Development of the urinary system

The permanent organs of the adult are preceded by a set of structures which are purely embryonic, and which with the exception of the ducts disappear almost entirely before birth.

These embryonic structures are on either side; the pronephros, the mesonephros and the metanephros of the kidney, and the Wolffian and Müllerian ducts of the sex organ.

The original evaginations form a series of transverse tubules each of which communicates by means of a funnel-shaped ciliated opening with the abdominal cavity, and in the course of each duct a glomerulus also is developed.

One end grows toward and finally opens into the Wolffian duct, the other dilates and is invaginated by a tuft of capillary bloodvessels to form a glomerulus.

The mesonephros persists and form the permanent kidneys in fish and amphibians, but in reptiles, birds, and mammals, it atrophies and for the most part disappears rapidly as the permanent kidney (metanephros) develops beginning during the sixth or seventh week, so that by the beginning of the fifth month only the ducts and a few of the tubules of the mesonephros remain.

In the female, on the other hand, the Wolffian bodies and ducts atrophy, leaving behind only remnants in the adult, involving e.g. the development of the suspensory ligament of the ovary.

The orifice of the invagination remains open, and undergoes enlargement and modification to form the abdominal ostium of the fallopian tube.

This fusion of the Müllerian ducts begins in the third month, and the septum formed by their fused medial walls disappears from below upward.

The ostium of the fallopian tube remains from the anterior extremity of the original tubular invagination from the abdominal cavity.

A ring-like outgrowth of this epithelium occurs at the lower end of the uterus and marks the future vaginal fornix.

The ureteric bud starts close to where the Wolffian duct opens into the cloaca, and grows dorsalward and rostralward along the posterior abdominal wall, where its blind extremity expands and subsequently divides into several buds, which form the rudiments of the renal pelvis and renal calyces; by continued growth and subdivision it gives rise to the collecting duct system of the kidney.

These last join and establish communications with the collecting duct system derived from the ultimate ramifications of the ureteric bud.

The remainder of the vesico-urethral portion forms the body of the bladder and part of the prostatic urethra; its apex is prolonged to the umbilicus as a narrow canal, the urachus, which later is obliterated and becomes the median umbilical ligament of the adult.

Until about the ninth week of gestational age,[3] the sex organs of males and females look the same, and follow a common development.