GENITAL SYSTEM DEVELOPMENT

1. GENITAL SYSTEM DEVELOPMENT

2. GENETIC FACTORS : In the nucleus of every cell of his or her body, a human being has 46 chromosomes. 22 chromosome pairs (numbered from 1-22) belong to the autosomes 1 pair to the sex chromosomes or gonosomes. They are denoted as X and Y. A female has two X-chromosomes A male an X and a Y-chromosome. SEX DETERMINATION

3. In a woman, the Barr Body is one of the two X-chromosomes The Barr Body formsheterochromatin (sex chromatin). The diagnosis of the genetic gender is made on this basis. This inactivation already takes place in the blastocyststage. BARR BODY

4. The sex information is encoded on the Y-chromosome. The SRY (sex determining region Y gene) gene is localized on this chromosome TheSRY operates very early in the development as a guide or "master gene". This small gene (a single exon), which is localized on the shorter arm of the Y chromosome (Yp), Certainautosomes (9, 11, 17, and 19) are involved gonadogenesis. GENETIC FACTORS

5. The SF1 gene (chromosome 9) is expressed in the gonad bud of both sexes, The expression of this gene provides the synthesis of steroids. The WT1 (Wilms Tumour ) gene (chromosome 11) is expressed in the urogenital ridge, Expression of the WT1 gene appear the mesonephros, the supporting cells (Sertoli) and the follicular cells of the ovary. This gene is required for the morphogenesis of the urogenital system and represents a regulation gene for the transcription of the SRY. It has been found in patients with a Wilms' tumor. The DAX1 gene (chromosome 10) is also expressed in the genital ridges and prevents the activation of SF1. GENETIC FACTORS

6. The gene expression of the SOX9 gene (chromosome 17) is found in the genital ridge of both sexes and in the supporting cells (Sertoli). This gene plays a role in the activation of the AMH gene (antimüllerian hormone). The mutation with a functional failure of a singlecopy of the SOX9 gene leads to a reversal of the gender of a male individual (46,XY) . The gene coding for AMH (antimüllerian hormone; chromosome 19) is expressed in the supporting cells (Sertoli), This is responsible for the atrophy of the paramesonephricduct (Müller). GENETIC FACTORS

7. During the embryonic period (from the 6th week) the interstitial cells (Leydig) in the fetal testes secrete the testosterone hormone that is responsible for the male differentiation of the genital primordium. Roughly, from the 7th week the antimüllerian hormone (AMH) is produced by the supporting cells (Sertoli), AMH leads to an atrophy of the paramesonephric duct (Müller). In contrast, the female sexual apparatus differentiates itself spontaneously. HORMONAL FACTORS

8. LEYDIG CELL FUNCTION • The interstitial cells (Leydig) probably have their origin in the mesonephros. • Leydig cell develop outside the testicular cord in the testes. • From the 7th week they secrete increasing amounts of testosterone. • The maximum is attained in the course of the second trimester, when the final differentiation in the direction of the male phenotype takes place.

10. The GnRH of the hypothalamus (Gonadotropin Releasing Hormone) affects the adenohypophysis and stimulates the release of LH (Luteinizing Hormone), which also has an influence on the testes. The LH binds itself directly on specific receptors of the interstitial cells (Leydig) and causes the release of androgens (testosterone). This fat-soluble hormone (testosterone) gets bound to a protein (ABP = Androgen Binding Protein). Testesterone also diffuseslocally via the basal membrane outwardly. Thus, the seminiferous tubules promotes the development of the germinal cells. The specific receptor protein for the androgen (ABP) synthesis in the supporting cells (Sertoli) This receptor is coded by a gene on the X chromosome. A mutation of this gene is responsible for the insusceptibility to androgens. LEYDIG CELL FUNCTION

11. LEYDIG CELL FUNCTION • In the target organ the testosterone diffuses through the cell membrane where it binds itself to the cytoplasmatic receptor proteins • Thus, the testosterone is transported into the cell nucleus this way. • It binds to a nuclear receptor. • This hormone-receptor-complex is ultimately able to bind itself to regions of the DNA. • Then this event regulates the transcription for a series of further proteins.

12. LEYDIG CELL FUNCTION • Testesterone is either active itself (mesonephric duct [Wolff], epdidydimis, seminal vescicle, etc.) or works as a precursor hormone • Testesterone transformation through 5 a-reductase into dihydotestosterone (DHT) (outer genital organs, urogenital sinus). • In addtion to its role in embryonic development testosterone is also important for the expression of the secondary characteristics of the masculine sex. • Urinary bladder • Deferent duct  • Seminal vesicle  • Prostate • Urethra • Epididymis • Testis • Pubic hair Red: testeron effects Blue: dihydrotestosterone effects.

13. The supporting cells (Sertoli) are located within the seminiferous tubules. Their task is the creation of a hemato-testicular barrier and the nourishment of the spermatozoa. They can only proliferate in the first year of life (their equivalent in the woman are the follicular cells). Sertoli cells are Christmas-tree-shaped and sit on a basal membrane. Laterally they stand in direct contact with one another and with the germ cells. The oval cell nucleus is indented and oriented perpendicular to the basal membrane and the nucleolus is clearly visible. SERTOLI CELL FUNCTION

14. Each supporting cell (Sertoli) is bound together with the neighboring cell through "tight junctions". This divides the germ epithelium into basal and adluminal compartments. The basal compartment contains spermatogonia up to the preleptotene stage (doubled DNA before the 1st meiosis). In the adluminal compartment are the spermatocytes, spermatids and spermatozoa. The "tight-junction" as a blood-testicle-barrier keeps spermatozoa from getting into the blood circulation or the lymphatic systems. This is important because the immune system would produce antibodies against the antigens on the membrane of the monoploid spermatozoa, leading to an autoimmune-orchitis and thus to sterility. SERTOLI CELL FUNCTION

15. The function of the supporting cells (Sertoli) is controlled by the FSH pituitary hormone (follicle-stimulating hormone). Sertoli cells synthesize ca. 60 various proteins that are connected with reproduction. The most important are inhibin, androgen-binding-protein (ABP) and the antimüllerian hormone (AMH). Antimüllerian hormone (AMH) is a glycoprotein of 560 amino acids and, together with inhibin, belongs in the TGF-b family. It is responsible for the atrophy of the paramesonephric duct (Müller) in men. The associated gene is localized on chromosome 19. SERTOLI CELL FUNCTION

16. In the "Persistent Müllerian Duct Syndrome" (PMDS) the paramesonephric duct (Müller) persists in a man that otherwise shows normal internal and external genitals. The reason is either a structural anomaly or a deficiency of AMH or its receptors. With the inception of puberty the amounts of AMH decrease greatly because testosterone, which is produced increasingly, inhibits the gene expression of AMH. Inhibin is also a glycoprotein that inhibits the secretion of FSH. Inhibin is released in varying amounts in concert with testosterone. ABP is a protein with a great affinity to testosterone and dihydrotestosterone. ABP is released under the influence of FSH and testosterone. SERTOLI CELL FUNCTION

17. The development of the genital apparatus accompanies that of the urinary system. It also has its origin in the intermediate mesoderm and urogenital sinus. The primordial germ cells share in the formation of the gonads, but have an ectodermal origin. In males, the development of the testes is closely joined with that of the mesonephros. In females, on the other hand, the mesonephros plays no role at all. ANATOMIC RELATIONSHIP BETWEEN THE GENITAL AND URINARY SYSTEM • Coelomic epithelium • Local mesenchyma (in proliferation) • Gonadal cord • Primordial germ cells (PGC) • Mesenchyma • Allantois • Omphalomesenteric duct • Intestine • Dorsal mesentery • Genital ridge • Nephrogenic cord • Mesonephric duct (Wolff) • Mesonephric tubule • Aorta

18. The intermediate mesoblast is the origin of the urogenital ridge (crista urogenitalis), Urogenital ridge lies on both sides of the midline between the lateral mesoderm and the root of the dorsal mesenterium of the embryo. It consists of two main components, The nephrogeniccord, Thegenital ridge as the origin of the gonads. The genital ridge extends from the upper thorax region to the level of the cloacae. The true gonad primordium develops only the middle area. The cranial and caudal parts of the urogenital ridge form the upper and lowergonadal bands, respectively, that as embryonic structures secure the gonads cranially and caudally. ANATOMIC RELATIONSHIP BETWEEN THE GENITAL AND URINARY SYSTEM • Coelomic epithelium • Local mesenchyma (in proliferation) • Gonadal cord • Primordial germ cells (PGC) • Mesenchyma • Allantois • Omphalomesenteric duct • Intestine • Dorsal mesentery • Genital ridge • Nephrogenic cord • Mesonephric duct (Wolff) • Mesonephric tubule • Aorta

19. The gonads arise from two very different kinds of cells that originate in the embryo:· The primordial germ cells (PGC) will form the gametes (sperm cells and oocytes). These cells come from the ectoderm. GONADS: INDIFFERENT STAGE • PGC • Allantois • Cloacal membrane • Epiblast • Pharyngeal membrane • Heart anlage • Umbilical vesicle (yolk sac) • Endoderm • Mesoderm

20. The somatic cells with nourishing functions surround the primordial germ cells and form the somatic gonadal blastema. In the testes the supporting cells (Sertoli) and the interstitial cells (Leydig) are involved. In the ovary the follicle cells and the theca cellsare involved. Somatic cells have three possible sources mesonephros, local mesenchyma, superficial epithelial cells (coelomic epithelium). GONADS: INDIFFERENT STAGE • Rectum • Omphalomesenteric duct • Allantois • Nephrogenic cord (pink) • Gonadal ridge (green) • PGC • Heart anlage

21. The primordial germ cells (PGC) already appear at the time of gastrulation in the epiblast and complete an emigration out of the embryo into the wall of the yolk sac (umbilical vesicle). This event is managed by three factors; the folding of the embryo, chemotactic factors amoeboid movements During their migration, which takes place between the 4th and 6th weeks. Theymultiply through mitosis. GONADS: INDIFFERENT STAGE • Mesonephric duct (Wolff) • PGC • Peritoneal cavity • Aorta • Mesonephric tubule • Local coelomic mesenchyme • Thickened coelomic epithelium • Intestine • Mesentery

22. The nephrogeniccord extends from the heart region to the location near the cloaca. In the time between the 4th and 6th week the middle section of this gonadal ridge develops into a gonad primordium The cells of the coelomic epithelium proliferate in this area and compose thickened zone. The immigrated PGC penetrate into this thickened zone of the coelomic epithelium. Thecoelomic epitheliumbecomes multi-layered and loses for now its basal membrane. Gonadal cords arise that surround the PGC and extend into the depths. Up to the 6th week male and female gonads cannot be distinguished. The gonadal cords and the PGC can be found both in the cortical as well as in the medullar zones of the future gonads. GONADS: INDIFFERENT STAGE • Mesonephric duct (Wolff) • PGC • Peritoneal cavity • Aorta • Mesonephric tubule  • Local coelomic mesenchyme • Thickend coelomic epithelium • Intestine • Mesentery • Anlage of the paramesonephric duct (Müller)

23. TESTES DIFFERENTIATION • The testes differentiate themselves earlier than the ovaries, in the course of the 7th week (44 days). • Responsible for this is the SRY gene on the Y chromosome that induces the development of the testes through the activation of a series of further genes Mesonephric duct (Wolff) PGC Peritoneal cavity Aorta Mesonephric tubule  Gonadal cords Coelomic epithelium Intestine Mesentery Anlage of the paramesonephric  duct (Müller) Mesonephric nephron

24. DEVELOPMENT OF THE TESTES PARENCHYMA • Mesonephric duct (Wolff) • PGC • Peritoneal cavity • Aorta • Mesonephric tubule  • Gonadal cords • Coelomic epithelium • Intestine • Mesentery • Anlage of the paramesonephric  • duct (Müller) • Mesonephric nephron • The differentiation of Sertoli'sform the first step in the organogenesis of the testes. • These cells come pluripotent coelomic epithelial cells of the gonadal ridge. • In the gonadal primordium, through the influence of genetic products that are activated by the SRY, • They form intercellular membrane connections and in this way surround more and more the primordial germ cells, • Gonadal cords is growing at the same time into the medulla.

25. DEVELOPMENT OF THE TESTES PARENCHYMA • In addition, in a male embryo, cells of mesonephric origin are involved as well in forming the gonadal cords, by accumulating on the outside of the gonadal cords and forming the peritubularmyoblasts. • From the gonadal cords the testicular cords form that then differentiate to become the convoluted seminiferous tubules (500 to 1000) and straight seminiferous tubules of the mature testicles. • Until puberty the coiled testicular cords are filled. • Mesonephric duct (Wolff) • Mesonephric nephron (atrophying) • Testicular cords surround the PGC • Aorta • Paramesonephric duct (Müller) • Mesonephric tubule  • Testicular cords that grow into themedulla • Tunica albuginea

26. TESTES DIFFERENTIATION • During puberty they form lumens and are from then on called convoluted seminiferous tubules. • The germ cells on the other hand divide mitotically, but their meiosis begins only with puberty. • The deep portions of the coiled testicular cords, which are delimited by septa, are stretched and are called straight seminiferous tubules. • Mesonephric duct (Wolff) • Testicular cords, surround the PGC • Aorta • Paramesonephric duct (Müller) (atrophying) • Mesonephric tubule (later efferent ductules) • Testicular cords  • Tunica albuginea

27. TESTES DIFFERENTIATION • These last go over into the rete testis, which is a labyrinth of small passages in the tunica albuginea. • The thin wall possesses a cubic epithelium. During the 9th week, from 5-12 mesonephric tubules the efferent ductules form that bind with the rete testis in the 3rd month. • Mesonephric duct (Wolff)  • PGC surrounded by supporting cells (Sertoli) • Aorta • Paramesonephric duct (derivative) • Efferent ductules  • Straight seminiferous tubule  • Tunica albuginea • Convoluted seminiferous tubule • Rete testis (testicular network)

28. In addition, in a male embryo, cells of mesonephric origin are involved as well in forming the gonadal cords, by accumulating on the outside of the gonadal cords and forming the peritubularmyoblasts. From the gonadal cords the testicular cords form that then differentiate to become the convoluted seminiferous tubules (500 to 1000) and straight seminiferous tubules of the mature testicles. Until puberty the coiled testicular cords are filled. DEVELOPMENT OF THE TESTES PARENCHYMA Mesonephric duct (Wolff)  PGC surrounded by supporting cells (Sertoli) Aorta Paramesonephric duct (derivative) Efferent ductules  Straight seminiferous tubule  Tunica albuginea Convoluted seminiferous tubule Rete testis (testicular network)

29. The efferent ductules form the connection between the rete testis and mesonephric duct. Towards the end of the 8th week, under the influence of testosterone, the cranial part of the mesonephric duct gets to be tightly coiled and so forms the ductusepididymidiswhich, outside the epididymis, continues as the deferent duct. After the 8th week certain mesenchymal cells between the testicular cords differentiate to become interstitial cells (Leydig), which produce testosterone. The testes thus represent an endocrine gland that produces androgens. The origin of these cells is still unclear – one suspects that a steroid-producing population of cells in the ventral part of the mesonephros differentiate and form both the origin of the adrenal cortex cells and also interstitial cells (Leydig) EXCREATORY DUCT DIFFERANTIATION

30. The male genital duct system develops from the mesonephric duct, And some mesonephric tubules near the gonads: MALE GENITAL SYSTEM DEVELOPMENT

31. The mesenchyma between the testicular cords congeals and forms connective tissue septa that subdivide the testicles into lobules (ca. 250-370). In stage 22 (ca. 53 days) this mesenchyma also forms a taut connective tissue layer between the testicular cords and the coelomic epithelium as well as the future tunica albuginea. Finally, the coelomic epithelium transforms itself into a mesothelium, just like the coelomic epithelium around the other serous cavities (peritoneum, pleura, pericardium). DEVELOPMENT OF STROMA

32. The differentiation of the ovaries happens later than that of the testes, taking place during the 8th week. Since females lack the Y chromosome, they have no SRY gene, except when a translocation of the gene onto the X chromosome occurs! Histologically two regions can be distinguished in an ovary: Cortex, containing all the elements of the parenchyma Medulla, which shares the elements of the stroma with the cortex OVARIES DIFFERENTIATION

33. Development of the stroma: In an ovary the majority of the gonadal cordsstay in contact with the surface coelomic epithelium. Those gonadal cords that go into the depths out of the thickened coelomic epithelium and lose contact with it atrophy. One also suspects there are signals from the ovary, though, which actively prevent the differentiation into male gonads. So, for example, WNT-4 functions partly as an anti-testis gene in that it suppresses certain developmental steps of differentiation in the direction of the testes. OVARIES DIFFERENTIATION

