Kidney development

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In humans, the metanephros (adult kidney) begins as an aggregate of mesenchymal cells that are detectable by the fifth gestational week as two small areas in the intermediate mesoderm close to the pelvic aorta. It is approximately at this time that the nephric duct produces a finger-like projection called the ureteric bud that has, or will soon invade, the aggregate of cells known as the metanephric mesenchyme.

The ureteric bud of the nephric duct is stimulated by genetic signals emanating from the metanephric mesenchyme and the ureteric bud brings with it new genetic signals that will help the kidney form. The ureteric bud will grow into, and branch several times within, the metanephric mesenchyme, like a plant root penetrating and growing within ground soil.

Eventually, the ureteric bud will form the collecting ducts of the kidney and the ureters of the excretory system. The primitive nephric duct (pronephros) will degenerate to some degree but portions of it will go on to form the correct gender specific organs (gonads).

Once the ureteric bud has invaded the metanephric mesenchyme it will stimulate certain cells within the metanephric mesenchyme to condense around the tips of the branches of the ureteric bud and these cellular condensations will eventually form nephronic units (vascularized glomeruli).

In humans, all of the branches of the ureteric bud and the nephronic units have been formed by 32 to 36 weeks of gestation. However, these structures are not yet mature, and will continue to mature after birth. Once matured, humans have been estimated to possess approximately one million nephronic units (approximately 500,000 per kidney) or more.

After inducing the metanephric mesenchyme the lower portions of the nephric duct will migrate caudally (downward) and connect with the bladder, thereby forming the ureters. The ureters will carry urine from the kidneys to the bladder for excretion from the fetus into the amniotic sac. As the fetus develops, the torso elongates and the kidneys rotate and migrate upwards within the abdomen which causes the length of the ureters to increase.