Book - Manual of Human Embryology 19-3
III. THE UROGENITAL UNION.
The efferent apparatus for the male sexual products is produced in both sexes by a union between the mesonephros and the testis. In describing the second period of degeneration of the mesonephros (p. 829) we have spoken of a division of its remains into the epigenitalis and the paragenitalis. The tubules of the epigenitalis, 5-12 in number and corresponding to the 58th-62nd or 69th mesonephric tubules, degenerate completely as far as the tubuli collectivi. The angle at which the transition of the tubulus secretorius into the tubulus collectivus occurs, and at which the interruption between the two first takes place, of all parts of the mesonephros, projects furthest medially or, what is of especial importance here, furthest towards the reproductive gland area. The blind ends of the tubuli collectivi become surrounded by the epithelial nucleus of the indifferent reproductive gland, so that each comes to lie in a bay within the nucleus. This portion of the epithelial nucleus gives rise, as has been seen, to the rete blastema; from the beginning, accordingly, the tubuli collectivi and the rete tubules lie wall to wall. This proximity of the two has led some observers who studied poorly preserved embryos — and these form the majority — to the erroneous conclusion that the tubules of the male and female rete, and even the tubuli recti of the testis and the medullary cords of the ovary, arose by the ingrowth of mesonephric tubules. There can be no question of such an origin. The rete tubules are developed before their union with the mesonephric tubules occurs, the tubuli collectivi of the latter end with broadened and thickened ends, sharply defined and distinctly marked off from the rete cords. The union of the two, the urogenital union, takes place at very different periods; I saw it for the first time in embryos of 60 mm. head-foot length. Up to this time the development in the two sexes is alike ; a further development now begins in the males, degeneration in the females.
The Further Differentiation in the Male Sex.
As soon as the tubuli collectivi have broken through into the rete tubules, we term them the ductuli efferentes testis. In the fourth to the fifth fetal month (Kolliker 1879) they begin to coil at their ends that are towards the primary excretory ducts, while towards the testis they remain straight. As a result they appear as wedge-shaped structures, whose apices are towards the testis and the bases towards the excretory duct; when, later, they are surrounded with a firm connective tissue membrane, they are known as the coni vasculosi. All the 5-12 tubules of the epigenitalis do not necessarily take part in the urogenital union, nevertheless they frequently persist, appearing as mesonephric remains in the epididymis. They may make secondary connections with neighboring parts, and especially, may break through into the tunica vaginalis propria, that is to say, into a portion of the abdominal cavity. Several coni may unite to form a duct before opening into the rete, the union resulting from the coni being connected in the first place with the rete and, secondly, with each other by means of longitudinal anastomoses, so that the individual tubules become independent of their connections with the rete and may lose them. Those that behave thus are usually the most caudal efferentes. The primary excretory duct becomes the canalis epididymidis and the ductus deferens. At first it is lined by a onelayered cylindrical, non-ciliated epithelium ; in its upper part it coils in the manner described in works on systematic anatomy (canalis epididymidis) and at its lower end it dilates almost to four times its original size and forms the ampulla; the dilatation ends just at the beginning of the horizontal portion (Fig. 625), so that even before the anlage of the vesicula seminalis, the ductus ejaculatorius, which is developed from the horizontal portion, is recognizable.
In fetuses of 50 mm. head-foot length there begins, first in the lower portion of the ductus deferens, a concentric layering of the mesenchyme around the epithelial cylinder, and thence this arrangement extends over the whole of the primary excretory duct. I believe that I was able to recognize the first cilia on the epithelium in embryos of 70 mm. head-foot length. The musculature of the duct must first appear very late, since no muscle fibres were present in a fetus of 240 mm. head-foot length.
