Paper - The development of the vagina in the rabbit (1933)
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The Development of the Vagina in the Rabbit
By James S. Baxter, M.Sc., M.B., B.Cu.
Senior Demonstrator in Anatomy, The Queen’s University, Belfast
The development of the vagina is a matter on which observers have come to different conclusions, most especially perhaps as regards the structures from which the organ is formed; and it is now generally considered that among the Mammals the developmental processes, so far as the nature of the structures which take part in them is concerned, show a considerable variation. Bloomfield and Fraser (1) reviewed the literature of the subject, and divided the descriptions of the origin of the vagina into three main classes: (a) those which describe the vagina to be a structure totally derived from the Mullerian ducts; (b) those which consider the vagina to be mainly Mullerian in its origin, but possessing an additional component derived from the Wolffian ducts at its lower end; and (c) those which make the vagina a derivative of the urogenital sinus to a greater or lesser extent. Each of these three modes of development has been considered, at one time or another, to be descriptive of the process which occurs in the human subject.
In the Rodentia, as is well known, the vaginal orifice occupies one of two situations. In the one position, as is exemplified in the rabbit, the vagina opens into the urogenital sinus some distance from the exterior; in the other position, as occurs in the rat, the vagina opens into the vulvar cleft on the external surface of the body. The difference between these forms, the one form being as it were complete and the other incomplete, suggested the possibility that these two forms of rodent vagina might have a different developmental history; and that if this difference were known, it would provide a basis, not only on which to compare the rodent vaginae, but also to establish comparisons with the vaginae of other forms.
The development of the vagina in the rabbit was described by Langenbacker (2), according to whom it is derived entirely from the Mullerian ducts. Neither the Wolffian ducts nor the urogenital sinus make any contribution to its formation. The development of the second form of vagina has been studied by Mijsberg(8) who worked on the rat. He concluded that in the rat the upper part of the vagina is derived from the Miillerian ducts; that the part next below this is formed by the fusion of the lower ends of the Wolffian ducts with the Miillerian ducts on each side and the subsequent union of these two masses with each other across the middle line; while the lowest part of the vagina is a portion of the dorsal wall of the urogenital sinus which has been separatell off from the remainder. The chief difference between these two descriptions is that in the rabbit the Miillerian ducts are said to open directly into the urogenital sinus while in the rat they open into the sinus through the Wolffian ducts and do not of themselves acquire a sinus connection; the absence of the Wolffian element in the rabbit is a striking omission.
Carleton (4) has shown that the epithelium of the vagina of the rabbit differs in the upper and lower parts of the organ. The upper two-thirds is lined with a columnar epithelium, of which some of the cells are ciliated and some are not, while the lower one-third has a stratified squamous epithelial lining. It seemed to me that these two regions in the rabbit vagina, especially after I had observed their relations in a developmental series, might have different derivations in spite of the previous work of Langenbacker. Consequently, in an investigation of the development of the female genital tract in the rabbit I paid particular attention to the formative processes of the vagina.
Materials and Methods
In this investigation I have studied female rabbit embryos and foetuses from 18 days of gestation (24 mm. c.R. length) until birth. These were available in serial transverse sections, either of the entire embryo or of the abdomen and pelvis. The vagina was also studied in its post-natal development. As a routine Bouin’s fluid was employed for fixation. Where decalcification was necessary 2 per cent. nitric acid in 70 per cent. alcohol was used. The sections were stained either with Delafield’s haematoxylin followed by 1 per cent. aqueous eosin or with Ehrlich’s haematoxylin and Biebrich scarlet.
At the stage of 24 mm. c.r. length the Millerian ducts have entered the genital cord but have not yet reached the urogenital sinus. No fusion has occurred. between the Miillerian ducts. The Wolffian ducts open on each side into the urogenital sinus lateral to a long low ridge which runs down the dorsal wall of the sinus.
The Mullerian ducts reach the urogenital sinus about the 80-mm. stage. Fusion now occurs between the Miillerian ducts in the caudal one-half of the genital cord resulting in the formation of a simple tube lined with a single layer of columnar epithelial cells (text-fig. 1, 35-mm. stage). The upper limit of this tube is the future cervico-vaginal junction. At its lower end this tube or vaginal anlage is in contact with the epithelium of the dorsal wall of the sinus, there being, however, no communication between its lumen and that of the sinus. On each side, too, at its lower end the vaginal anlage is in continuity with the epithelium of the Wolffian duct just above the opening of this duct into the urogenital sinus.
