. The greater part of the axial filament and its sheath is found in the flagellum. A relatively thick layer of cytoplasm surrounds the filament and its sheath in the chief-piece region of the flagellum, but, in the caudal tip or end piece, the axial filament seems to be almost devoid of enveloping cytoplasm. The end piece often is referred to as the naked portion of the flagellum.
In figure 79 is shown a diagrammatic representation of a urodele amphibian sperm. Two important differences from the mammalian sperm described above are to be observed, namely, the middle piece is devoid of mitochondria and is composed largely of centrioles 1 and 2, and the tail has an elaborate undulating or vibratile filament associated with the chief piece.
3) Spermiogenesis or the Differentiation of the Spermatid into the Morphologically Differentiated Sperm. The differentiation of the spermatid into the fully metamorphosed sperm is an ingenious and striking process. It involves changes in the nucleus, during which the latter as a whole contracts and in some forms becomes greatly elongated into an attenuant structure. (See figs. 79B-F; 85L-P.) It also is concerned with profound modifications of the cytoplasm and its constituents; the latter changes transform the inconspicuous
Spermatid into a most complicated structure. Some of these changes are outlined below.
a) Golgi Substance and Acroblast; Formation of the Acrosome. The Golgi substance or parts thereof previously associated with the idiosome of the spermatocyte (fig. 80A) proceeds to form the acrosome of the developing spermatid as follows: In the differentiating human sperm, the Golgi substance of the spermatocyte (fig. SOD) becomes aggregated at the future anterior end of the nucleus, as shown in fig. 80F, where it forms an acroblast within a capsule of Golgi substance. This acroblast later forms a large vacuole within which is the acrosomal “bead†(figs. 68B; 80G). The acrosomal bead proceeds to form the acrosomal cap, shown in figure 81 A, and the latter grows downward over the anterior pole of the nucleus (fig. 81 A, B). Most of the Golgi substance in the meantime is discarded (fig. 81 A, B). (Sec Gatenby and Beams, ’35.)
In the guinea pig the acroblast together with other Golgi substance, migrates around the nucleus toward the future anterior pole of the latter where the acroblast takes up its new position (fig. 80B, C, E). (See Gatenby and Woodger, ’21.) As shown in figure 80E, the acroblast is composed of inner and outer acrosomal substances. These inner and outer areas of the acroblast give origin respectively to the inner and outer zones of the acrosome (fig. 82). The peripheral or surrounding Golgi material of the acroblast detaches itself meanwhile from the developing acrosome (fig. 80E, H) and drifts downward toward the posterior end of the sperm. Eventually it is discarded with the excess cytoplasm and some mitochondrial material. In some animal species (e.g., grasshopper) the acrosomal substance arises from a multiple type of acroblast (Bowen, ’22). (See fig. 83.) Nevertheless, the general process of acrosome formation is similar to that outlined above.
b) Formation of the Post-nuclear Cap. All spatulate sperm of mam
Fig. 83. Formation of the acrosome from a multiple acroblast in the grasshopper. (After Bowen, Anat. Rec., 24.)
Fig. 84. The mitochondrial nebenkern and its elaborate development in Brachynema. (After Bowen, J, Morphol., 37 and Biol. Bull., 42.) (B-I) Division of the nebenkern
(A) and its elaboration into two attenuant strands extending posteriad into the flagellum.
mals possess a nucleus which has an acrosomal cap over its anterior aspect and a post-nuclear cap covering its posterior area. Both of these caps tend to meet near the equator of the nucleus (fig. 77).
The exact origin of the post-nuclear cap is difficult to ascertain. In the human sperm it appears to arise from a thickened membrane in association with centriole 1 (fig. 80G, post-nuclear membrane). This membrane grows anteriad to meet the acrosomal cap (fig. 81A-C). In the sperm of the guinea pig, a series of post-nuclear granules in the early spermatid appear to coalesce to form the post-nuclear cap (fig. 82A-C).
c) Formation of the Proximal and Distal Centrioles; Axial Filament. While the above changes in the formation of the acrosome are progressing, the centriole (or centrioles) of the idiosome move to the opposite side of the nucleus from that occupied by the forming acrosome, and here in this position the proximal and distal centrioles of the future sperm arise. In this area the neck granules also make their appearance (figs. 68B; 80F-H). The axial filament arises at this time and it probably is derived from the two centrioles simultaneously (fig. 80F, H). The centrioles soon become displaced along the axial filament, the caudal end of which projects from the surface of the cell membrane (fig. 80F-H). The axial filament grows outward posteriorly from the cell membrane in line with the two centrioles and the acrosome-forming material. The anterior-posterior elongation of the sperm thus begins to make its appearance (fig. 80H). The anterior centriole retains a position close to the nuclear membrane, but the posterior or ring centriole moves gradually posteriad toward the cell surface (figs. 81, 82A-C).
d) Mitochondrial Material and Formation of the Middle Piece OF THE Sperm. The behavior of the mitochondria in the formation of sperm varies greatly. In the spatulate sperm described above, a portion of the mitochondrial substance becomes aggregated around the axial filament in the middle-piece area (figs. 77, 82D). In certain amphibian sperm the middle piece appears to be formed mainly by centrioles 1 and 2 (fig. 79D-F). In certain insects the mitochondrial body or nebenkern, divides into two masses which become extended into elongated bodies associated with the flagellum (fig. 84). Some of the mitochondrial substance is discarded with the Golgi substance and excess cytoplasmic materials.
