Development of the urogenital system.

The urogenital system can be divided functionally into the urinary (excretory) system and the genital (reproductive) system. Embryologically and anatomically, however, they are closely connected. The urogenital system develops from the intermediate mesoderm which extends along the dorsal body wall of the embryo. The intermediate mesoderm later loses its connection with the somites and forms a longitudinal elevation of the mesoderm called the urogenital ridge. Part of the urogenital ridge giving rise to the urinary tract is known as the nephrogenic cord. Medial part of the urogenital ridge forms the gonadal (genital) ridge. The mesothelial lining of the peritoneal cavity and the endoderm of the urogenital sinus also take part in the development of the urogenital system.

Development of the urinary system.

During development the kidney arises from intermediate mesoderm situated in the posterior abdominal wall. Primitive nephrons develop from the cords of mesenchymal cells and acquire a lumen, and the blind dilated end of the nephron (the future Bowman’s capsule) is invaginated by a tuft of capillaries. The ureteric bud, a diverticulum arising from the mesonephric or wolffian duct, grows into the mass of developing kidney, or metanephros. The mesonephric duct later becomes associated with the genital system, and by differential growth, the ureteric bud originating from it is taken up into the developing urinary bladder. When the growing end of the ureteric bud reaches the metanephros, it undergoes a series of divisions, each terminal branch becoming continuous with a developing nephron. The fine branches of the ureteric bud thus become the various orders of collecting or excretory ducts, and its main branches become the minor and major colyces of the renal pelvis; the ureteric bud itself becomes the ureter. The first nephrons to develop are those in the deeper layers of the cortex. At birth the kidney is irregular in outline (fetal lobulation), and the outer cortical region consists of undifferentiated mesenchyme from which additional nephrons will develop for some months or even years after birth. When development is complete, the kidney outline becomes smooth.

Developmental abnormalities of the kidney and excretory passages are not incommon. Early division of a ureteric bud before it reaches the metanephros can result in conditions such as bifid or double ureter, and double kidney. Failure of individual nephrons to connect with terminal divisions of the ureteric bud results in cysts, usually multiple (multicystic kidney).

Development of the male and female reproductive system.

The primordia of the gonads arise as thickenings of mesodermal epithelium, the genital ridges, on the medial surface of the mesonephros. The epithelial cells of the ridge proliferate and form a band of tissue composed of two types of cells. Most cells are small, cuboidal elements and scattered between them are large, spheroidal cells, the primitive sex cells. The epithelial cells penetrate the underlying mesenchyme and form sex cords.

Sexual differentiation.In the male human embryo, the testis becomes recognizable at about seven weeks. The sex cords become more distinct and elongate to form the seminiferous tubules. Their peripheral ends anastomose and unite with a number of mesonephric tubules to form the rete testis. Prior to the onset of puberty component cells of the seminiferous tubules differentiate into spermatogonia and Sertoli cells.

Differentiation of the ovary does not commence until about the eighth week of gestation. The sex cords formed during the indifferent stage gradually disappear. The covering (germinal) epithelium continues to proliferate and produces the primitive cortex of the ovary. This mass of cortical cells is subdivided by strands of mesenchyme into clusters containing oogonia, probably derived from the primitive sex cells. Proliferation of cortical tissue continues into the latter half of fetal life.

Genital ducts.Genital ducts develop in close connection with the embryonic urinary system. They are laid down initially as two paired longitudinal ducts, the ducts of Wolff and of Mьller, the latter from mesoderm lining the coelomic cavity. In the male the wolffian duct is transformed into ductus epididymidis and ductus deferens. Connection of the ductus epididymidis with the rete testis is established by a number of mesonephric tubules which become the ductuli efferentes. The mьllerian duct involutes, leaving only small rudiments. In the female, the wolffian ducts regress and the mьllerian ducts transform into the female genital ducts. Caudally the two mьllerian ducts fuse to form a single tube which opens into the urogenital sinus (cloaca). The paired upper portions from the fallopian tubes and the unpaired terminal portion become the uterus and vagina.

In man, as in most mammals, the XY and XX sex chromosome complements determine male and female sex, respectively. In the former, the genetic determinant on the Y chromosome is responsible for differentiation of the indifferent gonad into a testis, which otherwise develops as an ovary. The differentiating testis produces substances that suppress mьllerian duct development and promote wolffian duct differentiation. In the absence of a testis, mьllerian ducts differentiate and wolffian ducts fail to undergo differentiation.

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