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The urogenital system develops from urogenital ridges of the intermediate MESODERM. First stage is pronephros. Next is mesonephros, in which primitive nephrons form and produce some urine (as early as 9 weeks gestation) which goes into the gut tube (cloaca) of the pre-human. The MESONEPHRIC DUCT becomes the ductus deferens. The metanephros is the final form of the kidney. It forms when the URETERIC BUD (aka metanephric diverticulum) branches out from the mesonephric duct and forms the collecting system (ureter, calyces, collecting tubules). It signals the surrounding mesenchyme to become the metanephric blastema, which blasts out the rest of the kidney including the rental tubules. Vascular cells migrate into the blastema and form glomeruli.

The urorectal septum meets the cloacal membrane to become the perineal body, separating the urinary tract from GI tract, ureter from rectum. The urogenital sinus becomes the bladder and urethra. The development of male and female genitalia was not covered.

DEVELOPMENT OF ABNORMAL KIDNEYS

If the URETERIC BUD divides before it forms the metanephric mesoderm, you get double kidneys.
If the ureteric buds are too close together they might form just one big kidney with two ureters.
If the ureteric bud doesn't develop and there isn't enough contact between bud and mesoderm, you don't get kidneys.
If the kidneys fuse and get trapped below the inferior mesenteric artery, they can't "ascend". Don't try to have babies with this.
If the kidneys just don't ascend you get a pelvic or ectopic kidney. Makes pregnancy tough.

Ascent of the kidneys is really just growth of the embryo caudal to the developing ureteric buds. The blood supply changes as the relative position of the kidneys changes.

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