Reproductive System Development
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Direct answer
The reproductive system passes through an indifferent stage: gonads develop in week five to six from mesodermal ridges covered by coelomic epithelium, primordial germ cells migrating in from the yolk sac, and two paired duct systems — the mesonephric (Wolffian) and paramesonephric (Mullerian) ducts — coexist until week seven. In the male, the SRY gene drives Sertoli cells to secrete anti-Mullerian hormone and Leydig cells to make testosterone, preserving the Wolffian ducts and, through dihydrotestosterone, masculinising the external genitalia. In the female, with no SRY, the gonad becomes an ovary, Wolffian ducts regress and the Mullerian ducts form the uterine tubes, uterus and upper vagina.
What you must remember
- Gonadal differentiation: the Y chromosome SRY gene produces testis-determining factor; Sertoli cells then secrete anti-Mullerian hormone (Mullerian inhibiting substance) and Leydig cells testosterone, from about week eight; without SRY, cortex-bearing ovaries develop by default, with meiotic entry delayed until puberty.
- Descent: testes descend from the lumbar region through the inguinal canal in the processus vaginalis, guided by the gubernaculum, reaching the scrotum by about week 32 to 36; failure gives cryptorchidism; in the female the gubernaculum becomes the ovarian ligament and round ligament.
- Wolffian duct derivatives (testosterone dependent): epididymis, vas deferens, seminal vesicle and ejaculatory duct.
- Mullerian duct derivatives: uterine tubes, uterus, cervix and upper vagina (sinovaginal bulbs from the urogenital sinus form the lower vagina, with the vaginal plate recanalised).
- External genitalia: the genital tubercle becomes the penis or clitoris; urogenital folds form the ventral shaft and raphe or the labia minora; labioscrotal swellings form the scrotum or labia majora; scrotal fusion and prostate growth need dihydrotestosterone from 5-alpha reductase.
- Disorders: androgen insensitivity (testicular feminisation) — XY with defective androgen receptors, female external genitalia with testes and absent uterus; 5-alpha reductase deficiency — ambiguous genitalia at birth, virilising at puberty; congenital adrenal hyperplasia — virilised XX female with salt loss; Turner syndrome (45,X) with streak gonads.
- Cloacal division: the urorectal septum splits the cloaca into urogenital sinus and anorectal canal; failure gives persistent cloaca or fistulae.
Common confusion
The two hormones of the testis act on different targets and are endlessly crossed: anti-Mullerian hormone destroys the female ducts, testosterone builds the male ducts, and only dihydrotestosterone virilises the external genitalia and prostate — hence a patient with 5-alpha reductase deficiency has epididymis and vas but female-looking external genitalia at birth. The lower vagina is urogenital sinus in origin, not Mullerian, which is why uterus and upper vagina anomalies track together.
Exam-focused takeaway
Theory questions ask for the indifferent stage structures and their two possible fates, the hormones controlling each step, derivatives of the Mullerian and Wolffian ducts, the mechanism of testicular descent, and the embryological explanation of androgen insensitivity, 5-alpha reductase deficiency and congenital adrenal hyperplasia. In the viva, be ready to explain hypospadias, cryptorchidism and bicornuate uterus from partial Mullerian fusion.
Frequently asked questions
What does the SRY gene do?
It encodes testis-determining factor, which commits the indifferent gonad to become a testis whose Sertoli and Leydig cells then drive all further male differentiation through anti-Mullerian hormone and testosterone.
What are the derivatives of the Mullerian duct?
The uterine tubes, uterus, cervix and upper one-third of the vagina; non-fusion of the ducts gives a bicornuate or septate uterus.
Which hormone masculinises the external genitalia?
Dihydrotestosterone, made from testosterone by 5-alpha reductase in the target tissues; it forms the penis and scrotum and later the prostate.
How do the testes descend?
Gubernacular shortening and abdominal pressure carry them from the lumbar region through the inguinal canal to the scrotum by weeks 32 to 36, preceded by the processus vaginalis; arrest on this path is cryptorchidism.