Fertilization & Implantation


– In this video,
I’ll be focusing on the very first
steps of pregnancy. Fertilization, where sperm
meets egg, and implantation, when the early
embryo embeds itself in the wall of the uterus. Throughout our
discussion of pregnancy, we’re really going to be
focusing on selected topics. We’ll talk about fertilization
and implantation. We’ll talk about the placenta. We’ll talk about labor and
delivery, and lactation. But we’re not going to
go into a lot of detail on embryonic development. What you’re looking at here is
the moment of fertilization. You have an egg, an oocyte,
surrounded by spermatozoa. But before you
get to this point, the sperm need to
become activated. And that’s a process
called capacitation. The capacitation of sperm
is a two-step process. The first is an increase
in the sperm motility. Basically, when the flagellum of
the sperm cell begins beating. This happens in the male when
the spermatozoa encounter seminal fluid in the
ejaculatory duct. So the sperm are formed in
the seminiferous tubules. They’re stored in
the epididymis. When they move through
the vas deferens up to the ejaculatory duct,
they actually aren’t moving. Their flagella aren’t beating. But once they mix
with seminal fluid, then they start
beating their flagella. And they become capable
of moving on their own. So that’s the first
step in capacitation. The second step
in capacitation is a weakening of the cell
membrane of the sperm. This makes it
easier for the sperm to dump its acrosomal enzymes
on the structures that surround and protect the oocytes. And this actually happens
once the sperm enters the female reproductive tract. There’s a molecule that
inhibits capacitation — that’s produced by the male. And that molecule
gets diluted out once semen enters the
female reproductive tract. Now, the second part
of capacitation, making it easier to release
the acrosomal enzymes, is necessary because
the oocyte is actually protected by a couple of
surrounding structures. You have the corona radiata. And the corona radiata are
the follicular granulosa cells that surround the oocyte. You can’t really see
too many on here, but if I could sketch
in where they are, they’d basically be
this halo of granulosa cells around the oocyte. So sperm cells need to be
able to penetrate that halo, and penetrate
between those cells, in order to gain
access to the oocytes. The other structure that the
spermatozoa need to penetrate is the zona pellucida. The zona pellucida is this
thick glycoprotein coat around the oocyte. If you’re looking on
here, this structure is the zona pellucida. It’s like, if the
oocyte is the yolk, the zona pellucida is
the white around it. So it’s glycoproteins, it’s made up of protein with
some carbohydrate molecules attached to it. It’s produced by both the
oocyte and the granulosa cells. It, actually– some of the
proteins in the zona pellucida act as receptors for
the sperm to bind so the sperm can kind of
latch onto the zona pellucida. And it’s almost sort of like a
soft shell around the oocyte. And, in fact, when the
fertilized ovum loses the zona pellucida before implanting
in the uterine wall, that process is called hatching. Even though it’s not quite the
same process as, say, a chicken hatching out of its egg. Now, there are multiple steps
in the process of fertilization. And this slide walks you
through each of those steps. You have the oocyte here. You have the zona
pellucida here. And you have the
corona radiata here. So first, the sperm cell needs
to get past the corona radiata. And this is helped
by an enzyme that’s present on the surface of the
sperm, called hyaluronidase. Hyaluronidase breaks
down hyaluronan, which is part of the matrix
of the corona radiata. So it kind of burrows in between
the cells separating them. Now, once the sperm cells get
through the corona radiata, they encounter the
zona pellucida. And, as I said,
the zona pellucida has receptors for sperm cells. Once the sperm binds
to these receptors, that causes an
increase in calcium in the sperm cell,
which triggers something called the acrosomal reaction. The acrosomal reaction
is just a breakdown of the plasma membrane and
the acrosomal membranes so that the acrosomal contents–
those enzymes in the acrosome –get released onto
the zona pellucida and can, kind of,
eat through it. Enzymes that will break
down the zona pellucida. So in step three,
those acrosomal enzymes eat through the zona
pellucida creating a pathway to the oocyte. Now, the zona pellucida
surrounds the entire egg. It takes hundreds of
sperm cells, at least, to actually get through
the zona pellucida. One sperm cannot carry enough
acrosomal enzymes to do it on its own. So even though it’s only one
sperm that actually delivers its genetic material
to the oocyte, it takes hundreds to get a
single sperm to the oocyte. But once that sperm
does make it through, it binds to receptors on
the surface of the oocyte. The plasma membrane of the
sperm and the oocyte fuse so that the nucleus and
the head of the sperm enter into the oocytes. Once the sperm nucleus does get
into the oocyte’s cytoplasm, that triggers a rise
in calcium levels that sets off the
cortical reaction. And in the cortical reaction
you get exocytosis of these pre-formed granules that
are sitting there just under the surface– just under the plasma
membrane of the oocyte. And once the sperm
nucleus goes in, these vesicles are released. They dump their
contents all over, completely around the
surface of the oocyte. And there are two main effects
