We’re gonna continue discussing reproductive development
and look at one of the most interesting
and just flat out mesmerizing aspects of human development
a time when we have neither testes, ovaries, uterus, or vas deferens,
but we have the primordia to make the organs of either gender.
So we’re neither male nor female at this stage but have the potential to become both.
So we have an undifferentiated or sometimes called indifferent gonad
present in the intermediate mesoderm that develops into either the testes or the ovaries
depending on what signals it’s exposed to.
Likewise, we’re gonna have the same duct at work present in both sexes.
The mesonephric duct or something called the paramesonephric duct
that can form either the ductus deferens or the oviducts and uterus.
The change that initiates one chromosomal sex
or the other to affect the phenotypic sex male or female development
is a single gene on the Y-chromosome and the expression of that single gene,
that testes determining factor kicks off an entire kind of sequence of events
that diverts development down the male pathway.
In the absence of that one gene, female development
will happen and will wind up with ovaries, uterus and oviducts.
So variation in the hormonal signals that are received during this time
drive the development of secondary sexual characteristics
such as the appearance of the external genitalia
and how the internal genitalia meet, fuse, or don’t fuse to produce the normal
or sometimes abnormal appearance of the genitalia.
So we’re gonna start by returning to the trilaminar embryo
and the intermediate mesoderm
which at this point, is going to give rise to both the testes or ovaries, and the kidneys.
So as we work our way in, we can see that the body wall has started forming
and we have the nephrogenic cord deep into the body
coming from the intermediate mesoderm
and on its surface we have a genital ridge.
The genital ridge is the structure that’s going to give rise to the supporting tissues
of the ovary or the testes but not the actual sperm or oocytes.
What we get to create those, we have a group of cells originating from the ectoderm
that migrate into the yolk sac.
These primordial germ cells which can form either spermatocytes or oocytes
originate in the epiblast and work their way to the yolk sac.
Then, during week four of development, once the genital ridges are present,
they migrate back from the yolk sac, along the allantois, and then, distribute themselves
into the intermediate mesoderm and associate with the genital ridge.
Now again, at this point, these cells could be triggered
to become either spermatozoa or oocytes.
The primordial germ cells move into that intermediate mesoderm
and then, they’re gonna form the medulla of the gonads.
The epithelium on the outside of the genital ridge
will start to invade the intermediate mesoderm
and associate with these primordial germ cells.
These invasions of the underlying mesoderm are called primitive or primary sex cords.
While this is happening, we have a couple of ducts developing nearby.
In the mesonephros, we had filtration of urine occurring through the glomeruli,
draining into the mesonephric ducts and then, to the urinary bladder.
The mesonephros itself dwindles away and leaves the mesonephric ducts orphaned.
Fortunately, they’re going to associate with the developing gonad
and if these mesonephric, sometimes called Wolffian ducts, are present in a genetic male,
they will associate more and more closely with the developing testes.
Nearby, we have some tissue invaginating from the embryonic body cavity,
the intraembryonic coelom and that will form the nearby paramesonephric duct,
sometimes called the Müllerian duct.
So this open-ended duct moves its way in from the intraembryonic coelom
and comes into close association with the indifferent gonad
and if this person will develop as a female, the paramesonephric duct
will stick around and the mesonephric duct will dwindle and disappear.
So temporarily, we have both; an indifferent gonad that’s neither a teste nor an ovary.
We have the mesonephric duct present
to possibly form the ductus deferens and male plumbing
and the paramesonephric duct present that may form the oviduct, the uterus,
and the proximal vagina if the person develops as a female
and we’ll follow the triggers that lead to one side of development
or the other in subsequent talks.
Before we follow the triggers that lead to male or female development,
let’s look at how the ducts intercombine or stay separate
to result in the mature anatomy of the genital tracts.
In women, the paramesonephric ducts are separate near the developing ovary
but they fuse to form a single uterine body before associating with the urogenital sinus.
Because of that, we typically have the vagina leading to a single uterine cavity
which then branches into two separate oviducts.
Opposed to that in men,
we have two separate mesonephric ducts that enter the urinary sinus
or the urogenital sinus separately and therefore,
in mature males, we have two separate
ductus deferens entering the posterior side
of the prostate and then, reaching the urethra.
Thank you very much for your attention
and we’ll see you on the next talk.