The vesicula seminalis arises from- the primary excretory duct, and from the lower portion of it which has dilated to form the ampulla. In this region a simple epithelial tube is gradually constricted off in the cranio-caudal direction. The constriction is first found in embryos of 60 mm. head-foot length; it ends at the beginning of the horizontal portion. This, therefore, from the beginning is the anlage of the later ductus ejaculatorius.
Degeneration of the Urogenital Union and of the Primary Excretory Duct in the Female.
The connection of the epigenitalis tubules with the rete is completed either at birth or even before it. Since the degeneration of the primary excretory duct has already begun some time before this, the epoophoron has become completely free. Notwithstanding its degeneration, it continues to grow and its tubules acquire a ciliated epithelium and subepithelial smooth muscle cells (Biihler 1894). The primary excretory duct is already degenerated in embryos of 30 mm., at the very time, it is interesting to note, at which the degeneration of the Miillerian duct begins in male embryos. The first degeneration is exactly the same as in the case of the Miillerian duct, it affects the portion between the caudal pole of the ovary and the beginning of the utero-vaginal canal. The cranial portion may remain connected with the epoophoron for a longer or shorter time, the distal portion is usually completely degenerated; all the remains of the primary excretory duct are known as Gartner's canal. This may run (H. Meyer 1909) "from the epoophoron through the ligamentum latum, at first parallel with the tube, then more obliquely as the base of a triangle whose opposite angle is formed by the uterine portion of the tube and the corpus uteri." "At about the level of the internal os uteri, the canal first sinks into the substance of the uterus, and running downwards through the muscle layers, it gradually approaches the mucous membrane medially, without reaching it, however, and then passes from the upper part of the portio vaginalis into the lateral fornix of the vagina to run caudally at about the middle of the lateral wall of the vagina." "In its lowest portion the canal lies usually rather far dorsally in the lateral wall and thence passes ventrally in the lateral part of the hymen to reach the free border of that structure." "Its opening to the exterior is immediately at or very close to the free border of the hymen, through the vestibular epithelium ; rarely it is more lateral, in the nymphohymenal sulcus of the vestibule." Gartner's canal is very seldom indeed present throughout its whole extent. Up to the end of the third fetal month one finds, without exception, remains of the canal in the uterus, in the vagina or in the hymen ; from the fourth month onwards the number of cases increases in which remains are no longer present: in new-born infants and in children, one always finds smaller or larger remains of the canal in about onequarter to one-third of all cases, and the proportion is not much less in adults (R. Meyer 1909).
If one compares the course of the primary excretory duct in the embryo with the course of Gartner's canal in the adult, the different relation of the two to the wall of the uterus at once reveals itself. The primary excretory duct of the embryo is completely surrounded by the mesenchymatous wall of the uterus that encloses the utero-vaginal canal; Gartner's canal enters the uterine musculature only at the level of the internal os uteri. The difference between these two arrangements is, however, only apparent; the uterine wall is formed not only by the mesenchyme of the uterovaginal canal, but also by the mesenchyme of the horizontal portion of the tubes (Fig. 623 b, p. 917). The entrance of Gartner's canal marks the point where, in the adult, the portion of the uterus formed from the tube passes over into that formed from the uterovaginal canal. I was, therefore, correct when (p. 918) I placed the boundarv between the two at the internal os.
The persisting Gartner's canal may also form in the fifth month an ampulla and a vesicula seminalis (for details, see R. Meyer 1899 and 1909), and the ampulla then lies at the level of the cervix uteri, of the upper part of the portio vaginalis, and extends to the fornix of the vagina and into the uppermost part of the lateral wall of the vagina.
The Ligaments of the Reproductive Glands.
