Development of the Urogenital system I
Urogenital system is predominantly derived from the intermediate
mesoderm
Intermediate mesoderm Mesonephros (intermediate excretory organ –
helps the baby while it is in an aqueous environment) metanephros
(mature kidney, as it forms, the mesonephros disintegrates)
Pronephros is a modified structure that helps with this transition but it’s
role is controversial (some people say it’s not needed in mammals)
Mesonephros is made up of the mesonephric duct (aka wolfian/ nephric).
This duct integrates the mesoderm and endoderm by entering the cloaca
(the caudal end of the gut, made up of endoderm and gives rise to the anal
canal).
The nephric duct integrates into the cloaca, whereas the cloaca gets split
into two (because of the action of a urorectal septum, this causes the
ventral aspect to be the urogenital sinus, and the dorsal aspect is the anal
canal)
The mesonephric duct drains into the urogenital sinus (which will later
become the bladder)
The metanephric rudiment forms – this is composed of the metanphric
blastema (intermediate mesoderm) and urteric bud. This development
starts at week 10 and continues until week 30. This means premature
babies (before week 30) are likely to develop nephrotic syndrome later
on in life.
Metanephric blastema gives rise to:
o Epithelia of nephrons
o Stroma,
o Endothelia
Urteric bud is the most important, if there is no urteric bud there is no
kidney. Gives rise to :
o Collecting tubule epithelia
o Epithelia of renal pelvis, minor and major calyx, and ureter.
As the kidneys grow, they ascend from the pelvis to their proper
placement in the abdomen. Because new vascular connections to the
aorta are made at the same time, sometimes the blood vessels may block
this ascension of the kidneys and cause abnormalities.
When one kidney hasn’t ascended, this is called the pelvic kidney. When
both don’t ascend and fuse, it is called the horseshoe kidney.
Although this is caused by abnormal vasculature, there is some evidence
that it is genetic as well. Foxd-1 deletion causes pelvic and horseshoe
kidney.
Abnormal vasculature / aberrant vessels also cause obstructions in the
pathway of the urine through the ureter and renal pelvis, this condition is
called hydronephrosis. This causes enlargement of the renal pelvis and
can happen in two places most frequently:
o The ureteropelvic junction most common urinary tract
obstruction, poorly understood, can be diagnosed by the
enlargement of ONLY the pelvis.
, o The ureterovesical less common, between the ureter and the
bladder, both the pelvis and the ureter are enlarged.
The junction of the ureter- cloaca has to be remodeled to make the
bladder:
o This occurs through incorporation of the ureters (from the ureteric
bud) integrating into the urogenital sinus (the bladder) and fusing
at week 6.
o Apoptosis is also involved in this process as both the wolffian duct
and the ureter integrate collectively into the urogenital sinus, but
separation needs to occur.
Signalling between the metanephric blastema , nephric duct, and the
urteric bud is essential for the development of the kidney.
Metanephric blastema secrets factors that mediate the outgrowth of
ureteric bud from the nephric duct, as well as continued branching. These
factors are reliant on C-ret and FGF receptors.
WNT genes (specifically Wnt 9b) were found to be sufficient for signaling
in absence of uteric bud.
The ureteric bud induces the metanephric blastema to go from
mesenchyme to epithelium. Two genes are required for this transition
The wilms tumor gene WT-1 (when abberant, causes tumors in the
kidney) as well as the Pax-2 transcription factors (when abberant causes
both kidney and eye problems)
When epithelium is established, the embryonic podocytes of the kidney
secrete factors VEGF to create the glomerular capillary network.
Negative signaling and inhibition is needed to make sure not more than
one ureter is present.
Early embryo is sexually indifferent. There are three tissues that need to
be formed to form the sexual organs:
o Gonadal ridge forms gonads
o Germ cells (yolk sac) migrate into the gonads and form gametes
o Two ducts:
Mesonephric / wolfian duct
Paramesonephric / mullerian duct.
The germ cells migrate along the dorsal mesentry into the gonads at the
5th / 6th week of development. This is because their differentiation is
reliant on the environment in the gonads abnormal migration = not a
suitable differentiation environment = teratomas.
Paramesonphreic ducts are made from an invagination in the coelomic
epithelium. This gives rise to a duct that is ADJACENT to the mesonephric
duct. At this point, there are two ducts and the embryo is sexually
indifferent.
