Talk:Paper - The development of the penile urethra and the homology of cowper's gland of male spermophile (1937)

THE DEVELOPMENT OF THE PENILE URETHRA AND THE HOMOLOGY OF COWPER’S GLAND OF MALE SPERMOPHILE (CITELLUS TRIDECEMLINEATUS) WITH A NOTE ON THE PROSTATIC UTRICLE

By M. A. H. SIDDIQI. Vincent Massey Fellow, University of Toronto

‘Tue spermophile, like most other members of the family Sciuridae,} possesses a pair of bulbo-urethral glands, an intermediate bulbar gland, and an unpaired and central long penile duct that drains the three bulbar glands into the penile urethra. The bulbo-urethral glands, which are covered only by fascia, are spherical and lie on each side of the anus at the root of the tail. A duct, whose epithelium is folded in a complex manner, passes from the ventral surface of each gland to the dorsal surface of the caudal part of the bulb of the urethra. The two ducts unite in the midline with the duct of the intermediate gland to form a long and tapering penile duct which extends through the corpus spongiosum ventral to the urethra, into which it opens in the region of the ventral flexure of the penis.

The homologies of Cowper’s glands, the median bulbar gland and the penile duct are the objects of this embryological investigation.

MATERIAL AND METHODS

Embryos of 11, 18, 17, 19, 27 and 835 mm. crown-rump length were embedded in celloidin and serial sections of 20 » thickness were mounted and stained with haematoxylin and eosin. The sections were studied by means of graphic projections and reconstructions. The region of the developing urethra, and the associated glands and ducts noted above, were studied separately in each case. The ectodermal and endodermal areas were carefully defined and charted.

Text-fig. 2 is a diagrammatic representation of the results obtained and it illustrates my interpretation of the human homologies of the structures concerned.

DESCRIPTION

In the embryo of 11 mm. crown-rump length, the urorectal septum has completely separated the rectum from the urogenital sinus. The epithelium of the urogenital sinus extends cranially towards the allantois and ventrally to the genital tubercle where it is in the form of a solid, sagitally disposed doublelayered plate of cells enclosing a potential cavity which is continuous with the

1 The male reproductive tract of the Sciuridae has recently been studied in detail by Mossman, et al. (1932). Their paper includes a comprehensive review of the literature on the subject. 110 M. A. H. Siddiqi

actual cavity of the urogenital sinus. The lower urinary tract at this stage is in the form of a U loop with the bend of the U directed caudally. The cranial end of the dorsal limb of the U terminates at the body stalk, and the cranial end of the ventral limb ends at the genital tubercle. The body stalk and the genital tubercle are very close together owing to the non-existence of a subumbilical ventral abdominal wall at this stage of development. The dorsal limb of the U is the allantois, and it is destined to give rise to the bladder, while the ventral limb takes part in the formation of the penile urethra. The cavity of the urogenital sinus, which is continuous around the U-shaped bend of the loop with that of the allantois, traverses only the caudal one-third of the ventral limb. Beyond this point the ventral limb is composed of the continuation of the urogenital sinus epithelium, and in apposition with the ventral surface of this a second solid mass of cells formed as it were by an infolding of

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Embryo 13 mm. 17 mm. 19 mm. mm. 35 mm.

ll mm. c.r.

Text-fig. 1. Graphic reconstruction from a series of spermophile embryos of the posterior end of the gut and the lower urinary tract. U.G.C. urogenital mass of cells; Hct.C. ectodermal mass of cells; Bl. bladder; U.G.S.’urogenital sinus; R. rectum; P.A.G. post anal gut; End.U. endodermal urethra; Ect.7. ectodermal tube; B.U.G. bulbo-urethral gland; P.D. penile duct; B. bulbar gland; O.P.D. orifice of penile duct.

the ectoderm, derived from the urogenital sinus. The potential linear cavity it encloses is continuous with the external cloaca. The endodermal and ectodermal elements of this composite structure are very easily identified under the microscope.

In embryos of 13 and 17 mm. crown-rump length the cavity of the urogenital sinus has extended farther into the ventral limb of the U, and at this stage the ectodermal plate is also canalized to form the cavity of the ectodermal cloaca. The cavities of the urogenital sinus and of the ectodermal cloaca are disposed as two parallel tubes. The caudal part of the septum separating the two breaks down, the process extending cranially towards the genital tubercle, so that the two tubes communicate freely with each other. In the 17 mm. The Development of the Penile Urethra 111

embryo the ectodermal tube opens to the exterior and the cavity of the urogenital sinus thus obtains an indirect external opening.

In the 19 mm. embryo the caudal communication between the two tubes is obliterated by the ingrowth of lateral folds comprising both ectodermal and endodermal layers, and the caudal end of the ectodermal tube once again becomes blind. In the 27 mm. embryo the ingrowth of the lateral folds has progressed much farther cranially. At this stage the blind caudal end of the ectodermal tube is greatly dilated to form an intermediate bulbar dilatation and has separated from the surface ectoderm. Moreover a pair of lateral ducts open into it, the ducts being those of Cowper’s glands, which start developing at about the 17 mm. stage as solid outgrowths of cells from the intermediate ectodermal bulbar enlargement.

