Embryology of kidney

what do kidneys develop from

intermediate mesoderm

• intermediate mesoderm forms a common mesodermal ridge along post abdo wall

• 3 overlapping kidney systems are formed in a craniocaudal sequence in the intrauterine life of humans

1st overlapping kidney system formed in intrauterine

• pronephros

• formed in cervical region of embryo

• disappears immediately

2nd overlapping kidney system formed in intrauterine

appearance of mesonephric tubules and the mesonephric duct in the thoracolumbar region of embryo

• mesonephric tubules elongate to form an S-shaped loop representing the nephron

• a tuft of capillaries develop - form a glomerulus at its medial extremity

• these tubules drain into the mesonephric duct (open into the neurogenital sinus of the hind gut)

• most of the mesonephric tubules in cranial part disappear but some of the tubules and duct in caudal part persist to form a part of the genital system

3rd urinary organ/ overlapping kidney system formed in intrauterine

• metanephros (definitve kidney)

• week 5

• its excretory units develop from metanephric mesoderm

• collecting ducts develop from ureteric bud (an outgrowth from mesonephric duct close to its entrance to hindgut)

• bud penetrates metanephric tissue - molded over its distal end like a cap

what happens once the ureteric bud penetrates the metanephric tissue

• distal end of bud dilates to form pelvis of ureter and then splits into a cranial and caudal portion to form major callyces

• undergo extensive branching to 12 or more generations forming the minor callyces and the collecting tubules

what are the collecting parts of kidney from collecting tubules down to minor callyces, major calyx, pelvis of ureter and ureters are developed from

• the ureteric bud which is an outpouching from mesonephric duct

what is each of the newly formed collecting tubules covered at its distal length by

metanephric tissue

what happens under the inductive influence of the tubule cells

• the metanephric tissue undergo differentation to form parts of the nephron

• renal vesicle is formed - elongates to form different parts of coiled tube

• distal end of tube acquires a tuft of capillaries to form glomerulus

• other end of tube becomes continuous with collecting tubule

what does the excretory part (nephrons to DCT) of the kidney develop from

• metanephrose - dervied from intermediate mesoderm

describe renal agenesis

• one of the kidneys is missing (e.g. right kidney)

• right kidney not differentiated on right side

• because of undifferentiation of metanephric mesoderm (mostly due to absence of formation of ureteric bud from mesonephric tubule)

left bifid ureter and double kidney

• normal pevlic calycelial system but 2 urters proximally but 1 ureter distally

• ureteric bud has undergone incomplete division leading to different pelvic calyceial systems and each of them differentaitng the mesonephric tissue around it to form separate kidneys

ascent of kidneys

• initially develop in the pelvic region and later ascend cranially due to differential growth of body walls

• while in pelvis, metanephros receives arterial supply from pelvic branches of aorta

• as it ascends to abdominal level, vascularised from arteries that originate from aorta at continuously higher levels and the previous arteries that used to give blood supply degenerates

• therefore common to see accesory renal arteries that supplied kidney in case the earlier arteries that supplied kidney doesn’t degenerate

what can the normal asecent of the kidneys sometimes be hindered by

• what is this condition called

branches of arota

e.g. umbilical artery joins aorta in pelvic region and the fork formed by the umbilical artery and aorta is a hindrance for the ascent of the kidney and sometimes the kidneys are arrested at pelvic level

• pelvic kidney - can persist into adult life

how is a horseshoe shaped kidney formed

kidneys are pushed so close together during their passage through the arterial fork that the lower poles fuse forming a horseshoe shaped kidney

• usually at the lower lumbar vertebrae since its ascent is prevented by the root of the inferior mesenteric artery