In this talk we will begin looking at how implantation is followed
by development of the supporting structures of the embryo,
the placenta and the umbilical cord.
So we've seen how the embryo develops from the morula and then the blastocyst,
but now we're gonna look at how another group of cells on the outside of it.
The trophoblast are gonna grow into the uterine lining
and actually set up the supporting tissues
that allow effective exchange of oxygen and other gases
between the mother and the fetus while not allowing their blood supply
to literally come into contact with each other.
The placenta and umbilical cord are the major products of this process
and as the placenta and umbilical cords enlarged,
they are able to handle the metabolic demands placed on them by the fetus.
As this is happening, fluid filled spaces
such as the chorion and the amniotic cavity
develop around the embryo and should the zygote split
or more than one zygote implants simultaneously
will have a variety of confirmations of the chorion
and the amnion and the membranes that surround them
when we have more than one birth
taking place and twins developing or multiple children developing.
So with implantation, we have essentially got a blastocyst
with an inner cell mass, often one side of the cell,
known as the embryoblast and it will become the developing embryo in fetus.
On the outside we have the trophoblast and those are the cells
we're gonna be following in detail during this lecture.
As implantation occurs, the trophoblast is going to allow this blastocyst
to migrate into the uterine lining to support the fetus.
The first thing that's going to happen
is that the blastocyst will contact the endometrium
and the follicular layer of the uterus.
You may recall that the uterus has very much thickened its wall
to allow for possible implantation and nutrition of the embryo to occur.
The cytotrophoblast is going to be the name of the group of cells
that surround the developing embryo,
but just outside of the cytotrophoblast we have another layer of cells
that do something very interesting.
These cells migrate into the uterine lining
and lose their individual cell membranes,
so we essentially get an invasive syncytiotrophoblast,
a group of cells that lose their cellular distinctions
and are essentially a migrating mass of freely floating nuclei
that move into the uterine lining
and allow the developing embryo to follow after them.
So as the syncytiotrophoblast erodes the uterine lining,
it helps pull the developing embryo in
and by day eight the blastocyst has begun migrating deeper and deeper into the embryo
and will soon start to encounter uterine vessels and uterine glands.
As this is occurring, the embryoblast is split into a hypoblast and epiblast,
therefore, it's entered the bilaminar embryonic phase
and we may even be able to see
a little bit of the amniotic cavity coming into existence
just above the epiblast cells.
As the syncytiotrophoblast continues to enlarge,
it begins to surround the developing embryo all together.
As this is occurring, it starts to erode nearby maternal vessels
and maternal blood will spill into the syncytiotrophoblast region.
These are gonna be called the trophoblastic lacunae
and they're basically islands full of maternal blood.
Now this sounds like it would be a good point for nutrition to occur,
but even though some oxygen and other nutrients
are coming through the maternal blood at this point,
it's not really developed to the point where there's a good linear flow
to bring gases and nutrients to the developing embryo.
That process will continue
as the syncytiotrophoblast and cytotrophoblast further develop.
As all of this is occurring, the extra embryonic mesoderm has developed inside
the developing embryo and is separating the bilaminar embryo
from the cytotrophoblast.
So the extra embryonic mesoderm is beginning to push the developing embryo
further away from its nutrient source, the cytotrophoblast.
By days 10 to 12, we've got the embryo finally migrated entirely
into the uterine lining.
So there may be a small closing plug marking the place where its moved in,
but otherwise its developing entirely inside the uterus at this point.
The trophoblastic lacunae have enlarged and various uterine glands
are now emptying into the syncytiotrophoblast.
While all of this is happening,
we're starting to get a little bit of organization to the flow of blood
into this trophoblastic lacunae,
and on the embryo side we're going to have spaces developed
in the extra embryonic mesoderm
and these lacunae are going to fuse and enlarge and create what's known
as the extra embryonic coelom or a space between the developing embryo,
the yolk sac and the cytotrophoblast on the outside.
By day 13, the cytotrophoblast starts to get involved
in organizing the placenta.
Growths of the cytotrophoblast,
that's the distinctive layer of cells on the outside of the developing embryo,
and its extra embryonic mesoderm grow upwards into the syncytiotrophoblast
and these upgrowths are called primary villi
and they're exclusively made of cytothrophoblast cells.
As that is all happening,
the extra embryonic mesoderm has split and the little cavities,
the lacunae that were present,
will then enlarge tremendously to form an extra embryonic coelom
and this is also gonna be called the chorionic cavity.
So the chorionic cavity is surrounding the developing embryo and the yolk sac.
The amniotic cavity is still exclusively hanging out
above the epiblast cells of the bilaminar embryo.
And then we can also change a couple of names
because we don't do that enough
and start calling the extronic means extraembryonic mesoderm
outside of the embryo,
the cytotrophoblast and the syncytiotrophoblast, the chorion.
So the chorion is gonna be the entire layer on the outside
with the developing embryo inside the chorionic cavity.
By day 14, we've moved the primary yolk sac into two pieces.
The secondary yolk sac remains in contact with the developing embryo
and remnants of the primary yolk sac have pushed themselves away from it
to the opposite side of the extra embryonic mesoderm
and are only tethered to the developing embryo by a small thread.
Likewise, a small thread of extra embryonic mesoderm
known as the connecting stalk
holds the embryo to the rest of the extra embryonic mesoderm and chorion
and this will eventually become the umbilical cord
and it's the only attachment of the developing embryo to the chorion
which is going to be the extra embryonic mesoderm outside of the cell,
outside of the embryo, pardon me,
and then the cytotrophobast and syncytiotrophoblast.
While all of this is happening,
those primary villi of the cytotrophoblast have grown taller and taller
and move closer and closer to the lacunae full of maternal blood.
These primary villi are exclusively made of the cytotrophoblast,
but the underlying extra embryonic mesoderm
is going to proliferate and actually fill in underneath them.
Once we have a core of extra embryonic mesoderm,
they're now called secondary villi and sooner or later
we're gonna have blood vessels forming within that mesoderm,
and once blood vessels are present we are gonna call those tertiary villi.
So primary villi are just cytotrophoblast,
secondary villi cytotrophoblast and extra embryonic mesoderm
and tertiary villi are all three,
the cytotrophoblast, extra embryonic mesoderm and blood vessels.
And these blood vessels develop at the same time
as blood vessels in the umbilical cord
and the developing heart and the vessels of the embryo
so that when the heart starts beating there's already a readymade pathway
for blood to travel to and from the placenta back into the embryo.
Now, moving a little further along,
the syncytiotrophoblast reaches its outer limit
and is going to form bumps that are in contact with the uterus.
These are caller cotyledons
and they are the outermost extent of the fetal placenta.
Within each cotyledon, the cytotrophoblast is gonna leave a little gap,
a little hole that allows maternal blood
to travel into the developing placenta
so maternal blood can move in through the cotyledons
and then perfuse the spaces around the placenta
and then pull the blood back out from maternal veins.
So these intervillous spaces are gonna be how maternal blood gets to and from
the close association that needs to have
with the embryonic blood for gas exchange to occur.
At this point, we can refer to the entirety of this construct, the cotyledons,
the intervillous spaces as the placenta
and we do indeed have blood moving from the embryo out and then back
and we have blood from the maternal side coming in to each cotyledon
and the intervillous spaces
and then leaving after it is given nutrients and oxygen
to the developing embryo.
Thank you very much for your attention.