The cranial part of the ectodermal tube, which lies immediately ventral to the endodermal tube (endodermal urethra), becomes the median unpaired penile duct. The ingrowth of the lateral folds which forms the unpaired penile duct stops short caudal to the genital tubercle, so that the penile duct is provided with an opening into the urethra, as occurs in the 27 mm. embryo. The 35 mm. embryo shows that distal to the orifice of the penile duct the urethra is developed from the ectodermal tube, for the cells of the urogenital sinus do not extend beyond this level.

The development of the region can be summarized as follows: The floor of the external cloaca becomes folded cranially to form an ectodermal tube and at the same time the cavity of the urogenital sinus extends towards the genital tubercle to form an entodermal tube which lies parallel and dorsal to the ectodermal tube. The septum which separates these two tubes breaks down and the cavities of the two tubes communicate with each other. At the same time the urogenital tube acquires an indirect opening to the exterior through the ectodermal tube. :

The septum which originally separated the two tubes reforms and the two tubes once again become separated except cranially where their communication persists. Thus the ectodermal tube, whose caudal end is blind, is again cut off from the urogenital sinus. Its caudal end dilates and forms the intermediate bulbar gland, while the rest of it becomes the median unpaired penile duct. The endodermal tube derived from the urogenital sinus gives rise to the penile urethra up to the orifice of the unpaired penile duct.

DISCUSSION

My observations lead me to agree with the views of Mossman, Lawlaw and Bradley on the homologies of the accessory glands. They write:

‘So while we have for the present thought it advisable to accept the typical sciurid Cowper’s glands as homologous to those of Primates and other mammals, it may be worth while to point out some of the possibilities as to the manner of development of the bulbar gland and penile duct which is what will eventually 112 M. A. H. Siddiqi

prove or disprove our assumption. We are inclined to the belief that they are true bulbo-urethral glands and that the bulbar gland and the penile duct simply are modifications of the penile portion of their ducts. Two other possibilities immediately present themselves, however. One is that the penile duct and bulbar gland really are developmentally a urethral sinus or diverticulum, as Tulberg supposed, split off from the urethra in some manner and carrying the openings of Cowper’s ducts. A second possibility is that the whole penile duct and bulbar gland with Cowper’s duct and glands are actually ectodermal structures, the opening of which has become incorporated into the distal part of the penile urethra.”

In spite of the danger of drawing homologies between mammals as widely separated as man and spermophiles, certain speculations on the subject appear — worthy of record.

Text-fig. 2. Diagram of the lower urinary tract in man (A) and spermophile (B) to show the contribution of endoderm and ectoderm towards the formation of the penile urethra and its associated glands. The continuous line represents the endoderm and the broken line the ectoderm. Bl. bladder; P. prostate; B.U.G. bulbo-urethral gland; End. endoderm; Ect. ectoderm; Hct.U. ectodermal urethra; Hnd.U. endodermal urethra; P.D. penile duct; B.G. intermediate bulbar gland.

By referring to Keibel’s reconstruction as it appears in text-books, to Frazer’s as it is given in his manual, and to Koff’s in Contributions of Embryology, vol. xxiv, I find a very close analogy between man and the spermophile as to the limits of extension of the urogenital cells towards the genital tubercle. The process of development up to the formation of an ectodermal tube by the fusion of the genital folds and the manner in which the urogenital sinus forms a communication with this tube by the disappearance of the intervening septum are the same in the spermophile and in man. The process involved in the later separation of the ectodermal tube by lateral folds as occurs in the spermophile does not seem to occur in man. During the development of the human urethra, the structures that correspond with those which in the spermophile are derived from the ventrally situated ectodermal tube The Development of the Penile Urethra 113

(bulbar gland and the penile duct) freely communicate along their entire length with the cavity of the dorsally situated endodermal tube, which in the spermophile alone forms the penile urethra. In terms of this homological interpretation, the proximal two-thirds of the mucosa of the dorsal part of the human penile urethra is endodermal in origin, while the bulb of the urethra, Cowper’s glands, the floor of the proximal two-thirds, and the whole of the terminal third of the penile urethra are ectodermal in origin. In this connexion, it would seem significant that lacunae occur on the dorsal wall but not on the floor of the penile urethra in man. The lacuna magna probably marks the anterior limit of endodermal extension.

Prostatic utricle

Careful examination of serial sections of the prostatic urethra of an adult male spermophile does not show the presence of any remains of the Millerian ducts. Mossman, Lawlaw and Bradley who examined three such serial sections give their findings thus, “a minute prostatic utriculus similar to that of the chipmunk in position and form but with no opening was found in one animal. None at all was found in the other two series of sections”.

In each embryo the Wolffian ducts closely accompany the Miillerian ducts and open into the lumen of the urethra. The Miillerian ducts in the 17 and 27 mm. embryos are in two parts. The cranial parts diverge laterally from each other, while the caudal parts lie in close apposition to each other in the midline, sandwiched, as it were, between the Wolffian ducts on either side. Very fine lumina are present for a short distance in the cranial parts of the ducts, but the caudal parts are not canalized. The cranial parts of the Miillerian duct are completely absent in the 85 mm. embryo. The caudal parts are represented by two columns of cells lying side by side.