e) The Cytoplasm, Axial Filament, Mitochondria, and Tail Formation. Synchronized with the above events, the cytoplasm becomes drawn out in the posterior direction, forming a thin cytoplasmic layer over the sperm head, and from thence posteriad over the middle piece and the chief piece of the flagellum. However, the end piece of the flagellum may be devoid of investing cytoplasm (fig. 77). As the cytoplasm is elongating posteriorly over the contained essential structures of the forming sperm, much of the cytoplasm and Golgi substance and some mitochondria are discarded and lost from the sperm body. It may be that these discarded bodies form a part of the essential substances of the spermatic (seminal) fluid. (See Chap. 1.) (See figs. 66; 68B-E; 81; 82; 85M-0.)
The centralized core of the tail is the axial filament which arises in relation to centrioles 1 and 2 and grows posteriad through the middle piece and tail
(figs. 80F-H; 81A-C; 82A-~C; 85M-P). A considerable amount of mitochondrial material may also enter into the formation of tail (fig. 84).
A peculiar, highly specialized characteristic of many sperm tails is the development of a vibratile membrane associated with the axial filament (fig. 79E, F). Its origin is not clear, but it probably involves certain relationships with the mitochondrial material as well as the cytoplasm and axial filament.
In the formation of the human and guinea-pig sperm, the nucleus experiences only slight changes in shape from that of the spermatid. However, in many animal species, spermiogenesis involves considerable nuclear metamorphosis as well as cytoplasmic change (figs. 69, 79, 85).
In summary it may be stated that while the various shapes and sizes of mature flagellate sperm in many animal species, vertebrate and invertebrate,
Fig. 85. Spermatogenesis in the common fowl. Observe extreme nuclear metamorphosis. (After Miller, Anat. Rec., 70.) (A) Resting spermatocyte. (B) Early leptotene stage. (C, D) Synaptene stage. (E) Pachytene stage. (F, G) Diplotene stage. (H) Diakinesis. (I) First division, primary sperm. (J-P) Metamorphosing sperm.
are numerous, there is a strong tendency for spermiogenesis to follow similar
lines of development. Deviations occur, but the following comparisons between
mammalian and insect spermiogenesis, somewhat modified from Bowen (’22),
illustrate the uniformity of transformation of the basic structures of the
primitive meiocyte:
Mammalian Sperm
Insect Sperm
Nucleus — head
Nucleus — head
Centrioles — originally double and arranged in a proximal-distal formation. The axial filament arises from both centrioles
Centrioles — same as in mammals
Mitochondria — form an elaborate sheath for the anterior portion of the axial filament
Mitochondria — form a somewhat similar sheath for the axial filament
Idiosome and Golgi apparatus (acroblast portion) — gives origin to a vesicle which contains a granule, the acrosome granule, which is involved in the production of the acrosome
Idiosome and Golgi apparatus — much the same as in mammals
Excess Golgi substance — cast off with excess cytoplasm
Excess Golgi substance — cast off with excess cytoplasm
Excess cytoplasm — cast off — may be part of seminal fluid or possibly may be engulfed by Sertoli cells
Excess cytoplasm — cast off — may be part of seminal fluid or possibly may be engulfed by epithelial cells of the sperm cyst wall
c. Cytoplasmic Differentiation of the Egg
The cytoplasmic differentiation of the egg involves many problems. These problems may be classified under three general headings, viz.:
(1) Formation of the deutoplasm composed of fats, carbohydrates and proteins,
(2) development of the invisible organization within the true protoplasm or hyaloplasm, and finally,
(3) formation of the vitelline or egg membrane or membranes.
In view of the complexity of these three problems and of their importance to the egg in the development of the new individual, the mature oocyte or egg is in a sense no longer a single cell. Rather, it is a differentiated mass of protoplasm which is capable, after proper stimulation, to give origin to a new individual composed of many billions of cells. As such, the differentiation of the oocyte within the ovary represents a relatively unknown period of embryological development.
Fig. 86. Young oogonia of the fowl entering the growth (oocyte) stage. (A) Idiosome from which the Golgi substance has been removed and stained to show the centrosphere (archoplasm). The centrosome has two centrioles. (B) Idiosome with surrounding Golgi substance. The mitochondria surround the Golgi substance and the nucleus. (After Brambell, ’25.)
Fig. 87. The so-called mitochondrial yolk body in the developing egg of the fowl. (A) Oocyte from 11 -week-old chick, showing mitochondrial cloud and Golgi substances I and II. (B) Oocyte from ovary of adult fowl, showing both types of Golgi substance and mitochondrial cloud. (C) Oocyte from ovary of adult fowl, showing the appearance of the mitochondrial yolk body within the mitochondrial cloud. (D) Oocyte from ovary of adult fowl, showing fragmentation of Golgi substance 1 and the association of the resulting Golgi granules around the mitochondrial yolk body. (After Brambell, ’25.)
Fig. 88. Portion of follicle and periphery of oocyte from ovary of the adult bird,
showing the mitochondria and their transformation into the M-yolk spheres of Brambell.