Fig. 1. A diagram showing three stages in the formation of the rabbit vagina based upon graphic reconstructions of the genital cord at the same magnification. V. vagina derived from the Miillerian ducts; W.D. Wolffian duct; O. opening of the Wolffian duct into the urogenital sinus; W.B. Wolffian bladder.
The openings of the Wolffian ducts into the urogenital sinus become more and more constricted in later stages, and finally are completely closed off. As this occurs, the lower ends of the ducts become dilated until definite hollow bulbs are formed on them. These retain a solid epithelial continuity with the urogenital sinus below. In their further expansion the Wolffian bulbs extend medially and intervene between the lower end of the vaginal anlage and its connection with the sinus. They eventually communicate with each other across the middle line and in this manner form a barrier between the vaginal anlage and the sinus (text-fig. 1, 48-mm. stage). While these changes are occurring, the extreme lower end of the vaginal anlage has become solid; and it is in continuity with the Wolffian bulbs on each side. On careful examination of this part I am convinced that both Wolffian and Miillerian cells are present in the common knot.
After this time the Wolffian ducts degenerate except in that region where they are related to the vagina (text-fig. 1, 66-mm. stage). This persisting portion of each duct is found to play an extremely interesting part in the formation of the lower end of the vagina. Its réle is well illustrated in text-fig. 1, 66-mm. stage, and in Plate I, figs. 1-4. At this time the Wolffian ducts are first recognised, as the sections are followed in a caudal direction, in the loose layer of tissue internal to the future circular muscle layer of the vagina. They are quite separate at this point from the epithelium of the vaginal anlage, or from what, as I shall show would be better termed the Miillerian vagina (Plate I, fig. 1). About three-quarters of the way down the vagina the Wolffian ducts pass obliquely caudo-medially and fuse with the epithelium of the lateral wall of the vagina (Plate I, fig. 2). Then the Wolffian ducts are found to break through the lateral wall of the vagina and to proliferate cells into its lumen. These cells are polygonal in form with pale cytoplasm and quite lightly staining nuclei. They thus present a marked contrast to the deeply staining columnar Miillerian epithelium which is seen lying external to them (Plate I, fig. 3). When the Miillerian cells are traced downwards they are found completely to disappear. At this region the Wolffian cells are arranged in the form of two hollow bulbs first placed side by side (Plate I, fig. 4), and then communicating with each other below (text-fig. 1, 66-mm. stage). The epithelial walls of these bulbs are fused below with the epithelium of the urogenital sinus.
After this time, the Wolffian bulbs quickly become solid (75-mm. stage) by proliferation of the cells which form their walls (Plate I, fig. 5). The Wolffian ducts, proximal to the bulbs, are no longer to be recognised as such, except for a small strand of cells which fuses with the vaginal wall at the junction of the solid and hollow portions of the vagina. At birth the lower one-quarter of the vagina is represented by a solid epithelial cord derived from the Wolffian ducts. This epithelial cord is horseshoe-shaped in transverse section and somewhat compressed antero-posteriorly; in this way there is indicated its bilateral origin. Canalisation of this epithelial cord commences very shortly after birth (Plate I, fig. 6) and this process is completed at the age of five weeks, by which time the animal is 23 cm. in length from vertex to rump. The epithelial cord gives rise by its canalisation to that part of the adult rabbit vagina which is lined with stratified squamous epithelium. This portion is, therefore, Wolffian in origin. The remainder (the upper threequarters of the adult vagina) is derived from the Miillerian ducts and retains its primitive columnar epithelium. .
In the new-born rabbit it was observed that the vaginal mucous membrane projects inwards from the lateral wall of the vagina in the form of a crescentic shelf on each side. This shelf lies just at the junction of the hollow and solid portions of the vagina and, therefore, just above the point where formerly the Wolffian ducts were connected with the vaginal epithelial wall. When the adult virgin rabbit vagina was examined to determine the presence or absence of these folds the condition shown in text-fig. 2 was found. The upper threequarters of the vagina is beset with vertical ridges arranged parallel to each other. The lower one-quarter shows a number of small folds arranged irregularly and interlacing everywhere with each other. At the junction of these two parts there is, on each lateral wall, a very distinct crescentic fold projecting transversely into the lumen!. These folds have been described by Owen (5) and although he made no statement regarding their nature, it seems to me possible to regard:them as homologous with the hymen of higher forms.
