ORGANOGENESIS OF THE KIDNEY

Wolffian duct

In the outer part of the intermediate mesoderm, immediately under the ectoderm, in the region from the fifth cervical segment to the third thoracic segment, a series of short evaginations from each segment grows dorsally and extends caudally, fusing successively from before backward to form the pronephric duct. This continues to grow caudally until it opens into the ventral part of the cloaca; beyond the pronephros it is termed the Wolffian duct. Thus, the Wolffian duct is what remains of the pronephric duct after the atrophy of the pronephros.

Pronephros

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. A secondary glomerulus is formed ventral to each of these, and the complete group constitutes the pronephros. In humans, the pronephros is just rudimentary, and undergoes rapid atrophy and disappears.

Mesonephros

On the medial side of the Wolffian duct, from the sixth cervical to the third lumbar segments, a series of tubules, the Wolffian tubules, develops. They increase in number by outgrowths from the original tubules. They change from solid masses of cells to instead become hollowed in the center. 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 tubules collectively constitute the mesonephros.

The mesonephros persists and form the permanent kidneys in fishes 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..

Metanephros and definitive kidney

The metanephros is the definite, permanent, but yet immature kidney. It arises from two directions. On one hand, the precursor of the ureter buds from the Wolffian duct, while on the other hand, the precursor of the renal tubules develop from the metanephrogenic blastema. The ureteric bud subsequently grows into the latter mass, forming the parts of the nephron. Other changes include e.g. the translocation of the ureteric opening directly into the cloaca.

Ureteric bud

The rudiments of the permanent kidneys make their appearance about the end of the first or the beginning of the second month. Each kidney originate as an ureteric bud from the caudal end of the Wolffian duct, which, in turn, originates from intermediate mesoderm. 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. The other, more superficial, portion of the diverticulum, on the other hand, becomes the ureter.

Metanephrogenic blastema

The renal corpuscles and renal tubules, in contrast, are developed from the metanephrogenic blastema instead of from the ureteric bud. The metanephrogenic blastema is moulded over the growing end of the latter, and becomes a part of the metanephros in this way. The renal tubules of the metanephros, unlike those of the pronephros and mesonephros, do not open into the Wolffian duct. Instead, the tubules rapidly elongate to form the parts of the nephron: the proximal tubules, the loops of Henle and the distal convoluted tubules. These last join and establish communications with the collecting duct system derived from the ultimate ramifications of the ureteric bud. In the other end, the renal tubules give rise to Bowman's capsules and glomeruli.