(After Brambell, ’25.)
Before considering the various aspects of cytoplasmic differentiation of the oocyte, it is best for us to review the types of vertebrate and other chordate eggs in order to be able to visualize the various goals toward which the developing oocyte must proceed.
1) Types of Chordate Eggs. Eggs may be classified according to the amount of deutoplasm (yolk, etc.) present in the cytoplasm as follows:
a) Homolecithal (Isolecithal) Eggs. True homolecithal eggs in the phylum Chordata are found only in the mammals, exclusive of the Prototheria. Here the deutoplasm is small in amount, and is present chiefly in the form of fat droplets and small yolk spherules, distributed in the cytoplasm of the egg (figs. 118A, B; 147A).
b) Telolecithal Eggs. In the telolecithal egg the yolk is present in considerable amounts and concentrated at one pole. Telolecithality of the egg in the phylum Chordata exists in various degrees. We shall arrange them in sequence starting with slight and ending with very marked telolecithality as follows:
(1) Amphioxus and Styela. In Amphioxus and Styela from the subphyla Cephalochordata and Urochordata, respectively, the yolk present is centrally located in the egg before fertilization but becomes concentrated at one pole at the time of the first cleavage where it is contained for the most part within the future entoderm cells (figs. 132D, 167A).
(2) In many Amphibia, such as the frogs and toads, and also in the Petromyzontidae or fresh-water lampreys among the cyclostome fishes, the yolk present is greater in amount than in the preceding eggs. As such, it is concentrated at one pole, the future entodermal or vegetal pole, and a greater degree of telolecithality is attained than in the eggs of Amphioxus or Styela (fig. 141 A).
(3) In many Amphibia, such as Necturus, also in Neoceratodus and Lepidosiren among the lung fishes, and in the cartilaginous ganoid fish, Acipenser, yolk is present in considerable amounts, and the cytoplasm of the animal pole is smaller in comparison to the yolk or vegetal pole (figs. 150, 151, 152).
(4) In the bony ganoid fishes, Amia and Lepisosteus, as well as in the Gyrnnophiona (legless Amphibia) the yolk is situated at one pole and is large in quantity (figs. 153B-F; 154).
(5) Lastly, in a large portion of the vertebrate group, namely, in reptiles, birds, prototherian mammals, teleost and elasmobranch fishes, and in the marine lampreys, the deutoplasm is massive and the protoplasm which takes part in the early cleavages is small in comparison. In these eggs the yolk is never cleaved by the cleavage processes, and development of the embryo is confined to the animal pole cytoplasm (figs. 46, 47).
2) Formation of the Deutoplasm. The cytoplasm of the young oocyte is small in quantity, with a clear homogeneous texture (figs. 68A; 86A, B). As the oocyte develops, the cytoplasmic and nuclear volumes increase (fig. 68F), and the homogeneity of the cytoplasm is soon lost by the appearance of deutoplasmic substances (fig. 68G, H). In the oocyte of the frog, for example, lipid droplets begin to appear when the oocyte is about 50 /x in diameter (fig. 72A). (See Brachet, ’50, p. 53.) A little later glycogen makes its appearance, and finally yolk protein arises.
The origin of fat droplets and yolk spherules has been ascribed variously to the activities of chondriosomes (mitochondria and other similar bodies), Golgi substance, and of certain vacuoles. Most observers place emphasis upon the presence of a so-called “yolk nucleus†or “yolk-attraction sphere†situated near the nucleus of many oocytes as a structure associated with fat and yolk formation. In general, two types of yolk bodies have been described. One is the yolk nucleus of Balbiani and the other the mitochondrial yolk body of Brambell. The yolk nucleus of Balbiani (fig. 86A, B) consists of the following:
- a central body, the centrosphere or archoplasmic sphere within which one or more centriole-like bodies are found, and
- surrounding this central body, a layer of Golgi substances and chondriosomes (i.e., mitochondria, etc.).
This cytoplasmic structure probably is related to the idiosome of the oogonia (fig. 68A).
The formation of the deutoplasm, according to the theory associated with the Balbiani type of yolk nucleus is as follows: The surrounding pallial layer of Golgi substance and mitochondria moves away from the central portion (i.e., away from the centrosphere) of the yolk nucleus and becomes scattered and dispersed as small fragments within the cytosome (fig. 68F, G). The yolk nucleus as an entity thus disappears, and its fragments become immersed within the substance of the cytoplasm. Coincident with this dispersion of yolk nuclear material, rapid formation of small yolk spherules and fat droplets occur (fig. 68H). It appears thus that the formation of the deutoplasm composed of fat droplets and yolk spherules is directly related to the activities of the Golgi substance and chondriosomes.
Fig. 89. (A) Cytoplasm of oocyte, showing formation of a second kind of yolk (the M-C-yolk) in a vacuole surrounding the M-yolk sphere. (After Brambell, ’25.) (B)
Passing of Golgi substance from the follicle cells into the ooplasm of developing oocyte of the fowl. (After Brambell, '25.)
