In this talk, we will follow the progress of the earliest steps in embryology.
From fertilization, to formation of the bilaminar embryo.
We´re gonna pick up right here.
At the very moment after fertilization has occurred.
We can see that a spermatozoa has just entered the oocyte.
It has punctured through the protective barrier of cells
and the zona pellucida to reach the oocyte and its cytoplasm.
As soon as the single spermatozoa has entered the cytoplasma of the egg,
it sets off a chain reaction of events
that prevent additional spermatozoa from making it in.
At that point, the one male pronucleus consolidates inside the cytoplasm
and then, it begins to associate with the female pronucleus.
Once these two fuse and come together,
we have now got the genetic material making up a brand new and distinct individual.
At this point, we have the diploid complement of chromosomes
that can be used as the blueprint for all the subsequent formations
and changes that will bring about the human being
that can grow from this single cell.
As these nuclei have formed, a series of events can begin
that have been on hold for quite a while in the egg.
In particular, mitotic division can start taking place again
and the chromosomes can go through the whole process
of metaphase, anaphase, telophase,
and produce subsequent generations of cells that are genetically similar,
in fact, identical to the progenitor cells.
Roughly 30 hours after fertilization,
the first split is going to occur and have two daughter cells
to take the place of the single zygote.
Now, at this point, subsequent divisions are going to occur
and the resulting cells will not grow appreciably.
In fact, they´ll take up as much space as the original single celled zygote did.
So they will get smaller and smaller even as they become more numerous.
At this point, we´re gonna refer to the entire assemblage
as a morula, a collection of cells,
and we´re gonna take a look at the nine cell stage
roughly two and a half days into development.
At this time, we´ve got something about the same size as the zygote
and it´s gonna consist of a group of cells clustered in the center,
the inner cell mass and the surrounding outer cell mass.
During this time, the protective barrier that existed around the developing zygote morula,
the zona pellucida begins to break down
and fluid begins to permeate into the spaces between these cells.
Thereafter, we´re gonna go a bit further to a stage of development called the blastocyst.
The fluid that has moved into the spaces between the cells of the morula
creates a cavity called the blastocyst cavity
and the entire assembly of fluid and cells is referred to as the blastocyst.
Now, the early blastocyst has roughly 58 cells
and it will continue to develop to roughly, the 107 cell stage after five days
which is gonna be called the late blastocyst.
It looks about the same with a fluid filled cavity surrounded by cells on the outside
and a cluster of inner cells pushed off to one pole of the blastocyst.
The inner cell mass and outer cell mass will form very distinct structures.
As we proceed form the morula to the blastocyst, and further on into development,
we´ll see that the outer cell mass transitions
into a set of structures called the trophoblast.
And that the inner cell mass is going to become the embryoblast
and the embryoblast is what will generate all of the tissues of the actual embryo and fetus
whereas the trophoblast will become the supporting tissues
of the placenta and the umbilical cord.
Now, let´s take a look at how development proceeds from the blastocyst
into the bilaminar embryo
and during this time, we have the embryo moving into the uterine wall
from day seven to 15.
Initially, the inner cell mass which will form the embryoblast
subdivides into a thin layer of cells in contact
with the blastocyst cavity called the hypoblast
and a taller group of cells further away from the blastocyst cavity
called the epiblast.
The epiblast and hypoblast cells have distinctive fates.
The hypoblast cells are going to be proliferating to surround the blastocyst cavity.
They are small and cuboidal.
Whereas the epiblast cells grow taller and taller,
and are going to be the cells
that actually create the entirety of the embryo from this stage on.
Now, moving into the uterus,
the developing blastocyst is going to have these epiblast cells
and hypoblast cells do a few different things.
In particular, a small cavity is going to form in the epiblast cells
called the amniotic cavity.
Now, this tiny little pinpoint of a cavity may not look like much right now
but it will be the cavity that surrounds the entirety of the embryo
and fetus forming the water that supports
and nurtures it throughout development.
So the amniotic cavity will eventually surround the entire fetus.
As we go further and further into the uterus,
the hypoblast cells are going to proliferate and spread out across the blastocyst cavity.
Once they´ve completely done so and extended
what´s sometimes called Heuser´s membrane throughout that space,
that entire cavity will transition to become the primary yolk sac.
Moving a little further along, the primary yolk sac
and embryo becomes separated from the surrounding trophoblast cells
by a distinctive set of cells called the extraembryonic mesoderm.
This extraembryonic mesoderm will not contribute anything of substance
to the actual embryo but allows it to develop fully inside the uterus
and separates it from the placenta.
This happens because some cavities
are going to start appearing inside the extra embryonic mesoderm
and these cysts or little cavities are going to form, fuse,
and become what´s known as the extraembryonic coelom.
A space that forms outside of the developing embryo
and as it enlarges and completely wraps around the yolk sac
and the embryo, it will form what´s called the chorionic cavity.
Initially, the chorionic cavity will surround the embryo
and only much later be replaced by the expanding amniotic cavity.
So as we move a little further along,
the expansion of the chorionic cavity pinches the yolk sac
in such a way that a portion of it moves away from the embryo
towards the opposite pole of the developing embryoblast
and the part of the yolk sac that remains in contact with the embryo
is going to be distinct
and it´s going to form what´s called the definitive yolk sac.
The portion that moves away is going to form remnants of the primary yolk sac
called exocoelomic cysts.
Now, the secondary or definitive yolk sac stays in contact
with the epiblast cells that are developing into the embryo
and have an ever enlarging amniotic cavity within.
Now, here, as the chorionic cavity and the placenta has grown larger and larger,
we can see that the extraembryonic mesoderm
is lining the developing placenta and chorionic cavity
and is only connected to the developing embryo
by a single small connecting stalk.
That connecting stalk is what connects the developing bilaminar embryo
to the developing placenta
and that is what will become the umbilical cord.
Thank you very much for your attention and I´ll see you during our next talk.