Fig. 2. Drawing of the “hymeneal” folds in the vagina of an adult virgin rabbit, x 8. H. hymeneal fold; M. Miillerian vagina; W. Wolffian vagina.
1 Microscopic examination of these folds shows, that while the muscle and the submucosa of the vaginal wall present distinct differences above and below the fold, the epithelium cannot be said to show a definite alteration in character. This is possibly due to the stage in the oestrous cycle at which this animal was examined.
The developmental processes which I have found to occur in the formation of the vagina of the rabbit place it in the second class of Bloomfield and Fraser. My preparations do not support the view of Langenbacker that the rabbit vagina is derived only from the Miillerian ducts. On the contrary, the Wolffian contribution is not inconsiderable, amounting to the lower one-fourth of the adult organ. The formation of Wolffian bladders in the rabbit is a very definite phenomenon, and the manner in which they grow medialwards, fuse with each other below the Miillerian vagina and so form a barrier of Wolffian tissue between the Miillerian vagina and the sinus, as well as the actual invasion of the lower part of the Miillerian vagina by Wolffian cells, points clearly. to the extensive growth processes which occur at the lower ends of the Wolffian ducts. It should be noted that this growth of the Wolffian bladders does not produce an invagination of the dorsal wall of the urogenital sinus. Increase in length of the lower part of the rabbit vagina seems to be due only to interstitial growth of the Wolffian bladders.
There has been no indication in my preparations that the urogenital sinus makes any contribution to the lower end of the vagina in the rabbit in the manner in which it forms the lowest part of that organ in the rat. It would appear, then, that there are two main modes of formation of the vagina in the Rodentia. In all rodents the upper part of the vagina is Miillerian in origin; the next portion is formed from the Wolffian ducts. Then in those rodents in which the vaginal orifice is on the outer surface of the body, the urogenital sinus contributes to the formation of the lowest part of all.
In the mode of origin of the vagina there would seem to be certain similarities in the formation of the single vagina of the rodents and the lateral vaginae of the marsupials. According to Buchanan and Fraser (6) each lateral vagina of marsupials is Millerian in the upper two-thirds; the lower one-third is in some forms Wolffian alone, in others it is formed from the united Wolffian and Miillerian ducts, and yet in others it is derived from an outgrowth of the urogenital sinus. Now these facts would seem to indicate that in the rodent vagina there are retained the same sources of origin as the lateral vagina of the marsupials. They would also support the view that the single rodent vagina represents the fusion (in its lower part at least) of the paired lateral vaginae of the Marsupialia.
The “‘hymeneal” folds which I have figured in the vagina of the rabbit have a situation which requires further comment. They are found just above the point where the Wolffian ducts formerly fused with the lateral epithelial wall of the vagina and so they bear the same relation to the Wolffian ducts as does the hymen in the human subject (cp. Bloomfield and Fraser (1)). If it The Development of the Vagina in the Rabbit 561
be held that in the human subject the vagina is derived only from the Millerian ducts, it is then evident that there is a structure contributing to the formation of the rabbit vagina which is not represented in that organ in the human subject. Further, that this loss of the Wolffian component in the human vagina is due to the peculiar invagination of the dorsal wall of the urogenital sinus by the lower end of the vagina that occurs in the human female. The Wolffian ducts are in this manner excluded from the vagina, and, if they persisted, would open on the distal surface of the hymen.
- The development of the vagina has been investigated in the rabbit (Lepus cuniculus). ,
- The Miillerian ducts reach the urogenital sinus at the 30-mm. stage and quickly fuse together in the lower one-half of the genital cord. This results in the formation of a simple epithelial tube which is the anlage of the upper part of the vagina.
