As the kidney develop in young embryo, it almost seems as if they are unable to “make up their mind” how to go about it. Three different sets of kidneys develop from the urogenital ridges, paired elevations of the intermediate mesoderm that give rise to both the urinary organs and the reproductive organs. Only the last set persist to become adult kidneys.
During the fourth week of the development, the first tubule system, the pronephros (“pre kidney”), forms and then quickly degenerates as a second, lower set appears. Although the pronephros never function and gone by the sixth week, the pronephrict duct that connects it to the cloaca persists and used by the later-developing kidneys. (The cloaca is the terminal part of the gut that opens to the body exterior)
As the second renal system, the mesonephros (“middle kidney”), claims the pronephric duct, it comes to be called the mesonephrict duct. The mesonephric kidneys degenerate (with remnants incorporated into the male reproductive system) once the third set, the metanephros (“after kidneys”), makes it appearance.
The metanephros starts to develop at about five weeks as hollow ureteric bud that push superiorly from the mesonephric duct into the urogenital ridge, inducing the mesoderm there to form nephrons. The distal ends of the ureteric buds form the renal pelvis, calyces and collecting ducts, and their unexpanded proximal part, now called ureteric ducts, become the ureters.
Because the kidney develop in the pelvis and then ascend to their final position, they receive their blood supply from succesively final position. They receive their blood supply from the succesively higher sources. Although the lower blood vessels usually degenerate, they sometimes persist so that multiple renal artery are common. The metanephrict kidneys are secreting urine by the third month of fetal life and most of the amniotic fluid that surrounds a developing fetus is fetal urine. Nonetheles, the fetal kidneys, do not work nearly as hard as they will after birth because exchange through the placenta allows the mother “urinary system to clear most of the undesirable substance from the fetal blood.
As the metanephros is developing, the cloaca sub divides to form the future rectum and anal canal and the urogenital sinus, into which the urinary and genital ducts empty. The urinary bladder and the urethra then develop from the urogenital sinus.