Book - Aids to Embryology (1948) 1
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Baxter JS. Aids to Embryology. (1948) 4th Edition, Bailliere, Tindall And Cox, London.
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Chapter I The Germ Cells
The Male Organs of Reproduction
The male genital glands are known as the testes. They are concerned with the production of male germ cells or spermatozoa. Also, certain cells of the testes elaborate an important internal secretion. The spermatozoa are conveyed to the exterior by a system of genital ducts with which are associated several accessory glands.
Each testis is enclosed in a strong hbro-elastic capsule, the tunica albuginea. This is covered by a serous membrane, the tunica vaginalis testis, which permits considerable mobility of the organ within the scrotum. From the capsule, septa pass inwards subdividing the testis into a number of lobules. Posteriorly the septa converge to a fibrous thickening, the mediastinum testis. Each lobule of the testis contains one or more seminiferous tubules, in the walls of which development of the male germ cells or spermatogenesis takes place. The seminiferous, tubule is coiled up when in the lobule ; when unravelled it measures from i to 2 feet in length. I11 between the tubules are found blood vessels, connective tissue and groups of interstitial cells of Leydig, which elaborate male sex hormone. The seminiferous tubules terminate by straight tubules in a system of channels in the mediastinum known as the rete testis ; these are continued in turn into the efferent ductules of the epididymis. The latter connect with the canal of the epididymis and this passes into the ductus deferens. Near its termination the ductus is joined by the duct of the seminal vesicle and the canal so formed, the common ejaculatory duct, perforates the upper posterior part of the prostate to end in the prostatic urethra.
Each seminiferous tubule has an outer fibrous coat. It is lined by several layers of cells, and these are of two kinds - sustentacular cells of Sertoli and male germ cells in various stages of development. The sustentacular cells are elongate, columnar elements projecting from the basement membrane towards the lumen of the tubule. They are in close relation with the developing spermatozoa, and probably provide nourishment for them.
Fig. i. - Two Stages in the Development of the Spermatozoon.
A. - i, Acrosome ; 2, nucleus ; 3, proximal centriole ; 4, distal centriole. B. — 1 ,?Perf ora tori um ; 2, neck; 3, body; 4, tail; 5, terminal filament.
Spermatogenesis
The series of changes through which the male germ cells pass from the immature to the fully formed state is termed spermatogenesis. It may readily be studied in any adult male animal, since every tubule of an active testis contains several stages of the process, and these changes occur in orderly rhythm along the length of the tubule.
The primitive germ cells lie next to the basement membrane and are known as spermatogonia. They are rounded cells with a large vesicular nucleus, and the cytoplasm contains many scattered mitochondria. The Golgi material forms a cap at one pole of the nucleus. Division of the spermatogonia occurs by the ordinary process of mitosis. One of the daughter cells resulting from such a division becomes displaced towards the tubule lumen, and is now known as a primary spermatocyte. The nucleus of this cell type is larger than that of the parent spermatogonium, and the mitochondria become collected around it. A centriole appears in the cell cytoplasm. Division of the primary spermatocyte by a special process (meiosis, see p. io) involving reduction of the chromosome content of the nucleus to one-half, results in the formation of secondary spermatocytes. Then, mitosis of the secondary spermatocytes gives rise to spermatids which metamorphose into the adult spermatozoa without further cell division. The nucleus of the spermatid is a dense chromatic structure eccentrically situated in the cell, and to one pole of it is closely applied part of the Golgi material, the acrosome. This is ensheathed by a thin layer of cytoplasm, the head cap, and the two are destined to become the perforatorium of the mature spermatozoon. The centriole divides into two which migrate to that part of the cytoplasm opposite to the acrosome. From the distal centriole an axial filament grows out through the surface of the cell. The proximal centriole remains close to the nucleus, but the distal centriole becomes ring-shaped and moves away from it along the axial filament. During this migration of the distal centriole the mitochondria become aggregated in spiral fashion around that part of the axial filament between the two centrioles. This is the middle piece or body of the spermatozoon. Further elongation of the axial filament results in the formation of a tail distal to the posterior centriole, most of which is clothed with a very thin film of cytoplasm, the extreme tip alone being naked. During this process of maturation much of the spermatid cytoplasm, together with some of the Golgi material and a few mitochondria is cast off into the tubule lumen and degenerates.
