When you have completed this section, you should be able to:
describe the sequential development of pronephros,
mesonephros, and metanephros from the intermediate mesoderm
describe normal development of the definitive
kidneys, ureters, urinary bladder, and urethra
list some of the more important abnormalities of
the urinary system.
You will notice that there is a considerable overlap
in the development of the urinary and reproductive systems. This is
particularly obvious in the male, where structures originally formed
with a urinary function are 'taken over' by the reproductive system
at a later stage. For example, the mesonephric duct, which at one
stage drains dilute urine from the mesonephros, becomes modified to
form the epididymis and ductus deferens linking the testis with the
ejaculatory duct. Also in the male, the urethra becomes the final
common pathway for urine and semen. Thus, it is not uncommon to see
or hear references to the 'urogenital system'.
The source of most of the urinary structures is the
column of intermediate mesoderm, which lies between the somites
(situated more medially) and the lateral mesoderm which is split
into two layers by the intra-embryonic coelom.
Three 'urinary systems' appear in sequence during
embryonic development, perhaps indicating the evolutionary sequence
by which the definitive kidneys arose:
mesonephros → metanephros
They are sequential systems in terms of:
time of appearance: pronephros first, mesonephros
next, and metanephros last
position within the embryo: pronephros in the
cervical region, mesonephros in the abdominal region, and
metanephros in the pelvic region
In the human embryo, the metanephros forms the
definitive kidneys, but in other animals one of the earlier systems
may be retained.
Pronephros, mesonephros, and
metanephros arising in cranio-caudal sequence.
In the human embryo, the pronephros regresses soon
after its formation, and is probably non-functional.
– probably probably functional
The mesonephros is known to be functional in embryos
of some mammalian species (eg: cat, rabbit, pig), and there is
reason to expect this to be the case in the human embryo too. The
mesonephros develops in the posterior wall of the abdomen, and forms
a pair of elongated ridges, one on each side of the dorsal
mesentery. A segmental series of excretory tubules develops within
each ridge and link up with a collecting duct - the mesonephric
duct. The mesonephric duct drains into the allantois close to the
cloaca. The excretory units - mesonephric tubules - arise in a
cranio-caudal sequence, and as the more caudal ones appear, the
normal cranial ones gradually degenerate and disappear. The
mesonephros reaches its maximum development in week 6, after which
it begins to involute. (If the embryo is male, a small number of
mesonephric tubules will be retained and incorporated into the
reproductive system, as you will see later.)
Recall that a fold of tissues arises from the cranial
end of the mesonephric ridge on each side and contributes to
development of the diaphragm - these are the pleuroperitoneal
- definitive kidneys
As the mesonephros regresses, the definitive kidneys
(ie: the kidneys seen postnatally) begin their development from the
metanephros. Two important components are required to produce each
the metanephric blastema (intermediate mesoderm)
the ureteric bud (an outgrowth from the
The metanephric blastema differentiates into the
functional units of the kidneys - the nephrons (approximately 1-2
million per kidney). However, it can only achieve this under the
inductive influence of the ureteric bud.
From the ureteric bud develops the collecting duct
system, including the collecting tubules which link with the
nephrons), the major and minor calyces, the renal pelvis, and the
‘ascent’ of the kidneys
Each metanephric kidney 'ascends' from the pelvic
region, where it originates, to its final position on the posterior
wall of the abdomen. It seems that much of this ascent is the result
of differential growth - the posterior wall of the abdomen grows
rapidly so that in a sense the caudal end of the embryo 'grows away'
from the kidneys. The arterial supply of each kidney changes during
this ascent, with new branches being formed from the aorta at
successively higher levels. Occasionally, arteries from lower levels
are retained as supernumerary renal arteries.
As the kidneys ascend, they have to pass beneath an
arch formed by the two umbilical arteries as they course ventrally
from the paired dorsal aortae to the umbilical region. This brings
the kidneys close together, and occasionally their lower poles
become fused forming an abnormal, U-shaped horseshoe kidney.
Note that while the kidneys are ascending, the gonads
are descending - their relationships become reversed!
The remaining parts of the urinary system are:
the urinary bladder
Their development will be considered next.
The allantois acts as a temporary bladder, but then
the definitive bladder is formed as the urorectal septum grows
caudally to meet the cloacal membrane. This septum subdivides the
urogenital sinus from the hindgut, and it is the 'upper' part of
this sinus which differentiates into the urinary bladder. Although
most of the allantois eventually regresses, it probably contributes
to the apical region of the definitive bladder.
Note how the ureters - initially outgrowths from the
mesonephric ducts - eventually gain their own entrances into the
The lower portion of the urogenital sinus - between
the developing bladder and the urigenital membrane - forms the
urethra: all of the urethra in female embryos, but only the proximal
portion in the male embryos (the distal portion is formed during
development of the external genitalia, as we shall see later).
Onset of function
The metanephric kidneys are functional by the
beginning of the second half of pregnancy. The copious dilute urine
(containing a little urea and urid acid) is released into the
amniotic fluid that surrounds the fetus. However, the quantity of
fluid in the amniotic cavity is regulated by an ingenious mechanism.
This mechanism also rehearses the functioning of the digestive
system and allows the reuptake of urea so that it can be disposed of
eventually across the placental membrane. The feturs swallows
amniotic fluid periodically, and the fluid is then re-absorbed from
the digstive system into the fetal circulation. You may remember
from our study of the developing digestive system that the debris in
the fluid is acted upon by the digestive enzymes.
agenesis of one or both kidneys
duplication of the ureter
Questions on the urinary system:
1 Is there any significance in the close
relationship between the intermediate mesoderm and the
2 What happens to the metanephric blastema if
the ureteric bud of that side fails to reach it?
3 What is the embryological explanation of
4 Why is the horseshoe kidney generally
positioned lower in the abdomen than normal kidneys would be?
5 In what way is the development of the
trigone different from other parts of the urinary bladder?
6 How are the major and minor calyces of the
7 What is a urachal fistula?
8 What is the embryological explanation of
exstrophy of the bladder?
9 What clinical problems might be associated
with duplication of the ureter?