We will continue our discussion of renal development
by looking at how the microscopic development
of the metanephric kidney takes place
which gives rise to the actual filtration system of the kidney.
We have two sets of kidneys formed during early life.
The mesonephros forms, develops a mesonephric duct
that actually starts secreting urine into the urinary bladder.
So the tubules of the mesonephric duct associate with a glomerulus,
filtration occurs into the renal or Bowman’s capsule,
and urine is released to the bladder from the mesonephric kidney.
However, our body has decided that that wasn’t quite good enough
and we have a ureteric bud come off of the mesonephric duct,
induce formation of the final kidney or the metanephros
which will then begin filtering urine and releasing it to the bladder.
The mesonephros dwindles away to pretty much nothing
but its ductwork and its tubules stick around
and associate with the nearby gonad.
Now, at this stage, the gonad is completely undifferentiated or indifferent.
It’s neither a testis nor an ovary yet
but in the male, the mesonephric duct will stick around
and form a continuous connection from the gonad, down into the urinary system.
We’ll follow that in an entirely separate talk
but right now, we’re gonna instead move back
to the ureteric bud and the metanephros
and see how the filtration system of the kidney finally develops.
As the ureteric bud goes into the metanephros,
it starts splitting in more and more finely woven little pathways
that invade the metanephros.
In the process, it will create the renal pelvis,
the major calyces, and even smaller level,
the minor calyces, and an even smaller level,
it will actually invade the substance of the metanephros
and form the collecting ducts and the collecting tubule of the kidneys.
So here, we can see at the very, very fine magnification that collecting ducts
have connecting tubules kind of budding off of them
and associating with the tissue of the metanephros.
The metanephric vesicle is going to expand outward and develop a lumen,
develop a little hollow space inside and at the same time,
a hollow space or lumen is developing in the collecting ducts and collecting tubules.
Now, this vesicle of the metanephros
that’s growing away from the ureteric buds collecting tubule is going to elongate
and it’s gonna start snaking around and developing a loop.
So this S-shaped vesicle will eventually turn into the spiral
and then, looped structure of the loop of Henle and the rest of the nephron.
So we have two embryonic origins for the urinary drainage apparatus.
The metanephros is giving rise to the nephron
and then, the collecting system is coming from the ureteric bud.
But they form a continuous lumen
that allows blood to be filtered at Bowman’s capsule
and then, solutes, sugar, and other substances
recovered from it before the urine is released into the collecting tubule, collecting duct,
and then, into more and more large portions of the drainage apparatus.
That’s going to be the minor calyces, major calyces,
renal pelvis, ureter, and finally, the urinary bladder.
Now, filtration of amniotic fluid actually does occur during life.
We breathe in and swallow amniotic fluid.
That fluid winds up in our tissues
and our embryonic and fetal bodies filter that amniotic fluid
and actually release urine back into the amniotic fluid.
Now, that sounds horrible.
The good news is the placenta does such a good job of filtering out
all the waste products
and other nasty things that are present in our body
that the urine that’s excreted in the amniotic cavity
is very, very non-threatening.
It doesn’t really do any damage.
It’s very much like amniotic fluid in, amniotic fluid out.
So we’re not polluting the amniotic sac much
even though that process is occurring throughout fetal life.
Nearby, we have the suprarenal or adrenal glands forming
and they come from two distinct sources.
Initially, we have the cortex migrate in from an area near the urogenital ridge
and form a little C-shaped cup that associates with neural crest cells
that are migrating in to form the adrenal medulla.
The portion that came from the cortex,
pardon me, the portion that gives rise to the cortex
is coming from near the urogenital ridge
and it happens not just once, but twice.
There’s a fetal cortex that forms around the adrenal medulla
and then, the adult cortex moves in
and this finally creates the trilaminar appearance of the adrenal cortex
and surrounding those neural crest cells, we’ll have the medulla.
So the medulla is derived from neural crest cells
and they’re essentially going to release epinephrine and norepinephrine
and they are essentially sympathetic nerve cells
that instead of migrating into the gut
to release norepinephrine and epinephrine at a synapse,
are going to release it into our blood supply.
So just remember that the adrenal cortex
is coming from an area near the urogenital ridge
but the adrenal medulla is derived from neural crest cells.
Now, things that can go wrong with kidney development include oligohydramnios.
Now, oligohydramnios essentially means too little amniotic fluid
and problems with the kidneys are only one cause of oligohydramnios.
Leakage of amniotic fluid can also cause the same problem
but renal agenesis or blockage of the urinary drainage system
is a fairly common cause of it.
And if you look at the picture here,
you can note that these kidneys don’t exactly look healthy but in particular,
you can see that the ureter on the one on the left side of the image is pinched.
Failure of urine to be filtered because there’s no kidney
or failure of urine to exit the kidney can cause Potter sequence.
Potter sequence is a problem that develops from oligohydramnios,
too little amniotic fluid.
The amniotic fluid acts like a little buoyant kind of support for the developing body.
We’re floating around in the amniotic fluid and it keeps us relatively cushioned.
If there’s too little amniotic fluid,
then the fetus is being kind of pushed down like gravity.
This can cause limb defects, hip dislocation, and also, facial defects
because the face is pushed up against the wall of the uterus
instead of being able to kind of float relatively free.
So Potter sequence is a sequence of events that follow from lack of amniotic fluid
and they’re basically structural problems
caused by a structural misalignment of the kidneys
or other problems of the amniotic fluid
where there’s just too little to support the body.
Thank you very much for your attention and I’ll see you in our next talk.