Hello! I am Dr Aizaz from medicovisual.com and
in today's visual lecture we will talk about fate map during the gastrulation process. As we
have discussed previously in the lecture of somatogenesis and changes in the intra-embryonic
mesoderm that if we peel off this ectodermal layer of the embryo the intra-embryonic mesoderm will
be revealed. Now what we have discussed already that this intra-embryonic mesoderm it is not
a single homogeneous undifferentiated layer instead it consists of different parts and those
parts are here is the neural tube and notochord. This neural tube of course is not the part of
intra embryonic mesoderm but just remember that it is there and along with that this is the notochord
which is also called axial mesoderm and then we have par axial mesoderm which runs parallel to
this axial mesoderm but slightly laterally and more lateral to paraxial mesoderm the most lateral
structure is the lateral plate mesoderm and in between the par axial and lateral plate mesoderm
is the intermediate mesoderm. So, what we have observed is that intra-embryonic mesoderm consists
of different components. Now how these different components of intra-embryonic mesoderm are formed
during the process of gastrulation? Here is the ectoderm and here is the endoderm and here I have
made this ectoderm slightly transparent. So, you can see what happens. So, during the process of
gastrulation in third week the cells that ingress downward from this primitive node or henson's node
they will form the notochord and pre-chordal plate and the cells that ingress downward from this
primitive streak they will form different components of intra-embryonic mesoderm, they will
form all those components the paraxial mesoderm, the intermediate mesoderm, the lateral plate
mesoderm and even some contribution towards the extra embryonic mesoderm. Now let's see what
happens. So, here is the ectoderm, here is the endoderm, the primitive node and primitive streak.
Let's make the endoderm transparent and now first cells that go from this primitive node they will
form the the prechordal plate and the notochord and now the cell that, that ingresses
downward from the cranial most part, from the cranial most part of primitive streak
and some cordolateral part of the primitive node these cells that ingress from this point they will
form the par axial mesoderm. Right, now the point to understand here is that the fate of these
ingressing cells is determined by the location from which they ingress downward. For example:
these cells that ingress downward from the cranial part of the primitive streak they will form the
par axial mesoderm and then let's move forward here are those cells which ingress from the
midstream, from the middle part of the primitive streak these cells will form the intermediate
mesoderm and then those cells which ingress from the caudal part of the primitive streak they
are going to form the lateral plate mesoderm and finally the cells that ingresses from
the caudal most part of this primitive streak they will contribute towards the extra embryonic
mesoderm. Now extra embryonic mesoderm it, it is thought to be formed from the hypoblast cells
during third week of development but there is also some contribution done by these ingressing
cells in that third, during the third week of development. Now what you can understand from
this diagram is that the medial most component, the medial most component of the intraembryonic
mesoderm that is notochord here it passes from the cranial most part of the of the primitive streak
and primitive node and lateral most part of, of this intra-embryonic mesoderm it is contributed
by those cells that ingress from the caudal most part. For example; here you can see that the
lateral most part is the lateral plate mesoderm and it is contributed by those cell which ingress
through the caudal most part of the primitive streak. So, the primitive streak and primitive
node they are thought to be the real guiding stars during this process of gastrulation. Now
by the way what is the extra embryonic mesoderm? We have already talked about extra embryonic
mesoderm during the second week of development but let's briefly reorient ourselves
with the extra embryonic mesoderm. Now you must be aware of the fact that this
ectoderm it continues dorsally with the amnion. Right? And this, this endoderm it
continues ventrally with another balloon-like structure that is called yolk sac. So, the
embryo is provided with two balloons the amnion and the yolk sac and these two balloons
along with this embryo whole the structure it is plastered outside, it is surrounded
outside by the extra embryonic mesoderm. So, this is the extra embryonic mesoderm. So,
extra embryonic mesoderm during it is formed during the second week of development and it is
thought to be mainly formed by the hypoblast cell but it is also contributed by the cells
that ingress from the caudal most part of the primitive streak. These cells they also
contribute to the extra embryonic mesoderm. So, this was about the fate map during gastrulation.
Thank you so much for watching this video.