The urogenital fold divides into a series of portions (pp. 785 and 787). First there is a division into the mesonephric and genital folds (Figs. 631 and 632), and the former is again divided into the tubar, gland and mesenterial portions (Figs. 632). Let us first consider the mesenterial portion. The most cranial portion of it fastens the mesonephros to the diaphragm for a limited period of the development and, therefore, bears the name of diaphragmatic ligament of the mesonephros (Fig. 631). During the development of the mesonephros this ligament is continually degenerating and continually being formed anew. This process depends on the degeneration of the mesonephros and of the mesonephric fold itself. Both undergo an extensive process of degeneration, as we have seen on p. 816, and, since the degeneration of the fold begins at the cranial end and proceeds caudally, it must at once affect the most cranial portion, the diaphragmatic ligament. This will disappear, only to be immediately formed again, since the succeeding portion of the mesonephric fold will now attach the new cranial pole of the mesonephros to the pillar of the diaphragm, which has in the meantime grown downwards, and will so become a new diaphragmatic ligament. This process will be repeated from segment to segment, until an obstacle occurs to end it, and this obstacle is supplied in the vasa spermatica interna. We may say, therefore, that the diaphragmatic ligament of the mesonephros continues to degenerate until it becomes the mesentery of the spermatic vessels; as such it persists throughout life, along with the rest of the mesonephric fold.
Fig. 631. — Model of the posterior abdominal wall of a human embryo of 19.4 mm. greatest length. (Embryo Ma. 2, from the collection of Professor Hochstetter, Vienna.) Model prepared by my pupils Massard and Chome. The urogenital fold is separated by a deep groove into the mesonephric and genital folds. The upper pole of both these folds is attached to the diaphragm by the diaphragmatic ligament of the mesonephros. The mesonephric fold is bent at two places, and we may recognize an upper sagittal, a horizontal, and a lower sagittal portion. At the first bend, between the upper sagittal and horizontal portions, the mesonephric fold is connected with the lateral wall of the abdomen by the inguinal fold.
Fig. 632. — Transverse section of the urogenital fold of a human embryo of 50 mm. head-foot length. (Embryo R. Meyer 272, from the collection of Professor R. Meyer, Berlin; slide 2, row 1, section 1.) The urogenital fold is divided by two deep grooves into the mesonephric and genital folds; both are connected by the mesogenitale. The mesonephric fold is again divided into the tubar, the gland and the mesenterial portions.
The mesenterial portion of the mesonephric fold is connected with the tubar portion and the mesogenitale by means of the gland portion. In female embryos the tubes and ovary become greatly developed, but the mesonephros degenerates and, in consequence, the gland portion collapses and appears to be merely the continuation of the mesenterial portion, which seems to split into the tubar portion and the mesogenitale. We speak, therefore, of a ligamenrinit ovarico-pelvicwm (the mesogenitale) and a lig amentum inf 'undibulo-pelvicum (tubar portion). Both become greatly thickened by acquiring blood-vessels, but they always remain connected by the former gland portion.
• Development of the Ligamentum Ovarii Proprium, the Ligamentum Uteri Rotundum, and the Chorda Qubernaculi.
The genital fold is also a structure that continually alters its form, growing caudally by apposition and shortening at the cranial end by degeneration. The indifferent reproductive gland does not occupy the entire length of the fold, but leaves a portion of it free at either end. One may therefore recognize three portions in the human fold: (1) the progonal portion, the upper end without the reproductive gland; (2) the gonal portion, the largest, middle portion with the reproductive gland, and (3) the epigonal portion, the lower end without the reproductive gland. The degeneration of the genital fold precedes that of the reproductive gland, and the growth of the reproductive gland that of the genital fold. As long, therefore, as growth in length and degeneration prevail in the reproductive gland, so long will gonal portions be transformed into progonal at the upper end and epigonal to gonal at the lower end. In female embryos the caudal pole finally reaches the true pelvis and lies on the posterior surface of the genital cord. The epigonal portion of the genital fold consequently unites the caudal pole of the ovary with the genital cord, but in the genital cord the formation of the primitive wall of the uterus (see p. 916) extends just so far that the point of insertion of the epigonal portion comes to lie on the surface of the uterine wall. Exactly opposite the insertion lies, as we will see shortly, the ligamentum rotundum. In the interior of the epigonal portion the mesenchyme tissue condenses to form a cord, the ligamentum ovarii proprium, the cells of which very quickly become connective tissue and smooth muscle fibres, all of which have a parallel course and stream out into the subserous musculature. From what has been said, it will be clear that the lig. ovarii proprium must arise from the caudal pole of the ovary and pass to the posterior surface of the uterus.
