SOM 203
Plenary Session 4
Gastro-intestinal and Urinary Systems
Embryology: Urinary System
Dr. B.V.E. Segwagwe
Block 247 Office 314
Ext: 5659
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Learning
Objectives
At the end of the
plenary session, the
student must be able to:
 Describe the
Embryology of
 Urinary System
Animated
Illistration
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https://www.youtube.com/watch?v=b8Pu6S3VUlI
The Urinary
System is derived
from
intermediate
mesoderm and
Urogenital sinus
Somites (outdated: primitive segments)
 are divisions of the body of
embryo.
 Somites are bilaterally paired
blocks of paraxial mesoderm
that form along the head-totail axis of the developing
embryo in segmented animals.
Introduction
Development of the urinary
system is closely related to
the development of the
reproductive system;
particularly during the earlier
stages – where they develop
from the same origin.
However, the urinary system
develops ahead of the
reproductive system.
• The urinary system consists
of the kidneys, ureters,
bladder and urethra. A
region of intermediate
mesoderm, known as the
urogenital ridge, gives rise to
these structures.
•
Development of the Kidneys
 In the embryo, the kidneys
develop from three
overlapping sequential
systems; the pronephros, the
mesonephros, and the
metanephros. They are all
derived from the urogenital
ridge, a derivative of the
intermediate mesoderm
Pronephros
it appears in the 4th week of development.
 Its development begins in the cervical region
of the embryo. Segmented divisions of
intermediate mesoderm form tubules, known
as nephrotomes. In total, 6-10 pairs of
nephrotomes are formed.
 These tubules join into the pronephric duct,
which is a duct that extends from the cervical
region to the cloaca (distal end) of the
embryo.
 This early system is non-functional and
regresses completely by the end of week 4.
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Mesonephros
 During the degeneration of the
pronephros, the epithelial buds at
the thoracic and lumbar levels
form the second pair of kidneys,
the mesonephros (plural,
mesonephroi).
 First, the presence of the
pronephric duct induces nearby
intermediate mesoderm in the
thoracolumbar region to form
mesonephric tubules.
Mesonephros

These tubules receive a tuft of capillaries
from the dorsal aorta – allowing for the
filtration of blood – and they drain into
the mesonephric duct (a continuation of
the pronephric duct).
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They act as a primitive excretory system in
the embryo, with most tubules regressing by
the end of the 2nd month.
Additionally, the mesonephric duct
sprouts the ureteric bud caudally, which
induces the development of the
definitive kidney.
Metanephros
The metanephros forms the definitive
kidney. It appears in the 5th week of
development and becomes functional
around the 12th week.
 The ureteric bud from the mesonephric
duct makes contact with a caudal region
of intermediate mesoderm – the
metanephric blastema.
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Metanephros
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Collectively, these blastema form the metanephric
system, which has two components:
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Collecting system – derived from the ureteric bud.
It dilates to create the ureter, renal pelvis, major and minor
calyces and collecting tubules – terminating at the distal
convoluted tubule.
If the uretic bud splits too early, two ureters, or two renal
pelvices connecting to one ureter may result.
Excretory system – derived from the metanephric blastema.
Each collecting tubule from the collecting system is covered by
a metanephric tissue cap which gives rise to the excretory
tubules.
These excretory tubules (along with the developing glomeruli)
form the kidney’s functional units – the nephron.
The proximal end of the excretory tubule forms the
Bowman’s capsule around a glomerulus, while the
distal end elongates to form the proximal convoluted
tubule, loop of Henle and distal convoluted tubule
Metanephros
 The definitive kidney initially
develops in the pelvic region
before ascending into the
abdomen.
 In the pelvis, the kidney receives its
blood supply from a pelvic branch
of the abdominal aorta and as it
ascends, new arteries from the
abdominal aorta supply the kidney.
The pelvic vessels usually regress,
but can persist as accessory renal
arteries.
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
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Metanephros
 Tubules develop from nephrogenic
cord 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 Nephrogenic Cord & extends
from 8th somite to urinogenital sinus
Mesonephros
Clinical Correlates – Horse shoe kidney
(Fusion Kidney)
Horseshoe kidney is a condition
in which the kidneys are fused
together, commenly at the lower
end or base . By fusing, they
form a "U" shape, which gives it
the name "horseshoe.“There are
different types of horseshow
kidney
During the fifth and sixth weeks of
development, the mature kidneys lie in the
pelvis with their hila pointed anteriorly.
 During the relative ascent, the kidneys further
rotate to 90 degree, thus causing the ventrally
facing hilum initially, to finally face medially.
 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.
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Relative Ascent of
Kidneys
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Initiallly the kidney is lobulated, but later
becomes smooth
 In the early stages they receive
blood from from some supply of
pelvic vessels (median sacral,
common iliac arteries
 Later they ascend and receive arteries
from abdominal vessels (abdominal
aorta)
Relative Ascent of
Kidneys
 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
Cloaca
Development of the Bladder and Urethra
 The bladder and urethra of
the urinary system are
ultimately derived from the
cloaca – a hindgut structure
that is a common chamber for
gastrointestinal and urinary
waste.
Development of the Bladder and Urethra
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In the 4th-7th weeks of development, the cloaca
is divided into two parts by the uro-rectal
septum:
Urogenital sinus (anterior) – divided into three
parts:
The upper part of the urogenital sinus forms the
bladder.
The pelvic part forms the entire urethra and
some of the reproductive tract in females, and
the prostatic and membranous urethra in males.
The phallic/caudal part forms part of the female
reproductive tract, and the spongy urethra in
males.
Development of the Bladder and Urethra
Anal canal (posterior)
 The urinary bladder is initially drained by
the allantois. However, this is obliterated
during fetal development and becomes a
fibrous cord – the urachus. A remnant
of the urachus can be found in adults;
the median umbilical ligament, which
connects the apex of the bladder to the
umbilicus.
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Development of the Bladder and Urethra
As the bladder develops from
the urogenital sinus, it absorbs
the caudal parts of the
mesonephric ducts (also known
as the Wolffian ducts), becoming
the trigone of the bladder. The
ureters, which have formed as
outgrowths of the mesonephric
ducts, enter the bladder at the
base of the trigone.
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 obliterated & 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
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Urinary Bladder
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 mother
 Fetal kidneys are not necessary for exchange of
waste products