Development of the Genitalia

 

Overview

The development of the genital system is closely linked with that of the urinary system. The urogenital system develops from intermediate mesoderm. A longitudinal elevation - the urogenital ridge - forms on each side of the dorsal aorta, with the nephrogenic cord leading to the urinary system and the gonadal ridge leading to the genital system.

 

Common Gonad Development

The mesenchymal gonadal ridge is soon joined by fingerlike epithelial cords, producing the indifferent gonad with a cortex and medulla. In females, the cortex differentiates into an ovary, while in males the medulla differentiates into the testes.

 

By the 6th week, the cloacal membrane forms the primary urethral groove, bounded by primary urethral (urogenital) folds. Genital (labioscrotal) swellings occur on either side and the genital tubercule ventral to it. Sexual differentiation of the external genitalia begins to occur in the 1oth week.

 

Primordial Germ Cells

Primordial germ cells originate in the yolk sac and migrate through the gut to gonadal ridges during the 6th week.

 

Sex Determination

The presence of a Y chromosome, with the SRY gene for testis-determining factor, induces male differentiation.

 

Female Genital Development

 

Ovary

The gonadal cord cortex forms folliculogenous cords, and primordial germ cells form oogonia. During the second trimester oogonia enter meiosis I and become surrounded by follicular cells, forming primordial ovarian follicles. Meiosis I completes during ovulation, while meiosis II occurs following fertilization.

 

Paramesonephric (Mullerian) duct

Two Mullerian ducts form and fuse to form the uterus, cervix, and upper vagina, with proximal, unfused tubes becoming the uterine tubes.

in the presence of anti-Mullarian hormone/Mullerian inhibiting factor, produced by Sertoli cells, the paramesonephric ducts regress

 

External genitalia

The genital tubercle becomes the clitoris, the genital swellings become labia majora, and urethral folds become labia minora.

 

 

Male Genital Development

 

testis

Under the influence of TDF, the gonadal ridge medulla differentiate into testicular cords containing prospermatogonia and future Sertoli cells. Interstitial Leydig cells begin secreting androgens early in the fetal period, leading to masculine differentiation of internal and external genitalia.

Mesonephric tissue grow into the medulla and form rete testis, while mesonephric tubules form efferent ductules connecting the rete testis to the duct of the epididymis.

Descent of the testes usually occurs during the third trimester and is androgen-dependent.

 

Wolffian (mesonephric) Duct

Mesonephric ducts develop in the 5th week, and in the presence of androgens, the epididymis, ductus deferens, seminal vesicle, and ejaculatory duct form. The mesonephric duct degenerates in females.

 

External genitalia

In the presence of androgens, the genital tubercule becomes the penis, the urethral folds form the ventral shaft, and the genital swellings fuse to form the scrotum.

 

 

Congenital Genital (ha) Problems

 

cryptorchidism

non-descent of testes can cause tubular atrophy, infertility, and testicular cancer