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Kidney development
Last reviewed: 04.07.2025
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The kidney develops from the middle germ layer (segmental legs - nephrotomes) in the form of three successively replacing each other paired rudiments: the pronephron, the primary kidney and the definitive kidney.
The pronephron (anterior or head kidney) is laid down in the human embryo in the 3rd week of embryonic development from the nephrotomes of the lower cervical and upper thoracic segments (mesoderm in the area of transition of somites into lateral plates) and consists of 5-8 tubules. The pronephron tubules have a short development period (they exist for 40-50 hours), and then are completely reduced. The excretory duct of the pronephron is preserved and becomes the duct for the next generation of the kidney - the primary kidney.
The primary kidney (mesonephros); (middle kidney, trunk kidney, Wolffian body) begins to develop in the human embryo at the end of the 3rd week from the nephrotomes of the thoracic and lumbar segments and consists of 25-30 segmental convoluted tubules. The blindly starting end of each tubule expands and forms a capsule (double-walled goblet), into which the vascular glomerulus is invaginated. As a result, the renal corpuscle is formed. The other end of the tubule opens into the excretory duct of the pronephricle, which becomes the duct of the primary (middle) kidney (ductus mesonephricus). The primary kidney develops in the area of the posterior wall of the body cavity, being part of a longitudinal elevation - the urogenital fold (plica urogenitalis).
The primary kidney is the first secretory organ that functions in the human embryo during the 1st and 2nd months of embryonic development. At the end of the 2nd month, the primary kidney tubules are partially reduced and cease to function. From the remaining tubules of the primary kidney and the duct of the primary kidney, the epididymis and vas deferens are formed in the male, and the ovarian appendages in the female. The primary kidney is replaced by the third generation of urine-forming organs - the definitive kidney.
The definitive kidney (metanephros; permanent kidney, pelvic kidney) arises in the human embryo in the 2nd month of embryonic development caudal to the primary kidney (in the pelvic region) from two sources: from the metanephrogenic tissue and the proximal end of the ureteric outgrowth of the duct of the primary kidney. From the fusion of these two rudiments and the tubular system developing in them, the cortex and medulla of the kidney are formed, with the metanephrogenic tissue initially concentrated near the ureteric outgrowth growing into it. As the ureteric outgrowth grows and branches, its proximal end expands, turning into the rudiment of the ureter, renal pelvis, renal calyces and collecting renal (urinary) tubules. The collecting tubules with their branches become overgrown with metanephrogenic tissue, due to which the renal tubules (tubules of the nephron) are formed. Beginning with the 3rd month of embryonic development, the definitive kidney replaces the primary kidney. The development of the definitive kidney ends only after birth. During the development of the definitive kidney, there is a kind of ascent of the kidney into the future lumbar region due to the uneven growth of the body segments. The ureter is formed from the ureteric outgrowth of the duct of the primary kidney.
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