of this cortical reaction. You get a hardening
of the zona pellucida and you get clipping off
of the sperm receptors. So, basically, what this
does is this makes it so that you only get one
sperm fertilizing that oocyte. That’s what you need to
make a viable embryo. If you had multiple sperm nuclei
entering into that oocyte, it wouldn’t be viable. You’d have too many chromosomes, you’d have too many genes. That would not be
a viable embryo. So as soon as one sperm makes
it through into the oocyte, the oocyte locks
everyone else out. Hardens the zona pellucida,
cuts off those sperm receptors, nobody else gets in. This figure summarizes
the very earliest stages of embryonic development. This first box here is
basically the last slide. So fertilization, the very first
step in creating an embryo. Fertilization triggers the
oocyte to complete meiosis. And you get formation of
the second polar body. You also get a bit
of re-organization and some minor
structural changes in the nuclei of both the
oocyte and the sperm cell, forming the female pronucleus
and the male pronucleus. As these pronuclei, male
and female, are forming, the spindle fibers start to
form, setting the cell up to begin cell division. Before that can happen
you have amphimixis. The pro-nuclei lose
their membranes and the genetic
material intermixes. So amphimixis is basically the
fusion of the two pronuclei, where you go from
two haploid nuclei to one full diploid
set of chromosomes. The first few cell
divisions are referred to as cleavage because they
happen very, very rapidly. And the ovum– the fertilized
oocyte is now called an ovum –it basically divides, and
divides, and divides rapidly without growing in size. The cells don’t actually
increase in size. So what you end up with is
numerous small cells still contained, at this point,
within the zona pellucida. This cleavage stage continues,
just dividing over and over and over until implantation. This figure goes
through the steps involved in implantation of an
embryo into the uterine wall. The embryo, or to
be more technical, the zygote, at this stage, is
referred to as a blastocyst. Zygote is the earliest
stage of development. And then you have the embryo. And then, later on,
you have the fetus. The zygote, at this stage, is
referred to as a blastocyst. The term “cyst” refers
to a fluid-filled cavity. And the reason why this
is called a blastocyst is because you have
a fluid center that’s contained by a layer of cells. This layer of cells is
called the trophoblast. This trophoblast
is what secretes human chorionic
gonadotropin that can be detected early on
in pregnancy, in the blood and urine of a pregnant woman. You also have, to one side
of this food filled cavity, you have the inner cell mass. And it’s this mass of cells
that develop into the fetus, and eventually become the baby. Now, implantation usually
occurs about a week after fertilization. There are ovarian
hormones– progesterone, primarily –produced by
the corpus luteum that prepare the endometrium
for implantation. That make sure that
it is well-developed, that the uterine
glands are producing the glycogen the zygote needs
in order to fuel development. It’s a complex process. The endometrium has to
produce the right hormones and the right growth factors to
promote attachment and invasion by the embryo. The majority of
pregnancy losses occur before or during implantation. It’s really not all that
uncommon for an embryo to not make a
successful implantation. During a successful
implantation the embryo basically digs its way into
the endometrium of the uterus. The trophoblast, cells
of the trophoblast, secrete enzymes that break
down the connective tissue between the uterine
epithelial cells so that the trophoblast itself
can invade into that wall. Then, the uterine
epithelium of the mother basically repairs
itself and builds a structure over the embryo. So the embryo, once it’s
successfully implanted, is completely surrounded
by endometrial tissue. So you have maternal tissue
there, maternal tissue surrounding the embryo there. And the embryo itself develops
here, completely embedded within the uterine
wall, out of contact with the uterine cavity. This is where the embryo will
continue to grow and develop through the next nine months. After spending some time
studying this video, you should be able to
address these first two questions, in particular. Talk about the processes
of fertilization and talk about what implantation is. I will say, implantation almost
always occurs in the uterus. If implantation occurs
anywhere other than the uterus, it’s referred to as
an ectopic pregnancy. And if it occurs in the
fallopian tubes, which really don’t have space for
a developing fetus, it can be very dangerous
for the mother. It’s not a viable
pregnancy for the child. And if it’s not taken out, it
can kill the mother, as well. Ectopic pregnancies
in other locations that do have some space
for the fetus to grow are still very, very dangerous
for both mother and child. Like in the abdominal
cavity, I mean. They’re still very
dangerous, but they have some potential
to be viable with careful medical supervision. All right, so you should
also be able to discuss some terminology. You should be able
to talk about what the corona radiata and
the zona pellucida are. You should be able to discuss
the process of capacitation, what it means for the sperm,
what it does to the sperm. And amphimixis,
what amphimixis is.

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