The result of these changes is the formation of a mature spermatozoon composed of three parts — head, middle piece or body and a tail. The development of these three parts may be summarized as follows :
Summary of Development of Spermatozoon
- The head is composed of the nucleus of the spermatid capped by an acrosome formed from part of the Golgi material. A very thin head cap derived from cytoplasm covers the acrosome, and the two together form the perforatorium. The head is oval and somewhat flattened in shape.
- The middle piece extends from the head to the ring centriole. Most of it is surrounded by the spiral filament formed from the mitochondria. Anterior to this is a short neck in which is located the anterior centriole.
- The tail or flagellum is formed by the axial filament with a very thin sheath of cytoplasm around it ; the terminal part of the tail is bare and formed by the filament alone.
The human spermatozoon measures about 60 microns * in length ; of this, the tail forms about five-sixths. Spermatozoa from the seminiferous tubules are not fully mature. They are non-motile, and it is not until they have become suspended in the secretions of the accessory genital glands to form the semen that they become functionally active. In this fluid medium they progress in a forward direction by lashing movements of their tails with a speed of from 14 to 23 p, per second. The number of spermatozoa per cubic millimetre of semen is estimated to be 60,000, and an average ejaculation contains about 200,000,000.
- A micron ( i a) is nrooth part of a millimetre.
Although it is claimed that spermatozoa may remain alive for a week or longer in the female genital tract, the power to fertilize an ovum lasts only a short time — 24 to 48 hours. They are very sensitive to changes in the reaction of the medium, an acid one being very deleterious to them.
Abnormalities in the process of spermatogenesis may result in bizarre forms with two or more heads or tails.
The Female Organs of Reproduction
The paired female genital glands are known as the ovaries. The associated genital ducts consist of the uterine tubes, the uterus and the vagina. Of these, the ovary alone will be considered at this time. These glands produce the female germ cells or ova, which, when fertilized by the spermatozoa, develop into new individuals. Like the testis in the male, the ovary is the site of formation of important internal secretions.
The ovary is attached at its hilum by a short mesovarium to the posterior surface of the broad ligament of the uterus. Covering the gland, and continuous with the peritoneum of the mesovarium, is a layer of cubical cells termed the germinal epithelium since it is the source of ova and their surrounding follicle cells. It was formerly believed that ova were only formed during the embryonic period of life, and that these, lying dormant until after puberty, gave rise to the mature ova liberated from the ovary during the reproductive period in the adult female. The work of Evans and Swezy (1931) has, however, demonstrated that these early ova degenerate after birth, and that the germinal epithelium is active after puberty in the production of new female germ cells or oogonia. The main mass of the ovary is divisible into a compact cortical layer and a more loosely arranged medulla. The cortex contains follicles in various stages of growth ; the medulla, on the other hand, contains no follicles but consists of connective tissue in which are many blood vessels.
Oogenesis
The oogonia, which correspond to the spermatogonia, arise by proliferation from the germinal epithelium ; but whether their source is the epithelium itself or cells which have migrated to it during embryonic life from some other part, is not certainly known. The cells resulting from this proliferation are the primary oocytes located in the peripheral part of the ovarian cortex (Fig. 2 A). Each is surrounded by some flattened cells, the follicular epithelium. The next phase is one of growth in which the oocyte increases in size, and the elements of the ovarian or Graafian follicle become elaborated around it (Fig. 2, B and C). The essential details are these ; a structureless membrane, the zona pellucida, is laid down immediately around the oocyte. The follicle cells proliferate, become columnar and form a many-layered follicle wall. This is termed the membrana granulosa. A cavity (antrum) now appears in the membrana granulosa filled with fluid (liquor folliculi). Continued expansion of this cavity occurs as the follicle becomes mature until it reaches a diameter of 5 to 10 mm. and causes a bulging on the free surface of the ovary. In this state the oocyte, surrounded by the zona pellucida and a number of granulosa cells, is attached to the inner aspect of the follicle wall forming an elevation, the discus proligerus. During the growth of the follicle surrounding cells of the ovarian stroma become differentiated around the membrana granulosa in two layers , an inner of epithelioid cells and blood vessels is named the theca interna, while an outer fibrous coat is the theca externa (Fig. 2 C).
