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• weeks 4-8
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Development of the Urinary System

 

• overview
• kidney development
• collecting system development
• congenital urinary system problems

 

Overview

The development of the urinary system is closely linked with that of the genital system. The urogenital system develops from intermediate mesoderm. A longitudinal elevation - the urogenital ridge - forms on each side of the dorsal aorta, with the nephrogenic cord leading to the urinary system and the gonadal ridge leading to the genital system.

 

 

Kidney Development

Three sets of kidneys develop in human embryos:

• pronephric kidneys appear early in the 4th week and form ducts that run to the cloaca
• mesonephric kidneys use pronepnric ducts to develop mesonephric ducts, which become efferent ductules of the testes
• metanephric kidneys develop from from a metanephric mass of intermediate mesoderm, induced by the ureteric bud (metanephric diverticulum), an offshoot from the mesonephric duct. These permanent kidneys begin functioning in the 9th week.

 

The ureteric bud/metanephric diverticulum is the beginning of the ureter, renal pelvis, calices, and collecting tubules.

 

Straight collecting tubules undergo repeated branching to form successive generations of tubules.

The end of each new collecting tubule induces clusters of mesenchymal cells to form small metanephric vesicles, which then elongate to form renal tubules and fuse with the collecting duct. The proximal ends are invaginated by glomeruli.

 

Each functional unit is therefore composed of two mesenchymal tissue types - the collecting duct from metanephric diverticulum and the nephron from metanephric mesoderm. Mesenchymal-to-epithelial transition is all over the place in the kidney.

 

Fetal kidneys are divided into lobes, which usually disappear as nephrons increase and grow. At term, each kidney contains 800,000 - 1,000,000 nephrons.

 

Kidney Ascent

Metanephric kidneys begin close to each other in the pelvis. As the embryo grows, they come to lie in the abdomen and move into the abdomen, primarily due to the caudal end of the embryo growing away from the kidneys. During 'ascent' ,the kidneys' blood supply changes, with caudal branches undergoing involution. When the kidneys come into contact with the adrenal glands their ascent stops, and renal arteries become permanent.

 

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Collecting System Development

The metanephric diverticulum, or ureteric bud, is the beginning of the ureter, renal pelvis, calices, and collecting tubules.

At the end of the 4th week, embryo folding produces the cloaca, or expanded hindgut. In the 5th week, the allantoic diverticulum and mesonephric ducts join the cloaca, while the urorectal septum (extraembryonic mesoderm) begins to divide the cloaca into the urogenital sinus and anorectum.

 

 

Bladder Formation

The urogenital sinus is then considered divided into the vesicourethral canal and the definitive urogenital sinus. The bladder is formed primarily from the vesicourethral canal, which also contributes to the urethra. The definitive urogenital sinus (the phallic part) forms portions of the genitalia.

 

The proximal mesonephric duct becomes incorporated into the dorsal wall of the urogenital sinus, forming the trigone. Mesonephric and metanephric ducts enter separately.

 

The urethra

The urethra is derived from endoderm of the urigenital sinus

 

 

 

 

 

 

Division of cloaca

cloaca becomes separated to form urogenital sinus and anorectal canal (check these)

 

 

 

 

Congenital Urinary System Problems

Renal agenesis

• unilateral is 1:1000; other kidney usually hypertrophies and compensates
• bliateral agenesis is associated with oligohydramnios due to lack of urine production. babies with no kidneys rarely survive longer than a few months.

Renal hypolasia

- unilateral is 1:500 adults

both can be asymptomatic

 

 

Potter Syndrome - bilateral renal agenesis

 

Renal ectopia

• asymptomatic, but can cause problems during pregnancy with compression of an abdominal kidney

 

Horseshoe kidney

• kidneys fuse and become 'caught' by the inferior mesenteric artery
• often asymptomatic, but can have increased pyelonephritis (as is lower) and kidney stones

 

cystic renal dysplasia

• problem with differentiation; kidney contains primitive ducts and nonrenal tissues such as cartilage, fat, hematopoietic tissue, and cysts

polycystic kidney disease

• autosomal dominant disease - occurs later in life, with an incidence of 1:700; cysts form from all segments of nephron
• autosomal recessive disease occurs in infancy and has high mortality, incidence of 1:5,000; cysts form in collecting ducts

cystic disease of renal medulla

• presence of multiple medullary cysts, cortical tubular atrophy, interstitial fibrosis. Can result in renal failure.

 

double, bifid, ectopic ureters

• double = double
• bifid = branched ureteric bud leads to partial ureteric duplication
• ectopic = ureter opens in area other than trigone; ie in urethra or vagina, prostate or seminal vesicle

 

vesicoureteral reflux

• abnormal flow of urine back into ureters; can be caused by bladder/ureter valve problem, or blocked ureter
• can cause pyelonephritis, direct damage to kidney
• common in children with UTI

 

urachal anomalies

• the urachus develops from the allantois, an extension of the urogenital sinus, and connects the bladder and umbilicus but degenerates at birth
• lack of degeneration can lead to infection, urine leakage, or cyst formation

extrophy of bladder

• malformation of bladder, which is turned inside out, flattened, and exposed to the outside

epispadias

• rare penis problem in which urethra opens on the dorsal side