Chapter 16: Urogenital System

The text details the temporal sequence of three kidney systems: the rudimentary pronephros, the briefly functional mesonephros, and the definitive metanephros, or permanent kidney, which begins formation in the fifth week. A critical focus is placed on the molecular regulation of kidney development, describing the reciprocal inductive interactions between the ureteric bud (which forms the collecting system including the ureter, renal pelvis, and calyces) and the metanephric mesoderm (which forms the excretory units or nephrons). Key genetic factors driving this process include WT1, which regulates GDNF and HGF to stimulate branching via RET and MET receptors, as well as PAX2 and WNT4, which facilitate the epithelialization of tubules. The chapter explains the ascent of the kidneys from the pelvis to the abdomen and the division of the cloaca by the urorectal septum to form the urogenital sinus, which differentiates into the urinary bladder and urethra. The narrative then shifts to the genital system, where sex determination is dictated by the SRY gene on the Y chromosome, triggering the differentiation of the indifferent gonads into testes through the development of medullary cords and Leydig cells that secrete testosterone. In the absence of SRY (XX genotype), WNT4 signaling guides the formation of ovaries, characterized by the degeneration of medullary cords and the proliferation of cortical cords into follicular cells. The text elaborates on the hormonal control of genital duct development, where Müllerian inhibiting substance (MIS) causes the regression of paramesonephric ducts in males, while testosterone stimulates the mesonephric (Wolffian) ducts to form the epididymis and vas deferens. Conversely, in females, the absence of testosterone and MIS allows the paramesonephric (Müllerian) ducts to develop into the uterine tubes, uterus, and upper vagina. The differentiation of external genitalia is described, noting how androgens drive the formation of the penis and scrotum in males, whereas estrogens promote the development of the clitoris and labia in females. Finally, the chapter covers the descent of the testes through the inguinal canal and addresses significant clinical correlates, including Wilms tumor, renal agenesis (Potter sequence), polycystic kidney disease, hypospadias, and disorders of sex development (DSD) such as congenital adrenal hyperplasia and androgen insensitivity syndrome.