Fig. 90. Diagrams showing contrasting theories explaining the organization of polarity
of the cytoplasm of the fully developing egg or oocyte. Diagram at left shows polarity
explained according to quantitative differences, while the diagram to the right shows
qualitative differences. A = animal pole; V = vegetal pole. E represents a substance or
a factor, while EN-1, EN-2, etc., represent different quantities of substance E distributed
from pole to pole. SEC, SEN and SM are different chemical substances assumed to be
responsible for the determination of the ectoderm, entoderm, and mesoderm of the developing embryo. (After Barth: Embryology, New York, Dryden Press.)
On the other hand, the interpretation and description of the yolk body and its subsequent activities given by Brambell (’25) present a different view. According to the latter author, the yolk body is composed entirely of mitochondria; the Golgi substance and centrosphere are absent. Yolk formation proceeds as follows: As the young oocyte grows, the mitochondria increase in number and form the mitochondrial cloud (fig. 87A, B). The transitory mitochondrial yolk body differentiates within this cloud (fig. 87C). The mitochondrial yolk body ultimately breaks up into a mass of mitochondria, and the latter becomes dispersed in the cytoplasm of the oocyte (figs. 68F, G; 87D). Some of these dispersed mitochondria transform directly into yolk spheres (figs. 68H, 88, 89). Following this, another kind of yolk is formed in vacuoles surrounding these original yolk spheres (figs. 68H, 89 A, yolk spheres plus vacuoles). The fat droplets (C-yolk) within the ooplasm are formed according to Brambell “possibly under the influence of Golgi elements†(fig. 68H, fat droplets). Relative to the function of the yolk nucleus and its mitochondria, Brachet (’50), p. 57, considers it significant at the beginning, but its real importance is still to be understood.
The relationship, if any, of the oocyte nucleus to the deposition of yolk materials is not apparent. One must not overlook the real probability that the germinal vesicle (i.e,, the enlarged nucleus of the oocyte) may be related to the increase and growth of the cytoplasm and to yolk formation, for it is at this time that the chromatin threads surrender their normal diplotene appearance and become diffusely placed in the germinal vesicle. They also lose much of their basic chromatin-staining affinities while the Feulgen reaction is diminished (Brachet, ’50, p. 63). With regard to the possible function of the germinal vesicle in yolk synthesis, the following quotation is taken from a publication by Brachet (’47):
It is well worth pointing out that Puspiva (1942), using a very delicate and precise technique, found no correlation between the dipeptidase content of the nucleus and the onset of vitellus synthesis: such a correlation exists, however, in the case of the cytoplasm where dipcptidase increases markedly when the first yolk granules make their appearance. These results suggest that there is not evidence that the nucleus is the site of an especially active metabolism; cytoplasmic dipeptidase probably plays a part in yolk protein synthesis; if the nucleus controls such a synthesis, it works in a very delicate and still unknown way.
However, the means by which protein synthesis is effected still is a problem which awaits explanation (Northrop, ’50). (The interested student should consult Brachet, ’50, Chap. Ill, for a detailed discussion of the cytochemistry of yolk formation.)
Another aspect of the problem of cytoplasmic growth and differentiation of the oocyte presents itself for further study. Brambell (’25) concluded from his observations that Golgi substance passes from the follicle cells into the ooplasm of the growing bird oocyte and contributes to the substance of the peripheral layer (fig. 89B). Palade and Claude (’49) suggest that at least some of the Golgi substance be identified as myelin figures which develop “at the expense of lipid inclusions.†Thus it may be that the Golgi substance which Brambell observed (fig. 89B) passing from the follicle cells to the oocyte represents lipid substance. In the growing oocyte of the rat, Leblond (’50) demonstrated the presence of small amounts of polysaccharides in the cytoplasm of the oocyte, while the surrounding zona pellucida and follicle cells contained considerable quantities. These considerations suggest that the blood stream using the surrounding follicle cells as an intermediary may contribute food materials of a complex nature to the growing cytoplasm of the oocyte.
The localization of the yolk toward one pole of the egg is one of the movements which occurs during fertilization in many teleost fishes. In these forms, the deutoplasmic materials are laid down centrally in the egg during oogenesis, but move poleward at fertilization (fig. 122). A similar phenomenon occurs also during fertilization in Amphioxus and Styela among the protochordates. In many other fishes and in the amphibia, reptiles, birds, and monotrematous mammals, the yolk becomes deposited or polarized toward one pole of the oocyte during the later stages of oocyte formation, as the cytoplasm and the germinal vesicle move toward the other pole (figs. 68H, 72F). The polarization of the deutoplasmic substances thus is a general feature of the organization of the chordate egg.
3) Invisible Morphogenetic Organization Within the Cytoplasm of the Egg.
Two general categories of substances are developed within the cytoplasm of the oocyte during its development within the ovary, viz.:
(1) the visible or formed cytoplasmic inclusions, and
(2) an invisible morphogenetic ground substance.
The former group comprises the yolk spherules, fats, and other visible, often pigmented bodies which can be seen with the naked eye or by means of the microscope. The morphogenetic ground substance probably is composed of enzymes, hormones, and various nucleocytoplasmic derivatives enmeshed within the living cytoplasm. However, although we may assume that the basic, morphogenetic ground substance is composed of enzymes, hormones, etc., the exact nature of the basic substance or its precise relationship to the various formed inclusions of the cytoplasm is quite unknown (see Fankhauser, ’48, for discussion). More recent experiments demonstrate that the yolk or deutoplasmic material not only serves as a reservoir of energy for embryonic development but also is in some way connected with the essential, basic organization of the egg.
