Urinary system

•Development of the Urinary System

•Urinary and genital systems are 

closely associated

•Both develop from 

intermediate mesoderm

–7th- 28th somite level(3-d week)

–Nephrogenic mass (cord)

•Dorsal side of coelom each cord 

produces a bulge into the coelom called

 the urogenital ridge

•Urinogenital Ridge

–Form the urinary and genital 

structures

–Nephrogenic tissue from 7-14th

somite breaks up into segments 

called nephrotomes

•Intermediate Mesoderm

•Cervical region

–Loses contact with the somite

•Forms nephrotomes which acquire a lumen and open medially into the intra-embryonic coelom

•Caudal growth unite and form longitudinal duct

•Branches off dorsal aorta form glomeruli (ext & int)

•Thoracic, lumbar, sacral regions

–Loses contract with coelomic cavity

•Ext glomeruli fail to develop

•Segmentation disappears (nephrogenic cord)

–2 or more excretory tubules per prior segment

•Urinary Tubules

•Associated with a vascular tuft = glomerulus

–Open tubules = external glomerulus

•One end opens into the coelom

•Other end opens into the collecting duct

–Closed tubules = internal glomerulus

•Open only into the collecting ducts

•Formation of 3 kidney systems

•Pronephros (simplest & most primitive)

–7-10 solid or tubular arranged cell groups in the cervical region (head kidney)

–Gone by the end of the 4th week

•Mesonephros (intermediate-more advanced)

–Appear during regression of pronephros

–10-26th somite level

•Metanephros (permanent kidney)

–Begins to develop early in 5th week, functions by the 11th week

•Mesonephros

•Tubules develop from nephrogenic cord (NC)

–Opens into the excretory/mesonephric duct

–Gone by week 10 in females, in males some tubules persist & become vas deferens

•Approximately 38 pairs of closed tubules

–S shaped bend

–Surrounds internal glomerulus

•Mesonephric duct develops laterally from NC & extends from 8th somite to urinogenital sinus

•Metanephros

•Nephrons/tubules develop from 

nephrogenic mass 

(26th-28th somite level)

–Located lateral to mesonephric duct

–Internal dense layer which forms 

tubules/nephrons

–Outer loose layer forms connective tissue 

capsule

•Duct system derived from ureteric bud

–Ureter, renal pelvis, calyces, collecting ducts

–Ureteric bud elongates and makes 

contact with nephrogenic mass which 

surrounds bud like a cap

•Tubules are closed (internal 

glomerulus)

•Migrate from pelvis to abdomen as 

fetus grows

–Blood supply from aorta changes as 

ascent occurs

•Becomes functional in second ½ of 

pregnancy

•Cloaca

•Caudal end of the hindgut (dilated)

•In 3 week old embryo the hindgut ends blindly at the cloacal membrane

•Blind end = cloaca

•Allantois and mesonephric ducts open into cloaca

•Cloaca is latin for sewer, a system of pipes used to transport human waste

•Urinary Bladder

•During 4th to 7th week cloaca 

subdivided

–Posterior portion = anorectal canal

–Anterior portion = primitive urogenital inus

•Bladder is formed from primitive 

urogenital sinus

–Bladder is upper and largest part of 

urogenital sinus

•Initially bladder is continuous with the 

allantois

–Allantois lumen obilterated & urachus

formed connecting apex of bladder with 

umbilicus

–

in adult urachus = median umbilical 

ligament

•Ureter is outgrowth of mesonephric duct

–Terminal ends of mesonephric ducts 

become part of bladder wall

–Ureter obtains separate entrance into 

bladder with time

•Production of urine by fetus

•Fetal urine mixes with amniotic 

fluid

•Amniotic fluid enters fetal 

intestinal tract where it is 

absorbed into bloodstream

•From the bloodstream to the 

which transfers metabolic waste 

to the mother

Development of the reproductive system

•Makes its appearance during 5th & 6th week

–Indifferent stage-sex cannot be determined

•Gonads (testes & ovaries) develop from

–Coelomic epithelium

–Inner mesenchyme tissue

–Primordial germ cells

•Thickening of ventromedial surface of urogenital ridge forming genital ridge

•Genital ridge

•Covered by coelomic epithelium

–Primary sex cords

•Grow into underlying mesenchyme

•Inner mass is composed of mesenchyme

•Outer layer called cortex

•Inner layer called medulla

–Males- medulla differentiates, cortex regresses

–Females-cortex develops, medulla regresses

•Primordial Germ Cells (PGC)

•Differentiate in the neck of the yolk sac

–Early in the 4th week

•Migrate to genital ridge

–Amoeboid movement

–By end of 6th week the PGC become incorporated into the primary sex cords

–migration of primordial germ cells

•Development of Genital Ducts

•Indifferent stage

–Both male and female genital ducts present

•Male develop from mesonephric/wolffian ducts

•Female develop from paramesonephric/mullerian duct

–Undifferentiated gonad

•Males:Mesonephric ducts form epididymis, ductus deferens, ejaculatory duct

–Cranial mesonephric tubules Þ efferent ducts

•Open into epididymis

– Process begins about the 3rd month

•Development of Genital Ducts

•Females: Paramesonephric duct/Mullerian duct develops on each side of the body

–Longitudinal invagination of coelomic epithelium on the lateral surface of mesonephros

–Ducts open into coelom

–Runs along side of mesonephric duct

–Fuse at caudal end

•Y shaped uterovaginal complex Þ uterus & vagina

–uterovaginal complex

•

•Development of testes

•Primary sex cords of testes 

containing the primordial germ 

cells = testes cords

–Well defined cords within the 

medulla

–Contain two types of cells

•Epithelial cells Þ Sertoli cells

•Primordial germ cells Þ 

spermatoblasts

–development of testes

•Testes cords remain solid until 

puberty

–Canalize to form seminiferous 

tubules (ST), tubuli recti, rete testis

•ST seperated from each other by 

mesenchyme that gives rise to 

interstitial cells (Cells of Leydig)

–

•Development of the Ovaries

•Primary sex cords are not well defined

–Extend into the medulla but later dissappear

•PGC migrate near the cortex (surface epithelium

–Forms cortical cords

–At about 16th week cortical cords break up into isolated clusters called primordial follicles

–development of ovary