Development of the Urogenital System
The developing urinary and reproductive systems tend to be viewed as a single entity because both arise mainly from a common embryological origin, the intermediate mesoderm. This differentiates to form the nephrogenic cord, a column of condensed mesodermal tissue that extends caudally from the future cervical (neck) region. The intermediate mesoderm extends towards, but does not reach, the urogenital sinus, which is initially continuous with the most caudal (and ventral) component of the embryonic hindgut, but which will later become separated from it (see below).
The initial renal anlage that develops from the most rostral part of each nephrogenic cord is termed the pronephros and is not believed to function in any mammal. It consists of a small number of pronephric (or nephrotomal) vesicles that drain into the pronephric portion of the nephric duct. Despite the later degeneration of the pronephros, the pronephric duct is retained and extends caudally to be used by the mesonephros (also termed the “Wolffian” body), and is only then termed the mesonephric portion of the nephric duct.
The mesonephros develops primitive glomeruli, 4-6 of which drain through a mesonephric tubule to the mesonephric portion of the nephric duct, and this temporary kidney is therefore believed to be functional, producing some amniotic fluid. Each mesonephros also produces about 35 more caudal tubules, which do not connect to the mesonephric duct in mice.
In both sexes, a gonadal ridge forms along most of the medial aspect of the mesonephros. Primordial germ cells, arising from the yolk sac, then invade the gonadal ridge by way of the wall of the hindgut and its dorsal mesentery. In male embryos, the primary sex cords differentiate from the surface tissue of the gonads, invade the subjacent tissue and give rise to the seminiferous cords within which the germ cells are located. The “drainage” system of each future testis (efferent ducts, epididymis, ductus (or vas) deferens, seminal vesicles and ejaculatory duct) is derived from the mesonephric portion of the nephric duct and mesonephric tubules. The prostate gland develops from the urogenital sinus. In female embryos, the primary sex cords regress, and are replaced by the ovigerous cords. Ovigerous cords represent strings of germ cells, in which later the intercellular bridges between germ cells breaks down and single oocytes become closely surrounded by a somatic epithelial monolayer of pre-granulosa cells. The oviducts, uterine horns and the upper one-third of the vagina are derived from the paramesonephric (or Müllerian) ducts which arise from the surface of each mesonephros. The more caudal part of the vagina is believed to form from the urogenital sinus.
The mesonephric duct continues to extend caudally until it makes contact with the urogenital sinus in the region that eventually corresponds with the supero-lateral part of the trigone of the future bladder. Its caudal part then produces a cranio-laterally directed diverticulum, termed the ureteric bud, which invades the most caudal part of the nephrogenic cord tissue and begins to bifurcate repeatedly therein to form the primitive collecting duct system (ureteric tree) of the metanephros (permanent kidney). The mesenchyme invaded by the bud forms excretory nephrons that connect to the collecting duct system and drain through it.