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Development of the Urinary System
• Urinary and genital systems are closely
• 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
Fig. 1 -
Transverse section and dorsal view of an embryo (trilaminar)
Transverse section of the three-layered embryo
towards the end of the 3rd week of development.
(ca. 21 days)
1.Paraxial mesoderm
3.Lateral mesoderm
6.Intraembryonic coelom
(mesoderm and
(mesoderm and
11.Neural groove
12.Neural ridge
Mesonephros enlargement point A
1.Neural tube
3.Aorta dorsalis
4.Dorsal mesentery
5.Intestinal tube
8.Inferior cardinal
9.Mesonephric duct
(Wolffian duct)
11.Urogenital ridge
The urinary tract develops from the 3rd week of the embryonic
period from the intermediate mesoderm as well as from the
urogenital sinus. The kidneys develop from the 4th week in
three steps: As a first one, a cranial anlage, the pronephros,
forms that then later atrophies in the 8th week and is never
active functionally. It is followed by a further anlage from the
intermediate mesoderm, the mesonephros, that is formed
between the 6th and 10th weeks, but is only transitory, and the
anlage of the definitive kidneys, the metanephros. They
develop from a metanephric anlage (mesodermal origin) and
the ureter anlage (that has its origin in the caudal part of the
wolffian duct).
The urine-excreting part of the kidneys, the nephron, mainly
arises from the metanephric anlage (glomerulus, proximal,
intermediate and distal tubules), while the rest of the upper
urinary tract (collecting ducts, calices, renal pelvis and ureter)
develop from the ureter anlage.
The lower urinary tract differentiates from the cloaca between
the 5th and 8th weeks in that it becomes subdivided by the
urorectal septum. The ventral part of the cloaca forms the
primary urogenital sinus, out of which the urethra forms in the
lower part and the bladder in the upper part. The ureter anlage
discharges into the upper posterior wall of the urogenital sinus.
In males, the wolffian duct remains present and forms a
connection to the genital tract in the lower part of the
urogenital sinus.
The numerous induction mechanisms between ureter anlage
and metanephric mesenchyma during the development of the
renal system, as well as the ascent of the kidneys,
originating at the level of the sacrum and moving up to the
diaphragm at the end of the development, make it possible for
a large number of abnormalities to arise. Many remain
asymptomatic whereas others are not compatible with survival.
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 =
– 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
Pronephros (forekidney): transitory structure
1.Nephrogenic cord
2.Mesonephric duct
3.Intestinal tube
6.Yolk sac (umbilical
8.Outflow of the
mesonephric duct into
the cloaca
• 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
Mesonephros: transitory kidney
1.Nephrogenic cord
2.Mesonephric duct
6.Yolk sac (umbilical
8.Outflow of the
mesonephric duct
into the cloaca
9.Ureter bud (anlage)
Mesonephros enlargement point A
1.Neural tube
3.Aorta dorsalis
4.Dorsal mesentery
5.Intestinal tube
8.Inferior cardinal
9.Mesonephric duct
(Wolffian duct)
11.Urogenital ridge
The metanephros: definitive kidney 5th week
2-Mesonephric duct
(Wolffian duct)
3-Nephrogenic cord
4-Ureter anlage
6-Liver anlage
Metanephric outflow
2.Ureter anlage
duct (Wolffian
• 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
Kidney at the end of its development
2.Renal pelvis
3.Vena renalis
4.Arteria renalis
5.Major calix
6.Minor calix
8.Capsula renis
9.Medullary rays
10.Papilla renalis
11.Sinus renalis
12.Columna renalis
Schematic cut through the kidney (sagittal section)
2.Major calix
3.Minor calix
4.Renal pelvis
7.Kidney lobe
Enlargement of the inset Nephron
8.Distal tubule
9.Proximal tubule
• 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 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 sinus
• 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 placenta
which transfers metabolic waste to the
• Fetal kidneys are not necessary for
exchange of waste products
Ascent of Kidneys
During the fifth and sixth weeks of development,
the mature kidneys lie in the pelvis with their hila
pointed anteriorly. As the pelvis and abdomen
grow, the kidneys slowly move upward. By the
seventh week, the hilum points medially and the
kidneys are located in the abdomen. As the
embryo continues to grow in a caudal direction,
the kidneys are left behind and eventually come
to lie in a retroperitoneal position at the level of
L1 by the ninth week of development. In the
meantime, the kidneys have completed rotation
and the hila now face anteromedially.
Development of the reproductive
• 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
– Undifferentiated gonad
• Males:Mesonephric ducts form epididymis,
ductus deferens, ejaculatory duct
– Cranial mesonephric tubules  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
– Ducts open into coelom
– Runs along side of mesonephric duct
– Fuse at caudal end
• Y shaped uterovaginal complex  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  Sertoli cells
• Primordial germ cells  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
– Forms cortical cords
– At about 16th week cortical cords break up into
isolated clusters called primordial follicles
– development of ovary