Although we know little concerning the exact nature of the morphogenetic organization of the egg or how it forms, studies of embryological development force upon us but one conclusion, to wit, that, during the period when the oocyte develops in the ovary, basic conditions are elaborated from which the future individual arises (Fankhauser, ’48). Within the cytoplasm of the mature egg of many chordates, this inherent organization is revealed at the time of fertilization by the appearance of definite areas of presumptive organ-forming substances. For example, in the egg of the frog and other amphibia, the yolk pole is the stuff from which the future entodermal structures take their origin; the darkly pigmented animal or nuclear pole will eventually give origin to epidermal and neural tissues; and from the zone between these two areas mesodermal and notochordal tissues will arise (fig. 119K). Similar major organ-forming areas in the recently fertilized egg have been demonstrated in other chordates, as in the ascidian, St ye la, and in the cephalochordate, Amphioxus. In the eggs of reptiles, birds, and teleost and elasmobranch fishes, while the relationship to the yolk is somewhat different, major organ-forming areas of a similar character have been demonstrated at a later period of development (Chaps. 6-9). This suggests that these eggs also possess a fundamental organization similar, although not identical, to that in the amphibian egg.
4) Polarity of the Egg and Its Relation to Body Organization and Bilateral Symmetry of the Mature Egg. One of the characteristic features of the terminal phase of egg differentiation in the chordate group is the migration of the gerrninal vesicle toward the animal pole of the egg (figs. 72F, 119A). As stated aboveTTn many vertebrate eggs the deutoplasmic jnaterial becomes situated at the opposite pole, known as the vegetal (vegetative) or yolk pole, either before fertilizatloiToTsEortty after. The relatively yolk-free protoplasm aggregates at the animal pole. Consequently the maturation divisions of the egg occur at this pole (fig. 1 19 A, B, D). The formation of a definite polarity of the egg, therefore, is one of the main results of the differentiation of the oocyte.
Various theories have been suggested in an endeavor to explain polarity in the fully developed egg or oocyte. All these theories emphasize qualitative and quantitative differences in the cytoplasmic substances extending from one pole of the egg to the other (fig. 90).
The animal and vegetal poles of the egg have a definite relationship to the organization of the chordate embryo. In Amphioxus, the animal pole becomes the ventro-anterior part of the embryo, while in the frog the animal pole area becomes the cephalic end of the future tadpole, and the yolk pole comes to occupy the posterior aspect. In teleost and elasmobranch fishes the yolk-laden pole lies in the future ventral aspect of the embryo, and it occupies a similar position in the reptile, bird, and prototherian mammal (see fig. 215). Studies have shown that the early auxiliary or trophoblastic cells in eutherian mammals lie on the ventral aspect of the future embryo. Consequently, it is to be observed that the various substances in mature vertebrate and protochordate eggs tend to assume a polarized relationship to the future embrydnic axis and body organization.
Many vertebrate and protochordate eggs possess a bilateral symmetry
which becomes evident when the fertilization processes are under way or
shortly after their conclusion. The appearance of the gray crescent in the
frog’s egg (fig. 119K) and in other amphibian eggs during fertilization and
the similar appearance of the yellow crescent in the fertilized egg of the
ascidian, Styela (fig. 132D) serve to orient the future right and left halves
of the embryo. Conditions similar to that of Styela, but lacking the yellow
pigment, are present in Amphioxus. Similarly, in the chick, if one holds the
blunt end of the egg to the left, and the pointed end to the right, the early
embryo appears most often at right angles, or nearly so, to the axis extending
from the broader to the smaller end of the egg, and in the majority of cases
the cephalic end of the embryo will appear toward the side away from the
body of the observer. There is some evidence that the “yolk†or egg proper
is slightly elongated in this axis. It appears, therefore, that the general plane
of bilateral symmetry is well established in the early chick blastoderm, although
the early cleavages do not occur in a manner to indicate or coincide with
this plane. In prototherian mammals, a bilateral symmetry and an anteroposterior orientation is established in the germinal disc at the time of fertilization, soon after the second polar body is discharged (fig. 136).
5) Membranes Developed in Relation to the Oocyte; Their Possible Sources of Origin. A series of membranes associated with the surface of the oocyte are formed during its development within the ovary. Three general types of such membranes are elaborated which separate from the oocyte’s surface at or before fertilization, leaving a perivitelline space between the egg’s surface and the membrane. They are:
( 1 ) A true vitelline membrane which probably represents a specialization or product of the ooplasmic surface. For a time this membrane adheres closely to the outer boundary of the ooplasm, but at fertilization it separates from the surface as a distinct membrane.
(2) A second membrane in certain chordates is elaborated by the follicle cells. It is known as a chorion in lower Chordata but is called the zona pellucida in mammals.
(3) A zona radiata or a thickened, rather complex, membrane is formed in many vertebrates; it may be considered to be a product of the ooplasm or of the ooplasm and the surrounding follicle cells.
