53. Embryology - Development of the Kidney, Bladder, & Suprarenal Gland

Mesonephric Kidney

• The mesonephric kidney forms as excretory tubules from the mesonephric duct
• The tubules elongate into a S-shaped loop
• A glomerulus forms at the end of the tubule from a tuft of capillaries
• The glomerulus is surrounded by a Bowman's capsule which forms from the excretory tubule
• The glomerulus and Bowman's capsule form a renal corpuscle
• The mesonephric kidney begins to degenerate by week 5, starting at the rostral end and moving caudally
• The mesonephric duct remains to be used for development of the metanephric kidney, genital ducts, and urinary bladder structures
• The mesonephric kidney has temporary function

Metanephric Kidney

• The metanephric kidney forms from an interaction of the collecting ducts and metanephric mesoderm
• Metanephric mesoderm condenses around collecting ducts and becomes the metanephric cap
• The metanephric cap develops under inductive influences of the collecting tubules
• The metanephric cap becomes a renal vesicle (see Figure 5)
• Each renal vesicle forms an S-shaped tubule
• A glomerulus develops from capillaries at the distal end of the tubule
• The proximal end of the tubule connects to the collecting duct
• The tubule develops into functional parts of the nephron from proximal to distal: distal convoluted tubule, descending and ascending limbs of Henle's loop, proximal convoluted tubule and ends at Bowman's capsule
• The glomerulus plus the rest of the tubule structure makes up the nephron
• The distal convoluted tubule connects to the collecting tubule
• Urine production begins around week 10 of gestation
• Nephrons continue to develop until birth, resulting in over 1 million nephrons per kidney
• Newborn kidneys appear lobulated
• By the 3rd month, the kidney moves to the pelvis and eventually to the abdomen

Molecular Regulation of Metanephric Kidney Development

• Development of the kidneys is regulated by reciprocal signaling between ureteric bud epithelium and the metanephric mesoderm
• WT1 is released from the metanephric mesoderm and makes it receptive to signaling by the ureteric bud
• WT1 causes the metanephric mesoderm to release GDNF and HGF
• GDNF and HGF cause the collecting tubule to release FGF2
• FGF2 prevents apoptosis, stimulates cell proliferation of the metanephric mesoderm and stimulates more WT1 release
• WNT9B and WNT6 are released from the ureteric bud
• The metanephric mesoderm upregulates PAX2 and WNT4 as a result of WNT signaling
• The metanephric mesoderm condenses

Urinary Bladder Development

• The allantois is a connection between the yolk sac and the developing fetus
• The allantois shrinks, forms a ligament and attaches to the apex of the bladder
• The proximal portion of the allantois forms the primitive urogenital sinus
• The urogenital sinus develops into the urinary bladder
• The neck of the bladder forms from the narrowed portion of the urogenital sinus
• The neck of the bladder forms the pelvic part of the urogenital sinus
• In males, the neck of the bladder develops into the prostatic and membranous urethra
• In females, the neck of the bladder only forms the membranous urethra
• The penile urethra in males forms from the terminal portion of the urogenital sinus

Suprarenal Gland Development

• Development begins in the 6th week
• The cortex is derived from mesothelial cells associated with the dorsal mesentery
• The adrenal cortex is formed by the proliferation of mesothelial cell
• The first cells formed are acidophil cells which form the fetal cortex
• The definitive cortex is formed by the second group of proliferating cells
• The medulla is derived from neural crest cells
• Neural crest cells migrate into the adrenal cortex to form the medulla
• The neural crest cells become chromaffin cells in the medulla