Hello, we’ll be discussing now the partitioning of the urogenital sinus
and its role in renal development.
Now, last time we discussed the urogenital sinus,
it was in the context of development of the hindgut.
You may recall that initially, we have a cloaca, a common chamber
that drains the GI urogenital systems
and it gets separated into a separate rectum and urogenital sinus
as a massive mesenchyme moves down called the urorectal septum.
The urorectal septum moves down and forms a little knot of connective tissue
at the base of the perineum called the perineal body.
And in the process, separates the cloaca into the rectum
and the urogenital sinus.
Any you may also remember
if you viewed the discussion on hindgut development
that the cloacal membrane also separates
into an anal membrane and a urogenital membrane.
At this point, I want to draw your attention to a thin little strip of endoderm
that’s extending off the urogenital sinus up into the umbilical cord.
That is the allantois or if you prefer the proper French pronunciation,
the allantois and it acts as a very, very early filtration system
for the developing embryo
but is otherwise not really used by the mammalian body
except that it can wind up causing some developmental anomalies
if it doesn’t rescind properly.
Typically, the allantois is going to dwindle, move in from the umbilical cord,
and just take up residence on top of the urinary bladder,
and eventually, form a fibrous cord that’s called the urachus.
So generally, it goes away, it doesn’t really leave much behind
except a fibrous cord extending from the umbilicus
down to the very tip of the bladder.
The urogenital sinus is thereafter gonna form the urinary bladder
as well as the urethra.
In women, the urinary bladder allows urine to leave to the urethra
and enter the vestibule alongside the opening of the vagina.
In men, there’s going to be some folds of tissue that surround the urethra,
so these urethral folds will wrap around it
and form the shaft of the penis and enclose the spongey urethra.
So what’s going to happen thereafter
is that we have glands developing off the endoderm of the urogenital sinus.
So any gland associated with the bladder or the urethra
actually has an endodermal origin from the urogenital sinus.
Now, moving forward, we’re now going to see
how the bladder develops and enlarges.
Now, as the body gets larger,
it needs to accommodate a greater volume of urine coming from the kidneys.
Initially, the urogenital sinus can handle whatever volume is coming in
but it’s going to need to enlarge, and rather than just ballooning,
it enlarges by pulling a portion of the mesonephric duct into its wall.
Now, if you have viewed the talk I did on development of the kidneys,
you’ll know that the ureteric bud comes off the mesonephric duct
and as the bladder enlarges, it pulls the mesonephric duct
and the ureteric bud into its own wall and the smooth nature of those ducts
is what makes the smooth trigone of the bladder on its posterior side.
So that’s what leads to the ureters emptying directly into the bladder.
In women, the mesonephric ducts go away.
In men, they are maintained as the vas deferens,
also known as the ductus deferens.
And if we have a ductus deferens present
entering the posterior side of the prostate
and thereafter, the urethra, it’s going to drape over the ureter
prior to doing so.
And if you have to locate and isolate the ureter from the vas deferens,
you want to note that the vas deferens is going to drape over it
on its way out to the testis.
Malformations involving the urogenital sinus
largely cluster around failure of the allantois to rescind properly.
If the allaintois does not form a fibrous urachus
leading to the top of the bladder, we can have a variety of problems.
Most prominently is a urachal fistula
in which there’s an inappropriate connection
between the urinary bladder and the umbilicus
and in an infant who is born with urachal fistula,
you will have urine dribbling out of the umbilicus.
Not quite as severe can be deep pouches
leading from the umbilicus into the body
or from the bladder up along the edge of your body wall.
Those are going to be umbilical urachal sinuses.
If it’s going from the umbilicus, in,
or if it’s going from the top of the bladder out,
that is gonna be called a vesicouracheal diverticulum.
Now, neither of these are particularly problematic
because you don’t have urine drain into the umbilicus
and may be relatively hard to catch
because they don’t cause too many problems.
However, you can also have cysts develop
where the allantois did not fully rescind
and become urachus and those fluid filled cysts
can be relatively non-problematic
unless they get inflamed, and enlarged, and cause pain.
Other problems in this area involve the failure of the ureteric buds
to enter the posterior side of the urinary bladder appropriately.
This is most common when we have an ectopic ureter
form from a double ureteric bud.
Now, an ectopic ureter is a ureter that is emptying urine
into the wrong location.
Most commonly, with these double ureteric buds
will have one enter the bladder appropriately
and the other will enter another nearby area inappropriately.
In particular, it may enter the urethra
and since the bladder is there to store urine
and only release it when the muscles that are at its base
relax and allow urine to flow,
there’s going to be a constant dribble of urine
from the urethra into the vestibule or out the shaft of the penis.
In this case, it may actually be relatively hard to spot if an infant has this
because you don’t typically notice if urine is constantly dribbling from an area.
You just notice whether the child has wet diapers or not.
But if a child has consistently always got wet diapers
even a few weeks into development,
it may be something to keep on your radar that there could be a problem
with an ectopic ureter releasing urine constantly.
In women, the same process can occur
but the ectopic ureter instead of releasing into the urethra
can release into the vagina.
The net effect clinically will be about the same.
A constant dribble of urine but instead of from the urethra,
it will be coming from the vagina
and it is also possible to have ectopic ureters release onto the rectum.
Although, that is far less common because of the proximity difficulty there.
It’s hard for the ureter to make it all the way back to the rectum.
Thank you very much for your attention and I’ll see you on our next talk.