34. Towards the end of the embryonic period one can distinguish the cortex with its gonadal cords and the medullarPGC in the ovary. Probably the cells of the mesonephros only reach the hilus area of the ovary and participate there in the weakly formed rete ovarii. In the course of the 4th month the gonadal cords dissolve - also in the cortex - due to blood vessels that are sprouting from the medulla and isolated cell accumulations surround the oogonia that increasingly divide synchronously (mitosis). Like the spermatogonia the oogonia form similar cell clones. The individual cells are connected with each other via cellular bridges. One can now distinguish various zones in the cortex: In the outermost zone proliferating oogonia are found; somewhat further inward one recognizes oocytes that have spontaneously entered into the prophase of the first meiosis (meiosis 1). DEVELOPMENT OF THE PARENCHYMA

35. From the 5th month a third zone becomes visible towards the medulla in which the oocytes have already completed the prophase of the first meiosis and are surrounded by a monolayer of cells that have differentiated out of the gonadal cord cells and now are now called follicle or granulosa cells. The primary oocytes that are enveloped by follicle cells are now designated primordial follicles and then remain in this stage of the first meiosis (dictyotene stage). DEVELOPMENT OF THE PARENCHYMA

36. During the early fetal period millions of primordial follicles arise through intensive mitotic divisions of the oogonia. The number of the primordial follicles at birth amount to between 300,000 and 2 million, but they decrease massively from then till puberty. At the beginning of puberty only ca. 40,000 still remain. Of these only ca. 300primary oocytes develop further between puberty and menopause into fertilizable oocytes. It is to be noted that follicles only form in the presence of the PGC. Without them, sterile gonadal cords are formed that then further degenerate and as a consequence the ovary then consists only of stroma. DEVELOPMENT OF THE PARENCHYMA

37. Phase A:A migration of the PGC into the genital ridge where they mitotically multiply through contact with the coelomic epithelium. Formation of the gonadal cords that partially degenerate (weeks 6-7). Phase B:Active proliferation phase of the PGC and differentiation into oogonia (weeks 9 – 22). The maximal number of PGC (7 million) is attained with 20 weeks. Phase C:The oogoniaenter spontaneously into meiosis and become arrested in the diplotene of the prophase of the first meiosis. One now designates them as primary oocytes (weeks 12-25). Phase D:Formation of primordial follicles (weeks 16 -29) Phase E:Progressing follicular atresiafrom the 16th week on. OVARIES DIFFERENTIATION

39. Between the 3rd month of pregnancy and its end the testes become transferred from the lumbar area (ventro-medial to the mesonephros) into the future scrotum. This transfer is due to a combination of growth processes and hormonal influences. The gubernaculum testis also plays a decisive role in this phenomenon. The gubernaculum testis arises in the course of the 7th week from the lower gubernaculum, after the mesonephros has atrophied. Cranially it has its origin at the testis and inserts in the region of the genital swelling (future scrotum). At the same time, at the inguinal canal along the lower gubernaculum, an evagination of the peritoneum arises, the vaginal process, on which the testes will slide through the inguinal canal. DESCENT TESTES

40. DESCENT TESTES

41. In that the vaginal process lengthens downwardly, it takes the muscle fibers of the oblique internal muscle and the transverse muscle with it. The muscle fascia of the transverse muscle is the innermost layer and in the scrotal region, it forms the internal spermatic fascia of the spermatic cord and the scrotum. The muscle layer of the musculuscremaster is formed from fibers of the oblique internal and transverse muscles. Externally, the external spermatic fascia is formed from the superficial aponeurosis of the oblique external abdominal muscle. DESCENT TESTES Epidermis Dermis (tunica dartos) External spermatic fascia Musculus cremaster Internal spermatic fascia Parietal lamina of the tunica vaginalis Virtual cavity between the two layers of the tunica vaginalis Visceral lamina of the tunica vaginalis Tunica albuginea Interlobular septum of the testis

42. The lower urinary system - composed of the bladder and the urethra - is formed from the endoderm of the hindgut. SEPERATING CLOACA In stage 13 the cloaca is the common end of the rectal tube and the urogenital tract. Towards the outside it is closed by the cloacal membrane. Between the 4th and 6th weeks the urorectal septum separates the cloaca into a primary urogenital sinus (ventrally) and the rectum (dorsally). LOWER URINARY SYSTEM DEVELOPMENT • Legend: • Urorectal septum • Cloacal membrane • Urogenital orifice • Anal orifice • Cloaca • Urogenital sinus • Rectum • Allantoïs • 8a. Future bladder

