10.5.11+-+Embryology+of+the+External+Genitalia


 * 1. Describe how the "indifferent genitalia" are formed, and then how gender- specific morphogenesis occurs. Name the androgen responsible for formation of the male-specific genitalia. **

Primordial germ cells from the yolk sac migrate to the genital groove, where they and the intermediate mesoderm differentiate to form the 4 cell types of the indifferent genitalia. Mesothelium on the surface of the intermediate mesoderm remains mesothelial as the cortex of the genitalia. Mesothelium within the groove forms the primary sex cords. Remaining mesoderm forms the stroma. In males the germ cells are engulfed by persistent primary sex cords maintained by the SRY gene which leads to the formation of androgens (Dihydrotestosterone) and Mullerian Inhibiting Substance. In females the primordial germ cells are wrapped by secondary sex cords, no SRY gene and none of the other stuff either. Other factors on the X chromosome contribute to the formation of female-specific genitalia.


 * 2. Define the following terms related to "Indifferent Genitalia." **

Glans: the yellow tip; derived from ectoderm; will become either glans of penis or clitoris Lateral tubercle: edge of teal blue; derived from anterior abdominal wall; will become either body of penis or clitoris Urogenital groove: groove in center of urogenital folds; derived from UG sinus; will either disappear between raphe of penis or remain open between labia minora Genital (labioscrotal) swelling: Will form on lateral sides of lateral tubercles; derived from anterior abdominal walls; will become either scrotum or labia majora Urogenital fold: Red paired structures; derived from UG sinus; will become either raphe of penis or labia minora Urorectal septum: Space where urogenital folds are permanently fused in both sexes; will form between UG triangle and anus; will become perineal body

**3. Describe how the following female structures are formed during subsequent morphogenesis:**
Glans clitoris: already covered Labia majora: already covered Sinovaginal bulb: A transitional structure. Formed from the intersection of the UG sinus and the paramesonephric tubes. It develops into the vaginal plate which in turn develops into the inferior portion of the vagina. Hymen: The hymen is formed from the same tissue as the sinovaginal bulb. As the vaginal plate moves superiorly, the remaining cells proliferate and form a partial covering over the vagina Labia minora: already covered Uterine canal: A transitional structure. Formed from the paramesonephric tubules after they meet the UG sinus. The tubules fuse and the septum that divides them degrades. The single canal that forms develops into the uterus, cervix, fornix, and the superior part of the vagina


 * 4. Explain how the following female pathologies can occur: **

Atresia of the cervix: If in the formation of the uterine canal, the uterine septum does not degrade fully inferiorly. Atresia of the vagina: If in the transition from the sinovaginal bulb or uterine canal into vagina, the septum does not degrade, atresia occurs. Uterus arcuatus: If in the formation of the uterine canal, the uterine septum does not degrade fully superiorly. Uterus didelphys with double vagina: If the septum of the uterus does not degrade, but instead as the sinovaginal bulb develops into the inferior portion of the vagina, it forms two openings.


 * 5. Describe how the following male structures are formed during subsequent morphogenesis: **

Glans penis: already covered Raphé of the penis: already covered Shaft of the penis: Develops by an extension of the lateral tubercle


 * 6. Explain why the male urethral ends up at the tip of the penis, whereas the female urethra ends up in the vestibule. Describe how hypospadia can occur. **

The male urethra ends at the tip of the penis due to an extension of the urogenital sinus into the glans penis. Simultaneously, apoptosis occurs in the ectodermal glans penis, this forms the epithelial cord, which will develop into the navicular fossa. Also, as the UG sinus extends through the glans penis, it closes ventrally. Apoptosis must connect the urethra to the surface of the glans penis before the sinus closes, otherwise pressure will build up and closure errors will occur. Hypospadia is a closure error, specifically, the UG sinus does not fully close, leaving openings on its ventral surface. The female urethra ends up in the vestibule because it stays inferior and there is no expansion of the ectoderm of the glans penis and anterior extension of the UG sinus.