Fig. 2. - Diagrams to show Growth of the Ovarian Follicle.
A. Primordial follicle, i, Nucleus of primary oocyte ; 2, follicle cells. B.— 1, Primary oocyte ; 2, zona pellucida ; 3, membrana granulosa j 4> ovarian stromal cells. C. — 1, Primary oocyte ; 2, zona pellucida ; 3, discus proligerus ; 4, follicle cavity , 5, membrana granulosa ; 6, theca interna 7, theca externa.
During the time of follicle growth the female germ cell has remained in the stage of the primary oocyte. This must undergo certain changes comparable with those in the male before it is mature and capable of being fertilized. The primary oocyte transforms into a secondary oocyte and a polar body by a cell division involving reduction of the chromosome number to one-half. In addition (and this is peculiar to the female germ cell), the cytoplasm is very unequally partitioned, almost all of it going to one daughter cell, the secondary oocyte. This subsequently divides iby the ordinary process of mitosis, once more with unequal segregation of the cytoplasm, and a mature ovum and second polar body are formed. When extruded from the follicle at ovulation the mature ovum is a large cell with a diameter of about 135 j a in the human. It is enclosed in the vitelline membrane and the zona pellucida. Attached to the outer surface of this are cells of the discus proligerus, now called the corona radiata. The mechanism of ovulation is discussed later (p. 25). [It may be noted that extrusion of the second polar body does not take place until after rupture of the follicle and not until a spermatozoon penetrates the ovum — perhaps an economy of effort.]
The growth of follicles in the ovary occurs in cycles during the reproductive period of life. While a number of follicles commence growth together, in the human only one normally reaches maturity and escapes from the ovary in each ovarian cycle. The remainder, partly grown, regress in a process called follicular atresia. The cavity of the ruptured follicle becomes invaded by cells to form a corpus luteum (see p. 20) which is an endocrine structure. The control of follicle ripening is discussed later (p. 23).
Maturation of the Germ Cells
By this term is understood the changes undergone by the nucleus whereby its chromosome content is reduced to one half. This is an essential preliminary to fertilization, i.e. the union of the spermatozoon with the mature ovum.
During the usual manner of division of cells (mitosis) the chromatin of the nucleus is arranged as a number of short threads or chromosomes. The number of these for any animal is constant : in the human subject it is 48 (Painter, 1923). The chromosomes in the germ cells are extremely important for they are concerned with the transmission of hereditary characters. The factors concerned in this process are known as genes and they are located on the chromosomes. Obviously then, the changes occurring in maturation must be understood, before the meaning of fertilization in the determination of sex and the genetic make-up of the new individual may be appreciated.
It is convenient to consider maturation of the ovum first. The early germ cells (like the general cells of the body) contain in their nucleus a number of chromosomes which is constant for each animal form. In the human there are forty-eight of these chromosomes, and they may be considered as arranged in pairs in the nucleus. Twenty- three of the pairs are ordinary chromosomes (autosomes), while one pair is concerned with the determination of sex. In the early female germ cells each member of this pair is termed an X chromosome. In the usual mode of cell division (mitosis), as seen, for example, in the stage of proliferation of the oogonia (Fig. 3), each chromosome splits lengthwise and one half passes to the nucleus of each daughter cell, which contains therefore 46 + 2X chromosomes. Some time in the development of the germ cells reduction of the nuclear chromosome number to half of this figure must occur. Otherwise in union of the ovum and spermatozoon at fertilization doubling of the chromosome number would result. Here, in the female, this reductional or meiotic division is that of the primary oocyte. There are two striking events : (a) the members of each chromosome pair blend, quickly separate again and an entire chromosome from the pair passes into each daughter cell ; the total for each being 23 -j- X ; ( b ) the cytoplasm of the parent cell is split so unequally that almost all passes to the secondary oocyte. The other daughter cell, called the first polar body, is very small and consists almost entirely of nuclear material. Although in Fig. 3 the first polar body is shown as capable of further division, it is most improbable that this occurs in man, degeneration soon taking place.