43. The bladder and the pelvic limb of the urethra arise from the primary urogenital sinus and the caudal portion of the urethra comes from the definitive urogenital sinus. The urorectal septum divides the cloacal membrane into two membranes: The urogenital membrane (ventrally) and the anal membrane (dorsally). These two membranes atrophy, like the bucco-pharyngeal membrane, in order to form the intestinal and urogenital openings. LOWER URINARY SYSTEM DEVELOPMENT • Legend: • Urorectal septum • Cloacal membrane • Urogenital orifice • Anal orifice • Cloaca • Urogenital sinus • Rectum • 8a. Future bladder

44. The urorectal septum is no longer regarded as an isolated cellular layer of mesoderm cells that slowly grow towards the cloacal membrane. It consists of two mesodermal structures that are fused together. An upper fold (Tourneux), located frontally, grows caudally. Near the cloacal membrane two lateral folds (Rathke) form that fuse at the median level. PERINEUM AND URORECTAL SEPTUM • Legend: • Peritoneal Cavity • Upper fold (Tourneux)(pink arrow) • Lower fold (Rathke) (blue arrow)

45. . They subdivide the cloaca and also grow in the direction of the upper fold (Tourneux) that is located frontally. A disorder in the formation of these two structures leads to recto-urethral or rectovesical fistulas. Connective tissue and the perineal musculature, which keep the pelvic organs in place, arise from the mesoderm, which surrounds the rectal tube. The central fibrous part of the perineum corresponds anatomically to the region between the anal and urogenital orifices PERINEUM AND URORECTAL SEPTUM • Legend: • PeritonealCavity • Upperfold(Tourneux)(pink arrow) • Lower fold (Rathke) (blue arrow) • Primary Urogenital Sinus Anal Canal Urogenital Orifice Anal Orifice Perineum

46. Formation of the Bladder • Where this urorectal septum approaches the proctodeal end of the cloaca, the cloacal membrane ruptures thus separating the digestive tube from the urogenital structures. • The proximal part of the allantois dilates as the urinary bladder. • As the bladder enlarges, the mesonephric duct is absorbed into the bladder wall. This allows the mesonephric and metanephric ducts to open into the urogenital sinus.

47. The bladder develops from the upper part of the urogenital sinus (UGS) and is connected with the allantois. The allantois is obliterated during the development and forms a fibrous cord, the urachus, which following birth becomes the median umbilical ligament. While the cloaca is being divided, the caudal, originally common part of the mesonephric duct (Wolffian duct) and the ureter anlage is taken up into the upper, postero-lateral wall of the urogenital sinus (future bladder). URINARY BLADDER DEVELOPMENT • Legend: • Primary urogenital sinus • Mesonephric duct (Wolffian duct) • Ureter anlage • Common drainage urinary passages • Pelvic limb of the urogenital sinus • A. Part of the genital tract • B. Part of the urinary tract

48. The rapid growth of the back wall of the urogenital sinus has the result that the common lowest part of the ureter and the mesonephric duct (Wolffian duct) are both taken up into the bladder wall. Further complicated growth processes have the result that the ureteral orifices and the orifice locations of the mesonephric duct (Wolffian duct) go through a cranio-caudal position exchange during the course of the further development. The ureteral openings appear to migrate thereby in a cranio-lateral direction and the mesonephros orifices appear to be shifted caudo-medially. The triangular zone that is thus created is termed the vesical trigonum. In males, the wolffian duct forms the future deferent duct on both sides. URINARY BLADDER DEVELOPMENT

49. Later, though, the trigonum will be completely covered by endodermal epithelial cells. The smooth musculature of the bladder develops during the 12th week from the splanchnopleural mesoderm, which coats the endoderm on the outside. The trigonum thus originates from the mesoderm while the ventral bladder wall has an endodermal origin. LOWER URINARY SYSTEM DEVELOPMENT • Legend: • Primary urogenital sinus • Mesonephric duct (Wolffian duct) • Ureter anlage • Common drainage urinary passages • Pelvic limb of the urogenital sinusA. Part of the genital tractB. Part of the urinary tract

50. The urethra forms itself from the lower part of the urogenital sinus (UGS). In a man the prostate and membranous part of the urethra arise from the pelvic part of the UGS. The spongy urethra comes from the phallic part (urethral plate). In a woman the whole urethra and part of the vagina arise from the pelvic part of the UGS The phallic part (urethral plate) forms the vestibule and the labia minora. URETHRA DEVELOPMENT