All of the above membranes serve to enclose the egg during the early phases of embryonic development and therefore may be considered as primary embryonic membranes. As such, they should be regarded as a definite part of the egg and of the egg’s differentiation in the ovary. A description of these membranes in relation to the egg and possible source of their origin in the various chordate groups is given below.
a) Chorion in Styela. A previously held view maintained that the chorion
Fig. 91. Formation of the chorion in the egg of Styela. (A) Chorion is shown along
the inner aspect of the follicular epithelium. The test cells lie in indentations of the
peripheral ooplasm. (B) Optical section of an ovulated egg. (Redrawn and modified
from Tucker, ’42.)
Fig. 92. Developing vitelline membranes of Scyllium canicula. Observe that two membranes are present in the young egg; later these membranes fuse into one membrane. (A) Surface area of young oocyte with a vitelline membrane and zona radiata. (B) Slightly older oocyte with the radiate zone not as prominent. (C) Older oocyte with a single, relatively thick, vitelline membrane. (D) Nearly mature oocyte with a thin vitelline membrane. (After Balfour, Plate 25, The Works of Francis Maitland Balfour, ed. by Foster and Sedgwick, London, Macmillan, 1885.)
and “test†cells of the egg of Styela were ejected from the surface cytoplasm
at the time of ovulation (Conklin, ’05). A recent view, however, maintains
that the test cells arise from follicle cells and come to lie in indentations of
the periphery of the egg outside of the thin vitelline membrane (Tucker, ’42).
The chorion is formed by the inner layer of follicle cells and comes to lie
between the test cells and the inner layer of follicle cells in the mature egg
(fig. 91A). At ovulation the chorion moves away from the surface of the
oocyte. At this time also, the test cells move outward from their indentations
in the peripheral ooplasm and come to lie in the perivitelline space between
the egg surface and the chorion (fig. 9 IB). An ooplasmic membrane which represents the thin surface layer of ooplasm is present. However, it does not
separate from the periphery of the egg at fertilization. During its early development, the embryo remains within this chorionic shell. The chorion thus
represents the primary embryonic membrane of this species.
Fig. 93. Vitelline membranes of certain teleost fishes. (After Eigenmann, 1890.) (A)
Pygosteus pungtius. Radial section through micropyle of egg about 0.4 mm. in diameter. (B) Radial section through micropyle of egg of Perea, the perch. (C) Vitelline membranes of Fundulus heteroclitus about 0.8 mm. in diameter.
b) Egg Membranes of Amphioxus. Two surface membranes are formed
and eventually separate from the egg of Amphioxus. The outer vitelline
membrane is elaborated on the surface of the egg and remains in contact
with this surface until about the time of the first maturation division. It then
begins to separate from the egg’s surface. (See Chap. 5.) After the sperm
enters and the second maturation division occurs, a second, rather thick,
vitelline membrane also separates from the egg. The first and second vitelline
membranes then fuse together and become greatly expanded to form the primary embryonic membrane. (See Chap. 5.) A thin ooplasmic membrane
remains at the egg’s surface.
c) Vitelline Membrane and Zona Radiata of Elasmobranch Fishes. In the egg of the shark, Scy Ilium canicula, two egg membranes are formed, an outer and an inner membrane. The outer membrane is a homogeneous vitelline membrane, while the membrane which comes to lie beneath this outer membrane has a radiate appearance and hence may be called a zona radiata. This latter membrane soon loses its radiate appearance and becomes a thin membrane along the inner aspect of the vitelline membrane (fig. 92 A, B). In the mature egg both of these membranes form a thin, composite, vitelline membrane (fig. 92C, D). At about the time of fertilization the latter membrane separates from the egg’s surface; a perivitelline space then lies between these structures and the surface ooplasm of the egg.
d) Zona Radiata of Teleost Fishes. The surface ooplasm in teleost fishes gives origin to a membrane which in many cases has a radiate appearance. In some species this membrane appears to be composed of two layers. This radiate membrane which forms at the surface of the egg of teleost fishes appears to be the product of the ooplasm, and, therefore, should be regarded as a true vitelline membrane. In the perch a true chorion also is formed as a gelatinous or filamentous layer produced external to the radiate membrane by the follicle cells (fig. 93B). In Fundulus heteroclitus there are apparently three distinct parts to the membrane which surrounds the ooplasm of the egg:
( 1 ) a zona radiata,
(2) a thin structureless membrane external to the zona, and,
(3) the filamentous layer whose filaments are joined to the thin membrane around the zona (fig. 93C).
These three layers are probably derived from the ooplasm of the egg (Eigenmann, ’90). Consequently, the filamentous chorion or gelatinous layer, if derived from the egg itself, is not a true chorion in this particular egg.
Fig. 94. Vitelline membrane of an almost mature egg of the frog.
Fig. 95. Zona radiata (zona pellucida) or vitelline membrane of Chrysemys picta. (After Thing, ’18.)
Fig. 96. Zona radiata of the egg of the fowl. (After Brambell, ’25.)
At one end of the forming egg, a follicle cell sends an enlarged pseudopodiumlike process inward to the surface of the egg. As a result of this enlarged extension of the follicle cell to the ooplasmic surface, an enlarged pore-like opening in the zona radiata is formed. This opening persists as the micropyle after the egg leaves the ovary (fig. 93A).