When there is a functional Y chromosome, by the 4th month:
o Mullerian duct disintegrates
o Nephric duct becomes the vas deferens.
o Determination occurs due to the SRY (sex determining region of
the Y chromosome) which gives rise to the testis.
o The testis then develop 3 cells:
Urogenital system is predominantly derived from the intermediate
mesoderm
Intermediate mesoderm Mesonephros (intermediate excretory organ –
helps the baby while it is in an aqueous environment) metanephros
(mature kidney, as it forms, the mesonephros disintegrates)
Pronephros is a modified structure that helps with this transition but it’s
role is controversial (some people say it’s not needed in mammals)
Mesonephros is made up of the mesonephric duct (aka wolfian/ nephric).
This duct integrates the mesoderm and endoderm by entering the cloaca
(the caudal end of the gut, made up of endoderm and gives rise to the anal
canal).
The nephric duct integrates into the cloaca, whereas the cloaca gets split
into two (because of the action of a urorectal septum, this causes the
ventral aspect to be the urogenital sinus, and the dorsal aspect is the anal
canal)
The mesonephric duct drains into the urogenital sinus (which will later
become the bladder)
The metanephric rudiment forms – this is composed of the metanphric
blastema (intermediate mesoderm) and urteric bud. This development
starts at week 10 and continues until week 30. This means premature
babies (before week 30) are likely to develop nephrotic syndrome later
on in life.
Metanephric blastema gives rise to:
o Epithelia of nephrons
o Stroma,
o Endothelia
Urteric bud is the most important, if there is no urteric bud there is no
kidney. Gives rise to :
o Collecting tubule epithelia
o Epithelia of renal pelvis, minor and major calyx, and ureter.
As the kidneys grow, they ascend from the pelvis to their proper
placement in the abdomen. Because new vascular connections to the
aorta are made at the same time, sometimes the blood vessels may block
this ascension of the kidneys and cause abnormalities.
When one kidney hasn’t ascended, this is called the pelvic kidney. When
both don’t ascend and fuse, it is called the horseshoe kidney.
Although this is caused by abnormal vasculature, there is some evidence
that it is genetic as well. Foxd-1 deletion causes pelvic and horseshoe
kidney.
Abnormal vasculature / aberrant vessels also cause obstructions in the
pathway of the urine through the ureter and renal pelvis, this condition is
called hydronephrosis. This causes enlargement of the renal pelvis and
can happen in two places most frequently:
o The ureteropelvic junction most common urinary tract
obstruction, poorly understood, can be diagnosed by the
enlargement of ONLY the pelvis.
, o The ureterovesical less common, between the ureter and the
bladder, both the pelvis and the ureter are enlarged.
The junction of the ureter- cloaca has to be remodeled to make the
bladder:
o This occurs through incorporation of the ureters (from the ureteric
bud) integrating into the urogenital sinus (the bladder) and fusing
at week 6.
o Apoptosis is also involved in this process as both the wolffian duct
and the ureter integrate collectively into the urogenital sinus, but
separation needs to occur.
Signalling between the metanephric blastema , nephric duct, and the
urteric bud is essential for the development of the kidney.
Metanephric blastema secrets factors that mediate the outgrowth of
ureteric bud from the nephric duct, as well as continued branching. These
factors are reliant on C-ret and FGF receptors.
WNT genes (specifically Wnt 9b) were found to be sufficient for signaling
in absence of uteric bud.
The ureteric bud induces the metanephric blastema to go from
mesenchyme to epithelium. Two genes are required for this transition
The wilms tumor gene WT-1 (when abberant, causes tumors in the
kidney) as well as the Pax-2 transcription factors (when abberant causes
both kidney and eye problems)
When epithelium is established, the embryonic podocytes of the kidney
secrete factors VEGF to create the glomerular capillary network.
Negative signaling and inhibition is needed to make sure not more than
one ureter is present.
Early embryo is sexually indifferent. There are three tissues that need to
be formed to form the sexual organs:
o Gonadal ridge forms gonads
o Germ cells (yolk sac) migrate into the gonads and form gametes
o Two ducts:
Mesonephric / wolfian duct
Paramesonephric / mullerian duct.
The germ cells migrate along the dorsal mesentry into the gonads at the
5th / 6th week of development. This is because their differentiation is
reliant on the environment in the gonads abnormal migration = not a
suitable differentiation environment = teratomas.
Paramesonphreic ducts are made from an invagination in the coelomic
epithelium. This gives rise to a duct that is ADJACENT to the mesonephric
duct. At this point, there are two ducts and the embryo is sexually
indifferent.
When there is a functional Y chromosome, by the 4th month:
o Mullerian duct disintegrates
o Nephric duct becomes the vas deferens.
o Determination occurs due to the SRY (sex determining region of
the Y chromosome) which gives rise to the testis.
o The testis then develop 3 cells:
