Welcome to this talk on the major events that occur during the process of embryonic development.
Now, what are the things that always bothered me when I was studying embryology?
Was that we could study the formation of the heart,
formation of the lungs, formation of the limbs,
but I never got a sense of how it all fit together
and how the various events are organized altogether
chronologically as we move through the embryologic period.
And that?s exactly what this lecture is going to do
is discuss how every organ system is developing as we proceed from a single cell zygote
all the way to the end of the embryonic period and the beginning of the fetal period.
Because of that, I heavily recommend you view several of the other topics first
and make sure you have a fairly robust understanding of embryology
before coming to this lecture which will hopefully bring it all together.
Now as we follow the process of pregnancy
we typically split it into three trimesters
and of roughly thirteen weeks or three months each.
The embryonic period only takes up the first eight weeks of that entire process.
The first eight weeks of the first trimester
is pretty much what we spend our entire time viewing
because that?s the time all of the organs system
are coming from nothing and creating their more or less
final appearance, and then, develop further, elongate and grow
during the rest of the fetal period.
Embryonic growth at this time is usually calculated
via the ultrasound technique
of measuring the crown to rump length which is illustrated on the screen.
Looking at the longest distance we can cover
between the crown of the head and the rump of the body.
Another common measurement that you used
as the fetus gets older is going to be measuring the crown to heel length
while we measure the line from crown to rump, rump to knee, and knee to heel.
So starting off on day 1, Fertilization.
We have fertilization.
The sperm meets the egg and the male and female pronuclei
come together as the sperms penetrates the zona pellucida.
On day 2, we have our first split and we enter the two-cells stage.
By the time we get to day 3, we have reached the morula.
At least eight cells comprising an inner and an outer cell mass.
By the time we get to day 4,
the zona pellucida has started to breakdown just a bit
and fluid has penetrated it.
This is gonna form the early blastocyst
as the blastocyst cavity is coming into existence.
As we move to day 5, we enter the late blastocyst stage
with the distinct blastocyst cavity but also a distinct embryoblast,
a cluster of inner cells on one side and trophoblast cells surrounding it.
As we move in to day 6 and 7,
we?re gonna begin the process of implantation.
For that to occur, the trophoblast is going to need to distinguish itself
into this cytotrophoblast,
which maintains the distinctions between nearby cells and syncytiotrophoblast,
which looses it?s individual cell walls
and becomes an invasive, multinucleated structure
that helps the embryo burrow into the uterine lining.
So implantation into the endometrium begins once the syncytiotrophoblast
begins migrating and pulling the embryo with it.
At this time, the blastocyst has the bilaminar embryo start to form,
so that embryoblast is gonna separate into a hypoblast and epiblast.
As we move into day 8, not only do we have hypoblast,
series of cells in contact with the early yolk sac
which is the remnant of the blastocyst cavity,
but we have taller cells up above them, the epiblast,
developing their own cavity,
and we can see that early amniotic cavity present in this picture end on day 8.
As we move into day 9,
we?ve implanted far more completely into the uterine lining
and the primitive yolk sac has formed
as the hypoblast cells have migrated around
and completely covered the blastocyst cavity.
Lacunae are beginning to form inside the syncytiotrophoblast
as it fills with maternal blood, allowing the maternal blood to reach
the developing embryo for the first time.
As we move into day 10, the embryo is fully surrounded by the endometrium,
it is fully moved in with maybe a small closing plug behind it.
The extra embryonic mesoderm is beginning to separate the epiblast,
the hypoblast, and the yolk sac from the surrounding cytotrophoblast.
Moving in to day 11, the lacunae,
full of maternal blood are getting larger and more distinct
as the cytotrophoblast is allowing the embryo
to stay distinct from the uterine lining,
but the syncytiotrophoblast keeps moving further and further in
opening up that maternal vessels and glands.
Inside the extraembryonic mesoderm we have little cavities beginning to form
that will enlarge and form the chorionic cavity,
which will eventually separate the developing embryo and yolk sac
from the developing placenta.
By day 12, the secondary yolk sac is formed
and we?ve moved the little remnant of the primary yolk sac
to the opposite side of the developing embryo.
Moving into day 13, we can see that the primary yolk sac is gone,
the secondary or definitive yolk sac is present
and little remnants of it are on the other side of the cell.
In terms of the placenta,
we start to see primary villi of the cytotrophoblast growing outward
into the syncytiotrophoblast.
Day 14, we?re gonna focus now on the embryo itself.
This view is showing us the roof of the amniotic cavity having been exposed,
and we?re looking at the epiblast cells.
The connecting stalk is holding this developing embryo
close to the cytotrophoblast and will eventually form the umbilical cord.
The prechordal plate is not visible, but it?s beginning to be noted
by the cells that are in the epiblast and the hypoblast,
and the developing embryo will orient itself relative to that prechordal plate,
and this marks where the eventual head is going to form.