As the teleost egg is spawned, the chorionic layer hardens when it comes in contact with the water. If fertilization occurs, the surface of the egg emits a fluid and shrinks inward from the zona radiata. In this manner, a perivitelline space is formed between the egg, and the zona is filled with a fluid. The egg is thus free to revolve inside of the zona (Chap. 5).
e) Vitelline Membrane (Zona Radiata) in Amphibia. In the amphibia, a vitelline membrane is formed probably by the surface ooplasm, although there may be contributions by the follicle cells of the ovary (Noble, ’31, p. 281). This membrane separates from the egg at the time of fertilization, forming a perivitelline space (fig. 94). The latter space is filled with fluid. Later the vitelline membrane expands greatly to accommodate the developing embryo. A delicate surface layer or membrane forms the outer portion of the ooplasm below the vitelline membrane. In some amphibia the vitelline membrane may have a radiate appearance.
f) Zona Radiata (Zona Pellucida) of the Reptile Oocyte. In the turtle group, the development of the zona radiata (pellucida) appears to be the product of the follicle cells (Thing, ’18). Filamentous prolongations of the follicle cells extend to the surface ooplasm of the developing egg (fig. 95). A homogeneous substance produced by the follicle cells then fills the spaces between these prolongations. The filamentous extensions of the follicle cells in this way produce a radiating system of canals passing through the homogeneous substance; hence the name, zona radiata. Bhattacharya describes Golgi substance as passing from the follicle cells through the canals of the zona radiata into the egg’s ooplasm in the developing eggs of Testudo graeca and Uromastix hardwicki. (See Brambell, ’25, p. 147.)
In contradistinction to the above interpretation, Retzius (’12) describes the homogeneous substance which forms the zona radiata of the lizard, Lacerta viridis, as originating from the ooplasm of the egg.
g) Vitelline Membrane (Zona Radiata) of the Hen’s Egg. The vitelline membrane, as in the turtle groups, appears to form about the young oocyte as a result of contributions from the surrounding follicle cells although the superficial ooplasm of the oocyte may contribute some substance. This occurs before the rapid deposition of yolk within the developing oocyte. It is probable that the follicle cells send small pseudopodium-like strands of cytoplasm through the numerous perforations of the very thin vitelline membrane around the oocyte’s surface into the superficial ooplasm in a similar manner to that which occurs in reptiles. The vitelline membrane (zona radiata) thus assumes a radiate appearance as it increases in thickness (figs. 47, 96).
Fig. 97. Kinoplasmic bead or droplet upon the middle piece of mammalian sperm.
(A) Pig sperm. (After Retzius, Biol. Untersuchungen, New Serfes, 10; Stockholm: Jena.)
(B) Cat sperm. (After Retzius, Biol. Untersuchungen, New Series, 10; Stockholm: Jena.) (C-D) Dog sperm. (C) Upper part of epididymis. (D) Lower or caudal part of epididymis.
When the vitelline membrane thickens, the loci where the cytoplasmic strands from the follicle cells pass through the membrane become little canals or canaliculi. As the oocyte increases in size, a thin space forms between the vitelline membrane or zona radiata and the follicle cells; it is filled with fluid and forms the follicular space. The egg now is free to rotate within the follicle. In consequence, the pole of the egg containing the blastodisc always appears uppermost. Due to the increasing pendency of the egg follicle as the egg matures, the blastodisc comes to rest, a short while previous to ovulation, at the base of the pedicle where the blood vessels are most abundant (fig. 47B). During the latter phases of oocyte development, the vitelline membrane constitutes an osmotic membrane through which all nourishment must pass to the oocyte, particularly in its later stages of growth. Tlie surface ooplasm forms a delicate surface membrane beneath the zona radiata.
h) Membranes of the Mammalian Oocyte. All mammalian oocytes possess a membrane known as the zona pellucida. It is a homogeneous layer interposed between the ooplasm and the follicle cells. By some investigators it is regarded as a product of the oocyte, while others regard it as a contribution of the ooplasm and follicle cells. The majority opinion, however, derives the zona pellucida from the follicle cells. In addition to the zona pellucida, the oocyte of the prototherian mammals has a striate layer lying close to the surface of the oocyte. This striated layer probably is derived from the surface ooplasm. This membrane later disappears, and a perivitelline space occupies the general area between the surface of the oocyte and the zona pellucida (fig. 46; Chap. 5). The zona pellucida separates from the egg surface after sperm contact.
5. Physiological Maturation of the Gametes a. Physiological Differentiation of the Sperm
Added to the nuclear and cytoplasmic transformations of the sperm described above, a further process of sperm ripening or maturing appears to be necessary. In the mammal, for example, the sperm cell must pass through the epididymis to achieve the ability to fertilize the egg. This is shown by the fact that sperm taken from the seminiferous tubules will not fertilize, although, morphologically, two sperm, one from the testis and one from the epididymis cannot be distinguished other than by the presence in some mammals of the so-called “kinoplasmic droplet†(figs. 82D, 97). These droplets do not appear in great numbers upon ejaculated sperm but are found on sperm, particularly in epididymides. It is possible that these droplets may arise from a secretion from the epididymal cells (CoIIery, ’44). In the dog, these droplets are attached to the neck of the sperm in the caput epididymidis but are found at the posterior end of the middle piece of the sperm in the cauda epididymidis and vas deferens and are probably lost at the time of ejaculation (Collery, ’44). Investigators differ greatly in interpreting the significance of this body. However, these droplets do seem in some way to be directly or indirectly concerned with the physiological maturing of the sperm. In this connection Collery (’44) notes that sperm are probably motile on leaving the seminiferous tubules, but active forward movement is not seen until the bead has reached the junction of middle piece and tail.
