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.