This kidney system is derived from the intermediate mesoderm form-
ing the upper thoracic to upper lumbar part of the nephrogenic cord. It begins to develop by end of the 4th week. The two (ie, right and left) mesonephroi grow rapidly during the 5th and 6th weeks and form a pair of elongated swellings located on either side of the midline. The mesonephric kidneys consist of mesonephric tubules and mesonephric ducts.
The mesonephric tubules appear in a craniocaudal succession and a total of 40 pairs are produced, so that several form in each segment. These tubules appear as small vesicles, which become elongated and form an S-shaped loop. The medial end of each tubule forms a double-layered, cup-shaped sac, called a Bowman's capsule, which becomes invaginated by a tuft of capillaries called glomerulus. The glomeruli are produced on branches of arteries sprouting from the dorsal aorta. The glomerulus and Bowman's capsule are collectively known as a renal corpuscle. Laterally each mesonephric tubule enters the collecting duct of the mesonephros, which is called mesonephric duct.
Each mesonephric duct (older name Wolffian duct) first appears as a solid rod of condensed mesodermal cells within the nephrogenic cord in the thoracic region, dorsolateral to the developing mesonephric tubules. The rod canalizes to form a duct and its caudal end grows to reach the lateral wall of the cloaca, which it perforates.
The mesonephric kidneys are functional from the 6th to the 10th
week. They produce dilute urine, which is conveyed by the mesonephric ducts to the cloaca to be passed into the am-niotic cavity.
After 10 weeks the meso-nephroi cease to function and then regress. By the end of 12th week most of the mesonephric tubules and glo-meruli disappear. In the male some of the caudal mesonephric tubules and the mesonephric duct persist and form important elements of the male genital duct system. However, in the female the mesonephric tubules and mesonephric duct normally disappear completely.
The metanephros or definitive kidney develops from two sources: (1) the excretory units of the metanephros develop from the intermediate meso-derm of the sacral region, ie, caudal-most part of the nephrogenic cord. This mesoderm is called metanephric blastema, (2) the collecting ducts of the metanephros develop from the ureteric bud, which arises as a diver-ticulum from the mesonephric duct. The metanephric kidneys, ie, permanent kidneys, begin to develop early in the 5th week and start to function in the 9th week of development.
During the 5th week the right and left mesonephric ducts terminate in the urogenital sinus (which is a subdivision of the cloaca). A small distance away from its termination, each mesonephric duct gives rise to a dorsal outgrowth called ureteric bud. This bud
grows in a posterosuperior direction and soon its distal expanded end comes in contact with the sacral part of the nephrogenic cord. The caudalmost part the nephrogenic cord now surrounds the distal end of the ureteric bud as a cap of mesodermal tissue, which is called metanephric blastema. Under the inductive influence of the ureteric bud, the metanephric blastema gives rise to the excretory units (nephrons) of the permanent kidney, whereas the uteric bud itself gives rise to collecting tubules, renal calyces, renal pelvis and ureter.
During the 5th and 6th weeks the ureteric bud divides in a dichoto-mous manner; each branch of the ureteric bud shows an expanded end called ampulla. As the ureteric bud branches each new ampulla acquires a cap-like aggregate of the metanephric blastema tissue, giving^ the metanephros a lobulated appearance.
Initially the ureteric bud divides into two branches, which enlarge to form the major calyces. The branches of the second, third and fourth generations enlarge and coalesce to form the minor calyces. The subsequent eight generations of the ureteric dichotomy elongate considerably and give rise to the collecting tubules, which converge on the minor calyces and form the renal pyramids.
The blind end of each arched collecting tubule is covered by a cap of metanephric blastema tissue. Under the inductive influence of the collecting
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tubule, cells of this tissue cap give rise to small vesicles called renal vesicles (also called metanephric vesicles). Each renal vesicle elongates to form a metanephric tubule, which gradually assumes an S-shaped appearance. The proximal end' of each metanephric tubule becomes invaginated by capillaries, which form a tuft called glomerulus. The proximal end of the tubule itself forms a double-layered cup around the glomerulus. This cup is called Bowman's capsule; this capsule and the glomerulus are collectively known as a renal corpuscle. The distal end of the metanephric tubule becomes connected to an arched collecting tubule, so that the lumina of the two tubules become continuous with each other. Thus, a passageway is established from the renal corpuscle to the collecting system. Continuous lengthening of the metanephric tubule between its proximal and distal ends results in the formation of the proximal convoluted tubule, loop of Henle and distal convoluted tubule. The complete excretory unit of the kidney consisting of the Bowman's capsule, proximal and distal convoluted tubules and loop of Henle is called a nephron.
As is obvious from the abovegiven description, the nephrons are derived from the mesoderm of the metanephric blastema, whereas the collecting tubules are derived from the ureteric bud. This is a classical example of epithelial-mesenchymal interaction. The epithelium of the ureteric bud induces the metanephric blastema
to differentiate into nephrons. On the other hand, branching of the ureteric bud is dependent upon induction by the mesenchyme of metanephric blastema. It has been found that the WT1 (ie, wild-type 1) gene, encoded on chromosome 11p13, is expressed during em-bryogenesis in the developing kidneys.
Nephrons continue to differentiate throughout the fetal life. In a full term fetus about one million nephrons are present in each kidney. No nephrons are formed after birth. The glomerular filtration and, hence, production of urine starts in the 10th week of development. The urine passes down the collecting system into the bladder, from where it is passed into the amniotic cavity through the urethra.
The external lobulated appearance of the kidneys becomes less marked toward the end of the fetal period, but the lobes are still discernible in the kidneys of a1 newborn infant. The lobulation gradually disappears during infancy and early childhood. The increase in kidney size after birth results from the enlargement of the tubular portions of the nephrons, mainly the proximal convoluted tubules and loops of Henle. The interstitial tissue of the kidney also increases in amount with the growth of the organ.