- The Wolffian ducts lose their openings into the urogenital sinus (about the 40-mm. stage) and become dilated at their lower ends in the form of two little bladders ; and these grow medially between the lower end of the Miillerian vaginal anlage and the urogenital sinus until they meet in the middle line and fuse with each other. A communication is, in this manner, formed between the bladders across the middle line.
- The Wolffian ducts degenerate except in that portion which is related to the vagina. At the 66-mm. stage the Wolffian ducts are found to fuse with the lateral wall of the Miillerian vagina at the junction of the upper threequarters with the lower one-quarter of the genital cord. Then the ducts break through the wall of the Miillerian vagina and proliferate cells into its lumen. Lower down, the Wolffian bulbs grow and become solid by proliferation of the cells forming their walls.
- There is formed, in this manner, a solid epithelial cord which is canalised after birth and so gives rise to the lower one-quarter of the adult vagina. This portion of the vagina is Wolffian in origin and in the adult is that part lined with stratified squamous epithelium. The upper Miillerian part of the vagina retains its primitive columnar lining.
- Just above the opening of the Wolffian duct into the vagina there is formed on each side a crescentic fold projecting into the lumen. This fold persists when the Wolffian duct has completely vanished. If this fold is regarded as homologous with the hymen it is then suggested that the rodent vagina possesses a component that is no longer present in the human subject; further, that this Wolffian component of the rodent vagina has been exteriorised in the human owing to the invagination of the dorsal wall of the urogenital sinus by the lower end of the developing vagina. 562 James S. Baxter
- The close similarity between the components of the single vagina of the rodents and the lateral vaginae of the marsupials is indicated.
I should like to thank Prof. Walmsley for his interest and criticisms during this investigation. He suggested the problem to me in the first place and he has aided me greatly during its course by his kind advice. I am indebted to Dr R. H. Hunter for the drawing of text-fig. 2.
(1) BLoomrtexp, A. and Fraser, J. E. (1928). J. Anat. vol. txu, p. 9.
(2) LancenBackeEr, L, (1882). Arch. fiir Mikr. Anat. Bd. xx, S. 92.
(3) Mrsspera, W. A. (1925). Zeit. fir Anat. und Entwicklungsgeschichte, Bd. Lxxvu, S. 650.
(4) Carueton, H. M. (1931). Proc. Roy. Soc. B. vol. cvm, p. 1.
(5) Owen, R. (1868). Comparative Anatomy and Physiology of Vertebrates, vol. 111, p. 687.
(6) Bucuanay, G. and Fraser, E. A. (1918). J. Anat. vol. Lin, p. 35. .
Explanation of Plate I
Fig. 1. Transverse section at the junction of the middle and caudal thirds of the genital cord of a 66 mm. female rabbit embryo, x 120. 1, Miillerian vagina; 2, Wolffian duct.
Fig. 2. Transverse section of the same embryo 90 microns caudal to the preceding, x 120. The Wolffian ducts are shown fusing with the lateral walls of the Miillerian vagina. 1, Miillerian vagina; 2, Wolffian duct.
Fig. 3. Transverse section of the same embryo 45 microns further caudal, x 120. Cells derived from the Wolffian ducts are seen proliferating into the lumen of the Miillerian vagina. 1, Miillerian vagina; 2, Wolffian duct; 3, Wolffian cells filling up the lumen of the Miillerian vagina.
Fig. 4. A section 45 microns caudal to that shown in fig. 3, and at the same magnification. The Miillerian vagina as such has disappeared and in its place are seen the two Wolffian bulbs. A few Miillerian cells are to, be seen on the dorsal wall of the vagina. 1, Wolffian bulb; 2, Miillerian epithelium.
Fig. 5. Transverse section through the lower part of the genital cord of a 75 mm. female rabbit embryo, x 120. Proliferation of the Wolffian cells has given rise to a solid epithelial vaginal cord.
Fig. 6. Sagittal section through the pelvis of an 11 cm. rabbit, x 27-5, This shows the two component parts of the vagina. The canalisation of the lower solid cord has just commenced. 1, Miillerian vagina; 2, epithelial cord derived from the Wolffian ducts now showing the commencement of canalisation; 3, urethra.
The uniformity of the colour of the sections after staining required the insertion of a boundary line around the epithelium; this is not intended to represent the basement membrane. Journal of Anatomy, Vol. LXVITI, Part 4 Plate I
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