In male embryos the homologue of the lig. ovarii proprium, the lig. testis, is also found, but it has a different form and a different course. The difference in form depends in the first place on the greater length of the epigonal portion (the caudal pole of the testis lies higher than that of the ovary), and, in the second place, in its being constricted off from the rest of the mesonephric fold; the difference in the course depends, firstly, on the position of the caudal pole of the testis, which does not enter the true pelvis, and, secondly, on its non-union with the 'wall of the uterus; consequently the insertion of the epigonal portion, that is to say, the lig. testis, into the mesonephric fold must be at the boundary between the true and the false pelvis. That the points of insertion of the
Fig. 633. — Sagittal section of the lower end of the body of a human embryo of 40 mm. trunk length. (Embryo R. Meyer 259, from the collection of Professor R. Meyer, Berlin; slide 4, row 2, section 2.) X5.5. The section just cuts the right wall of the primitive true pelvis, and shows the foramen obturatum with the m. obturator internus and the levator ani. The testis lies just at the entrance into the true pelvis and is connected with the mesonephric fold, which is drawn out into a loop, by the lig. testis.
lig. testis and lig. ovarii proprium into the mesonephric fold or genital cord are homologous, is evident from the fact that both are opposite the insertion of the chorda gubernaculi or the round ligament (see below).
The urogenital fold in both sexes is united by the inguinal fold (p. 793) with the crista inguinaiis, situated on the posterior surface of the anterior abdominal wall (Fig. 634 a and b). This connection constitutes a bridge between the urogenital fold and the entrance into the inguinal canal, for this is marked quite early by the base of the crista inguinaiis. In the interior of the crista there is from the beginning a cord of compact mesenchyme, the chorda gubernaculi (Fig. 634 a and b), which is evident even before there is any indication of a differentiation of the abdominal musculature. It has a conical shape in transverse section, the apex being towards the crista inguinaiis and the base almost at the integument (Fig. 634). When, later, the abdominal musculature begins to develop, it must grow around the chorda gubernaculi, and thus there is necessarily formed a canal (the inguinal canal, Fig. 635), whose contents were there from the beginning. The developing muscles behave differently with reference to the chorda gubernaculi. The transversus and obliquus internus give off fibres
Fig. 634 a and b. — Two transverse sections of the urogenital fold at the level of the plica and crista inguinalis of a human embryo of 22.5 mm. greatest length. (Embryo R. Meyer 303, from the collection of Professor R. Meyer, Berlin; slide 34, row 2, section 1, and row 4, section 1.) The plica inguinalis arises from the lateral surface of the urogenital fold at the boundary between the false and the true pelvis; opposite the plica on the lateral abdominal wall is the crista inguinalis (a) . The plica and crista grow together (b) .
