3 primitive kidneys throughout development, all of which arise from the intermediate mesoderm:
- Pronephros
- Mesonephros
- Metanephros
As the embryo undergoes folding, the intermediate mesoderm separates from the paraxial mesoderm, migrating towards the intraembryonic coelom (future peritoneum).
At the same time, there is progressive craniocaudal development of the bilateral longitudinal mesodermal masses, called nephrogenic cords – each cord bulges into the posterior wall of the coelomic cavity – producing the urogenital ridge
The useless pronephros arises from the nephrogenic cord in the cervical region and is seen between the late 3rd week and degenerates completely by the 5th week.
The somewhat useful mesonephros is also transient but does have some excretory function while the metanephros develops.
- The mesonephric (Wolffian) ducts start as a pair of solid longitudinal tissue condensations about day 24, developing parallel to the nephrogenic cords dorsolaterally.
- Its blind distal ends grow caudally to the cloaca, fusing with it about day 28.
- This fusion creates a lumen, with canalisation then progressing cranially – transforming the solid tissue condensations to definitive mesonephric ducts capable of excretion
- Mesonephric tubules form, up to 40 pairs, but only 30 visible at any time as the cranially located tubules start to degenerate around the 5th week
- These differentiate into primitive nephrons which drain to mesonephric duct
- By the 16th week the mesonephros has almost completely disappeared except for a few elements which do develop into mature parts of reproductive tract
The remnants of the mesonephric (Wolffian) duct are:
- In men:
- Epididymis, vas deferens, seminal vesicles, ejaculatory ducts and efferent ductules of testis
- In women:
- Epoophoron and paraoophoron, Gartner’s duct
The metanephros is the definitive kidney, and forms in the sacral region
- Forms as a pair of new structures, the ureteric buds, sprout from the distal portion of the mesonephric duct and contact the metanephric mesenchyme (or metanephric blastema – a condensation of the intermediate mesoderm) at about the 28th day / end of the 4th
- The ureteric bud penetrates the metanephric mesenchyme and divides dichotomously to start forming the collecting system
- The ureteric bud and the metanephric mesenchyme have reciprocal inducive effects on each other
- The nephron is thought to derive from the metanephric mesenchyme, and the collecting system from collecting duct through to ureter is thought to derive from ureteric bud
- Nephrogenesis is generally completed around 32 – 34 weeks, although ongoing renal maturation does take place postnatally.
There is tentative renal function at the 9th – 10th week, and urine is produced from the 10th week.
The eventual pelvicalyceal system and its pattern and renal lobules arise from the dichotomous branching of the ureteric bud.
- The first 9 branches are formed by 15 weeks
- Ureteric bud branching complete by 20 – 22 weeks
- Collecting duct development occurs by extension of peripheral branches thereafter
- At 22 – 24 weeks (think viability) the cortical and medullary domains are established
Renal ascent occurs between the 6th and 9th week, with the kidneys ascending to a lumbar site just below the adrenals.
- Precise mechanism unknown, but likely not only true ascent but also excessive differential growth in the caudal part
- As the kidneys migrate, they are vascularised by a succession of transient aortic sprouts arising at progressively higher levels
- These arteries don’t lengthen to follow the ascending kidneys, but instead degenerate and replaced by new higher arteries, with the final pair becoming the renal arteries
- Persisting lower arteries become accessory lower pole arteries
- Failure of ascent results in a pelvic kidney, or a defect in ascent results in an ectopic kidney
- The inferior poles can also fuse, forming a horseshoe kidney anteriorly crossing the aorta
- The fused lower pole causes arrest of the ascent by the IMA
- Rarely the kidney can fuse to the contralateral kidney and ascend across the midline – crossed fused ectopia
- The kidney rotates 90 degrees as it ascends so its convex border faces laterally not dorsally – failure of the rotation gives a malrotated kidney