Figure 1.7 (a) The primitive hindgut is enclosed within the embryonic tail fold. (b) The developing urorectal septum grows dorsally and caudally from the rostral limit of the allantoic diverticulum. (c) The fusion of the urorectal septum with the cloacal membrane divides the hindgut into the urogenital sinus and the rectum.
Figure 1.8 (a) The paired primordia of the genital tubercle lie immediately caudal to the umbilical cord. (b) Migration of tissue towards the midline from both sides separates the umbilical cord and cloacal membrane, causing fusion of the primordia to form a midline genital tubercle and establishing bilateral cloacal folds and genital swellings. (c) Fusion of the urorectal septum with the cloacal membrane separates the anterior genital region from the posterior anal region.
Figure 1.9 The indifferent human embryo possesses mesonephric and paramesonephric ducts. The terminal paramesonephric ducts fuse within the urorectal septum and reach the urogenital sinus at the sinus tubercle situated between the openings of the two mesonephric ducts.
Figure 1.10 (a) The mesonephric duct, within the urorectal septum, opens into the urogenital sinus. (b) The caudal limit of the mesonephric duct gives origin to the ureteric bud. (c) The metanephric cap forms at the growing end of the ureteric bud or duct. (d) The mesonephric duct gives origin to the ureter and forms the trigone of the bladder and the posterior wall of the urethra.
Carnegie stage 19
The mesonephric ducts terminate in the urogenital sinus on either side of the sinus tubercle at 49 days. At the rostral end of the sinus tubercle, the urogenital sinus is referred to as the vesicourethral canal, and the bladder and the whole of the female urethra arises from it. The portion of the urogenital sinus caudal to the sinus tubercle continues to be referred to as the urogenital sinus and is subdivided into pelvic and phallic portions.
Carnegie stages 20–23
The indifferent gonad gradually develops into an embryonic ovary in the 45–55 day embryo, and by day 56, the external genital primordium has developed, but is still indeterminate.
End of the female embryonic period and further development (week 8 onwards)
At the end of the embryonic period, the foetus has gonads that are recognisable as ovaries, but still has indifferent external genitalia, and both mesonephric and paramesonephric duct systems are still present. Subsequent sexual differentiation of these ducts in the female develops because of a lack of anti‐Müllerian hormone. The mesonephric ducts (Wolffian ducts) degenerate, but occasionally remnants may be left behind. A remnant of the cephalic mesonephric duct and adjacent vesicles is a constant finding associated with the ovary. A more caudal portion of the mesonephros may be encountered in the broad ligament as the paroophoron, while remnants of the terminal mesonephric duct may persist lateral to the uterus and vagina or be incorporated into the cervix. Remnants of this duct found adjacent to the lower genital tract are referred to as Gartner’s ducts.
The fallopian tubes, uterus, and cervix develop from the upper un‐fused portions of the paramesonephric ducts and the vagina from the lower fused portion (Figure 1.11a). The arrival of the caudal end of the genital canal on the urogenital sinus stimulates cellular proliferation of the sinus epithelium to form three projections (sinuvaginal bulbs) which displace the genital canal dorsally (Figure 1.11b). Failure of these bulbs to develop results in vaginal agenesis. These sinuvaginal bulbs become solid and together with the solid end of the genital canal form the vaginal plate, which is complete at 19 weeks (Figure 1.11c). The sinuvaginal bulbs later fuse, but eventually undergo apoptosis to form a lumen. Sometime between 14 weeks and 20 weeks, the vagina opens into the pelvic portion of the urogenital sinus, forming the vaginal vestibule (Figure 1.11d)
Figure 1.11 (a) The fused paramesonephric ducts form the genital canal, the solid caudal end of which abuts on the posterior wall of the urogenital sinus at the sinus tubercle. (b) Cellular proliferation of the sinus epithelium generates the sinuvaginal bulbs, which displace the genital canal dorsally. (c) Further cellular proliferation converts the sinuvaginal bulbs into solid tissue projections, which participate in the formation of the vaginal plate. (d) Extensive caudal growth of the vaginal plate brings its lower surface into the primitive vestibule.
Further feminisation of the external genitalia begins between 63 and 77 days, when the genital tubercle lengthens to form the phallus. This then bends caudally to form the glans of the clitoris. During this period, the anogenital distance remains unchanged, there is no fusion of the genital folds, and the urogenital sinus remains open. The urethral and vaginal openings separate later. The phallus becomes the clitoris, being incorporated within the fused anterior ends of the genital folds, which develop into the labia minora. The genital swellings lateral to the labia minora become the labia majora and are continuous with the future mons pubis. The labia minora develop from the genital folds and divide anteriorly into the prepuce and frenulum of the clitoris (Figure 1.12).