to its surface, which bend around and accompany it to the crista inguinalis ; by these fibres a muscular mantle is formed around the chorda, and this is known in comparative anatomy as the conus musculosus inguinalis. The obliquus externus presents at first a distinct foramen corresponding to the base of the chorda (Fig. 635), and through this foramen the chorda stands in connection with a second mesenchymatous cord (Fig. 635), which has long been known as the lig. scroti. This extends from the outer open of the inguinal canal to the integument and to the base of the genital tubercle. Later the aponeurosis of the external oblique grows around the lig. scroti, which in the mean time lias completely united with the chorda gubernaculi, and so the aponeurosis forms a sort of thin hernial sack, which becomes the fascia terica; this fascia, accordingly, is not a true fascia, but an aponeurosis. Up to this point the development is the same in both the male and the female embryo. In the male the union of the gilbernaculum with the mesonephric fold is exactly opposite the insertion of the lig. testis (Fig. 635). In the portion of the mesonephric fold between the two insertions there develops another mesenchyniatous cord, which connects both with the lig. testis and with the chorda gubernaculi, and so unites the two. On the completion of this union there is a continuous cord extending from the lower pole of the testis through the inguinal canal and terminating in the integument at the base of the genital tubercle. I shall term this cord the chorda gubernaculi in a broader sense (Fig. 636). Its individual parts are (Fig. 636) : the pars mesorchica, formed by the lig. testis, the pars mesonephridica, the mesenchymatous cord in the mesonephric fold and the inguinal fold, the pars intermuscularis, formerly the chorda gubernaculi in the narrower sense and, finally, the pars scrotalis, formerly the lig. scroti.
Fig. 635. — Portion of a transverse section of a human embryo of 26 mm. greatest length. (Embryo R. Meyer 321, from Tj*.; the collection of Professor R. Meyer, Berlin; slide 49, row 5, section 3.) X ca. 30. The section shows the left mesonephric fold: it has formed a secondary fold, the plica inguinalis. On the posterior surface of the anterior abdominal wall a crista inguinalis has formed, and with it the plica inguinalis has united. The point of union is still distinctly recognizable. Within the crista a cord of compact mesenchyme has appeared, the chorda gubernaculi in the narrower sense. It may be followed through the abdominal wall to an opening in the aponeurosis of the obliquus abdominis externus. From this opening a second cord of compact mesenchyme, the lig. scroti, extends to the integument. The anterior abdominal wall has thickened except at the point of insertion of the crista inguinalis; thus the crista comes to lie in a groove, the anlage of the saccus vaginalis.
Fig. 636.— In a transverse section of the urogenital fold, of the inguinal fold, and of the body wall of a male embryo of 26 mm. greatest length, the various parts of the chorda gubernaculi are represented diagrammatically. The chorda gubernaculi in the broader sense consists of: 1, the lig. testis (pars mesorchica), 2, a mesenchymatous cord in the mesonephric fold and its plica inguinalis (pars mesonephridica), 3, a mesenchymatous cord in the crista inguinalis and between the abdominal muscles (pars intermuscularis), and 4, the lig. scroti (pars scrotalis).
In the female embryo the affair is simpler. The lig. ovarii proprium, which corresponds to the lig. testis, and the chorda gubernaculi in the narrower sense, which in this case is termed the lig. rotundum uteri, are no longer separated by the mesonephric fold, but by the primitive uterine wall. Consequently a union of the two is imposible, but both have an insertion into the wall of the uterus, and it is evident from the development that these insertions must take place in the posterior or anterior wall of the uterus. Consequently there also results in the female embryo a union of the lig. rotundum with the lig. scroti, or if we would be precise, with the lig. labiate. The lig. rotundum extends, acordingly, from the anterior wall of the uterus through the inguinal canal to the base of the genital tubercle; it consists of a pars intermuscular is, the former chorda gubernaculi in the narrower sense, and a pars labialis, the former lig. labiale.
If one would draw homologies, the male chorda gubernaculi in the wider sense corresponds to the following parts in the female : (1) the lig. ovarii proprium, (2) a part of the wall of the uterus, (3) the lig. uteri rotundum. The lig. ovarii corresponds to the former lig. testis or the pars mesorchica of the chorda gubernaculi in the wider sense. The lig. uteri rotundum corresponds by its pars intermuscularis to the similarly named portion of the chorda gubernaculi, by its pars labialis to the pars scrotalis. Finally, one might homologize a portion of the subserous uterine musculature, which is connected on the one hand with the lig. ovarii proprium and on the other with the lig. uteri rotundum, with the pars mesonephridica of the chorda, gubernaculi.
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