In the fowl, Domm (’30, p. 318) suggests the probability that the sperm may undergo an aging or ripening process essential for reproduction somewhere in the reproductive system other than the seminiferous tubules. The work of Lipsett quoted in Humphrey (’45) suggests that the accessory reproductive system also is necessary for a ripening process of the sperm in urodele amphibia.
On the other hand, in the frog, sperm taken from the testis have the ability to fertilize eggs. In this case, the sperm probably undergo a physiological ripening in the testis along with morphological differentiation.
The foregoing considerations suggest that a physiological maturation of the sperm is necessary to enable the sperm to take part in the fertilization process.
b. Physiological Ripening of the Female Gamete
The physiological maturing of the oocyte is linked to factors which influence the developing egg at about the time the maturation divisions occur. Sea-urchin sperm may penetrate the egg before the maturation divisions occur (Chap. 5). However, development does not take place in such instances. On the other hand, sperm entrance after both maturation divisions are completed initiates normal development. In the protochordate, Styela, marked cortical changes transpire at about the time the egg leaves the ovary, and as it reaches the sea water, the germinal vesicle begins to break down. The oocyte becomes fertilizable at about this time. In Amphioxus, although the first polar body is given off within the adult body, the egg apparently is not fertilizable until it reaches the external salt-water environment. The secondary oocyte of the frog presumably must remain within the uterus for a time to ripen in order that ensuing development may be normal. These and other instances suggest that physiological changes — changes which are imperative for the normal development of the egg — are effected at about the time that the maturation divisions occur.
D. Summary of Egg and Sperm Development
From the foregoing it may be seen that the development of the gametes in either sex involves a process of maturation. This maturation entails changes in the structure and constitution of the nucleus and cytoplasm, and, further, a functional or physiological ripening must occur. The comparative maturation events in the egg and sperm may be summarized as follows:
Egg ( in Oogenesis)
Sperm (in Spermatogenesis)
1. Nuclear maturation
a. Homologous chromosomes synapse and undergo profound changes during which parts of homologous chromosomes may be interchanged; ultimately, the chromosome number is reduced to the haploid number
b. Nucleus enlarges, and contained nuclear fluid increases greatly; ultimately the nuclear fluid is contributed to cytoplasm upon germinal vesicle break down
c. Nuclear maturation occurs simultaneously with cytoplasmic differentiation
1. Nuclear maturation
a. (Similar to the female)
b. Nucleus remains relatively small and
enlargement is slight; nuclear fluid
small in amount; during spermiogenesis the nucleus may contract into
a compact mass; considerable elongation of nucleus occurs in many
species
c. Nuclear maturation occurs before spermiogenesis or cytoplasmic differentiation
SUMMARY OF EGG AND SPERM DEVELOPMENT
Egg ( in Oogenesis)
Sperm (in Spermatogenesis)
2. Cytoplasmic maturation
This involves:
2. Cytoplasmic maturation
This involves:
a. Polarization of cytoplasmic materials and the nucleus in relation to the future maturation phenomena; the nucleus becomes displaced toward one pole, the animal pole, and the yolk, and other cytoplasmic materials; in many eggs becomes displaced toward the opposite or vegetal pole
a. Polarization of nucleus and cytoplasmic materials along an elongated antero-posterior axis, with the head, neck, middle piece, and tail occupying specific regions along this axis. The nucleus occupies a considerable portion of the anterior region or head
b. Formation of deutoplasm or stored food material, varying greatly in amount in different animal species. The deutoplasm is composed of fats, carbohydrates, and protein substances
b. Little food substances stored within cytoplasm; food reserve in seminal fluid
c. Cytoplasm increased in amount; formation of basic organ-forming areas or cytoplasmic stuffs from which the future embryo arises
c. Discarding of a considerable amount of cytoplasm, some Golgi elements and mitochondria. Retention of some Golgi elements, centrioles, mitochondria, etc.
d. Formation of primary embryonic membranes
d. No specific membranes formed around sperm, although elaborate membranes for motile purposes are formed in some sperm
3. Physiological maturation or the development of a fertilizable stage
This involves:
a. Formation of an organization which when stimulated by external influences initiates and carries on the processes necessary for normal embryonic development
3. Physiological maturation or the development of the ability to contact and fertilize the egg
This involves:
a. Development of an organization which, when stimulated by proper external substances, responds by a directed movement resulting in locomotion; also capable of being attracted by egg substances
b. Acquisition of ability to enter into a developmental union with a sperm
b. Acquisition of ability to fertilize, i.e., to enter into a developmental union with an egg or oocyte
c. Development of ability to form and secrete gynogamic substances which aid in the fertilization process. (See Chap. 5)
c. Acquisition of ability to produce and secrete androgamic substances which aid in the fertilization process
d. Assumption of an inhibited or dormant condition during which metabolic processes proceed slowly in anticipation of the fertilization event
d. Assumption of an active metabolic state
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