Fig. 3. - Diagram to illustrate Oogenesis Spermatogenesis.
During the reduction division in the male the chromosome number is so changed that half the spermatozoa carry 2 v4- X chromosomes and the other half 23 + Y chromosomes. The mature ovum has 23 + X chromosomes in its nucleus.
The secondary oocyte divides again by the ordinary process of mitosis. The nucleus of each resultant cell will therefore contain the reduced number (23 -j- X) chromosomes ; but the partition of the cytoplasm is once more unequal ; in fact, a second polar body containing 23 -j- X chromosomes and little else, is cast out from the secondary oocyte. The latter may now be termed the mature ovum, a large inert cell in which the cytoplasmic volume is excessive in relation to that of the nucleus.
The spermatogonia in the male contain forty-six ordinary chromosomes and two concerned with sex determination . One of these is like the X chromosome of the female, but the other is quite small and is known as the Y chromosome. During maturation the nuclear changes resemble those already described for the female. Reduction of the chromosome number is found during the division of the primary spermatocyte and one secondary spermatocyte thus contains 23 + X chromosomes : the other has 23 + Y chromosomes. Ordinary mitotic division of the two secondary spermatocytes gives four spermatids which transform into spermatozoa. Important differences from the female are : (a) four functional spermatozoa are derived from each primary spermatocyte ; (b) half of these contain twenty-three ordinary chromosomes with one X chromosome, and the other half contain the same number of autosomes with an additional Y chromosome (instead of X) ; (c) during meta morphosis of the spermatid there is elimination of cytoplasm so that the mature spermatozoon contains very little.
Determination of the sex of the embryo occurs at fertilization. From what has been said it is obvious that the chromosome number of the mature ovum is 23 + X while spermatozoa are of two kinds, some containing 23 -f- X, other 23 -j- Y chromosomes. When a spermatozoon with an X chromosome fertilizes an ovum the resulting embryo will be female, but the spermatozoon containing a Y chromosome will, on union with the mature ovum, give rise to a male individual.
Fertilization
Fertilization is the union of two mature germ cells, an ovum and a spermatozoon, to form a single uninucleated cell, the zygote. While this event has not actually been observed in the human, the known facts of comparative embryology indicate that the processes are fundamentally similar in higher mammals, and they may be described as follows :
In the human, fertilization normally occurs in the lateral third of the uterine tube. It was formerly believed that the spermatozoa deposited in the upper vagina at coitus, actively swim upwards through the uterus and uterine tube to meet the ovum. Hartman and Ball (1933) have shown that sperm transport in the rat is much too quick for this to be the only process involved, for in this animal less than two minutes elapse from the moment of ejaculation until spermatozoa reach the distal ends of the uterine cornua. The transporting mechanism is believed to be the powerful contractions of the uterine musculature which cause rapid disposal of the spermatozoa throughout the fluid-filled uterine cavity. That there is any comparable mechanism for sperm transport in the human is, at present, not proven, but it is believed that spermatozoa reach the outer part of the human uterine tube within a few hours after intercourse.
The first spermatozoon to reach the ovum penetrates the zona pellucida and the head with the middle piece enter into the cytoplasm. Other spermatozoa attempting to enter later become entangled in the zona pellucida owing probably to some change that takes place in the nature of this membrane. After the sperm head has penetrated the ovum it becomes swollen and is termed the male pronucleus. The nucleus of the mature ovum may, correspondingly, be called the female pronucleus. Union of these two structures occurs to form the segmentation nucleus. This results in two things : (a) the reduced chromosome number of the ovum (sometimes called the haploid number) is restored to normal : ( b ) the ovum is now stimulated to enter on a process of division or segmentation.
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Cite this page: Hill, M.A. (2024, April 16) Embryology Book - Aids to Embryology (1948) 1. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Aids_to_Embryology_(1948)_1
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
