Implantation of Embryo Video Lecture - NEET

>>Dr. Ketchum: Okay, so this segment now is
going to focus on fertilization, implantation, and
pregnancy. What we’re going to cover
in this series of notes is looking at the
events of fertilization and then we’ll talk
about early embryonic development and implantation,
as well as embryonic and fetal development
and then hormonal changes of pregnancy. So
we’re going to be covering all four topics.
So in fertilization, fertilization usually—I
put usually because that’s what the normal
is—it’s normal for it to take place in
the uterine tube, okay? As I’ve mentioned
in, in previous video lectures. Sperm would
have to be introduced into the female reproductive
tract within five days before ovulation. Timing
is everything, all right? The sperm are viable
for five days, and the secondary oocyte is
viable for 12 to 24 hours. So the sperm actually—they’re
initially incapable of fertilization, and
they become capable of fertilization through
a process called capacitation.
So what capacitation is, it’s where actually
the tail or the flagellum of the sperm moves
faster. So that actually increases motility,
and the plasma membrane of the sperm is altered,
and it’s altered in such a manner that it
can fuse with the secondary oocyte. Let’s
just look at sort of a, a pictorial view of what
we were just talking about. So we’ve already
talked about how the secondary oocyte gets
produced in the ovary, and we’ve mentioned
that it takes it four days to get to the uterus.
So while it’s traveling through that uterine
tube, it may encounter sperm. So the sperm
have to enter through the female reproductive
tract (so I’m using a blue color for sperm)
and the sperm have to encounter the secondary
oocyte, and that’s where fertilization takes
place. Okay so like I said earlier, timing
is everything, right? Okay, so fertilization
takes place in the uterine tube. Remember
that implantation takes place in the uterus.
All right, so here’s how this process happens.
So we’re going to look at sperm penetration
of the secondary oocyte. So how does the sperm
penetrate?
So there’s a whole bunch of sperm that actually
reach the ovum. Remember that for every ejaculation
there’s anywhere from 50 to 500 million
sperm that get released. So that means a whole
bunch of sperm can actually reach the ovum.
So once they do reach the ovum, and this actually—
write secondary oocyte here, because that’s
actually a more correct term. So when the
secondary—or when the sperm reach the secondary
oocyte, this is when the acrosomal reaction
begins. So the acrosomal reaction begins where
a whole bunch of enzymes get released, and
the enzymes start breaking through the zona pellucida.
And so here you can see in step
two, all right, the enzymes that are being
released. So you can see how that red area
is indicating that. And then we have step
three breaking through the zona pellucida.
Okay, because remember these have to penetrate
into the egg. So now sperm have gained entry
into your secondary oocyte. And once they’ve
gained entry into the secondary oocyte, the
first sperm to bind to the receptor on the
oocyte wins. That’s the one that fertilizes
the egg, okay? It’s got to bind to the receptor,
however. Once that happens, the sperm can
get transported to the cytoplasm of the oocyte,
and then the zonal pellucida will harden.
And that prevents what’s called polyspermy.
So you don’t want multiple sperm fertilizing
eggs.
This picture will look familiar to you once
again. So now where we are in this diagram—remember,
this was the diagram of oogenesis—we are
at this level here, after fertilization now.
So that has stimulated meiosis II. Fertilization
stimulates meiosis II, that’s where then
we go from a secondary oocyte to an ovum.
So the sperm plasma membrane then disintegrates.
The chromosomes from the sperm and from the
ovum migrate to the center, and now we have
a zygote. So from that fertilized egg then
the zygote is going to start developing. So
remember where we are; we’re in the uterine
tube. Here’s the zygote, which is the fertilized
ovum. Now that zygote is going to undergo
mitosis. And when it undergoes mitosis, the
result is a four-cell stage. Then it’s going
to undergo mitosis again, all right? And then
it will become a morula.
So a morula is literally solid ball of cells.
If you look at this image, it’s nothing
more than a solid ball of cells. That morual
then will undergo more development, and it
will develop into a blastocyst. A blastocyst
is essentially a hollow ball of cells.
Here's the little area that’s got some cells, and
the rest of this—the blastocyst is actually
hollow. The trophoblast will actually—it
will eventually become the placenta, or help
form part of the placenta at least.
It’s at this time that that blastocyst
becomes an actual embryo. So with the blastocyst,
the reason we’re saying that this becomes
an embryo is that the cells begin to differentiate.
And what I mean by the cells beginning to
differentiate is that they’re now committed
to becoming certain types of tissues. Maybe
they’re committed to becoming neural tissue
or maybe they’re committed to becoming muscle
tissue, and so on.
Then, finally here we have implantation at
day six post fertilization. And the trophoblast
is also very important or plays a role in
implantation. Okay, so a lot of times we get
the question, “Well how are twins derived?”
Okay, so if you get identical twins, what’s
happened with identical twins is that the
morula that we talked about on the previous slide,
the morula splits. And when the morula
splits, you wind up with two different embryos.
So you have identical twins. Fraternal twins,
on the other hand, is where you actually have
two different eggs that are fertilized by
two different sperm. So those are fraternal
twins. So now we have the blastocyst. So the
blastocyst will begin the process of implantation,
okay? So we’re in the uterus, and the blastocyst
is going to start to implant. So first off,
the endometrium—it swells and that increases,
and it increases glycogen stores. Why do you
think we’re going to need glycogen? Why
is that important? Continuing on with this
process of implantation, the trophoblast starts
secreting enzymes, and that’s going to start
dissolving away the wall of the endometrium.
This is showing how the enzymes are eating
away at that part of the wall. We’re digesting
the cells and we’re providing nourishment
for the embryo. At the same time, there’s
also paracrines that are secreted. And so
what that’s going to do is it’s going
to increase the capillaries in the area so
that way you can bring more oxygen and nutrients
delivered to the area. You can also then remove
more carbon dioxide and remove waste from
the area as well.
In late embryonic development—so at three
weeks you can see what the embryo looks like
at three weeks. You can see what the embryo
looks like a t five weeks; you can make a
comparison there. At five weeks, the trophoblast
actually thickens. And when the trophoblast
thickens, it becomes a chor—the chorion,
and the chorion encapsulates the embryo.
All right, and then by eight weeks, now we have
what we call a little human, right? And now
this thing resembles a human, and now it’s
appropriate to call it a fetus. So at this
time, by eight weeks, the placenta has developed.
We’re not going to go into details on the
exact physiology of the placenta, but the
placenta is allowing for exchange of gases
and nutrients between the fetus and the mother.
Okay, so it’s between the circulation of
the fetus and the circulation of the mother.
Or you could say maternal, all right, maternal
and fetal circulation.