In each of the embryos investigated, the caudal extremities of the ducts terminate abruptly and are embedded in a mesodermal mass which demarcates them from the mucous membrane of the urogenital sinus in that situation. In each of the three stages examined the distance between the terminal ends of the Miillerian ducts and the level of entry of Wolffian ducts into the urogenital sinus is 40 p.

A comparison of the sizes of the ducts and of their histological appearance in the present series shows that the ducts begin to retrogress before the 27 mm. stage is reached and that the divergent cranial parts have completely disappeared in the 85 mm. stage and that even the caudal parts are in a markedly retrogressive state. In none of the three embryos examined did the cells of the Miillerian ducts come into contact with the cells of the urogenital sinus, nor was there any appearance of cellular proliferation analogous with the sinovaginal bulbs which are formed by the epithelium of the urogenital sinus in man (Koff, 1988). ,

Only one of the four adult specimens examined by Mossman and his coworkers possessed a minute utricle, and even in this case the utricle did not

Anatomy LXxII 8 114 M. A. H. Siddiqi

open into the urethra. Its failure to do so was conceivably due to the nonformation of sino-vaginal bulbs, and it may be concluded that the normal absence of the utricle in the spermophile is due both to the non-formation of vaginal (Frazer) or sino-vaginal (Koff) bulbs, and the complete retrogression of the Millerian ducts.

SUMMARY

Serial sections of spermophile embryos of 11, 18, 17, 19, 27 and 35 mm. crown-rump length were investigated in order to study the development of the penile urethra, the bulbo-urethral and bulbar glands and the penile duct. The conclusions arrived at are as follows:

(1) The bulbar gland, the bulbo-urethral gland and the penile duct are purely ectodermal in origin.

(2) The proximal penile part of the urethra up to the orifice of the penile duct is purely endodermal, and the distal terminal part is ectodermal in origin. ,

(3) A consideration of the homologies between the above-mentioned structures of the spermophile and the corresponding organs in man suggests that the human bulbo-urethral gland, the bulbar enlargement of the human urethra, and the floor of the proximal two-thirds of the urethra and the whole of the terminal one-third are ectodermal, while the roof of the proximal two-thirds is purely endodermal in origin.

(4) Miillerian ducts begin to develop between 18 and 17 mm. stage and start retrogressing in the male before the 27 mm. stage is reached.

(5) Absence of a utricle in the spermophile is probably due to the nonformation of vaginal (Frazer) or sino-vaginal (Koff) bulbs associated with the complete retrogression of the Miillerian ducts.

ACKNOWLEDGEMENTS

I wish to express my best thanks to Dr H. W. Mossman for sending to me the embryos and for his very useful instructions. I am also grateful to Professors J. C. B. Grant and W. H. Piersol for their hopeful interest and to Mr H. E. Lemesurier for his assistance in preparing the sections and photographs. Journal of Anatomy, Vol. LX XII, Part 1 Plate I

17 mm., level 1

27 mm., level 3 27 mm., level 2

27 mm., level 4 35 mm., level 5

SIDDIQI—TueE DEVELOPMENT OF THE PENILE URETHRA The Development of the Penile Urethra 115

REFERENCES

Baxter, J. S. (1935). Contr. Embryol. Carneg. Instn, vol. xxv, p. 15. Enotg, E. T. (1926). J. Mammal. vol. vit (2).

Frazer, J. E. (1935). J. Anat., Lond., vol. yxrx, p. 455.

(1928). J. Anat., Lond., vol. Lx, p. 9.

—— (1931). A Manual of Embryology. London: Bailliére, Tindall and Cox. Hunter, R. H. (1930). Contr. Embryol. Carneg. Instn, vol. xx, p. 91. Kerra, Sir A. (1933). Human Embryology and Morphology, 5th ed. Kuaar, J. & Krasa, F. C. (1921). Z. ges. Anat. Bd. Lx1.

Korr (1933). Contr. Embryol. Carneg. Instn, vol. XXIv, p. 61. Kro.uine, O. (1921). Z. ges. Anat. Bd. Lx1.

Mossman, Lawiaw & BraDuey (1932). Amer. J. Anat. vol. ‘LI, p. 89. ZUCKERMAN & ParkzEs (1935). J. Anat., Lond., vol. Lxrx, p. 484.

EXPLANATION OF PLATE I

Microphotographs from serial cross-sections of the spermophile embryos. U.G.C. urogenital mass of cells; Ect.C. ectodermal mass of cells; U.G.S.and Hct.T. common cavities of urogenital sinus and the ectodermal tube; Hnd.U. endodermal urethra; Ect.P.D. ectodermal penile duct; S. septum forming to separate the ectodermal tube from the urogenital sinus; Ect.B. ectodermal bulb; O.P.D. orifice of penile duct into the endodermal urethra.