Hello! I am Dr Aizaz from medicovisual.com and today we will talk about the development of
notochord. How the notochord develops? So, let's start the video.
So, here is the trilaminar germ disc of the embryo at the start of third week of development and
as you know the notochord it start forming during the third week of development
so here is the primitive node and inside it is the depression called primitive pit and this
is the primitive streak along with the central midline depression called primitive groove and
of course this is the oropharyngeal membrane. Now let's cut a section here. Let's remove
this part and let's see from the front not from the front from the side that how it looks like.
So, here is the cross section of the embryo. As we have discussed previously that this structure
is called prechordal plate and this is the primitive node along with this depression called
primitive pit and this is the prechordal mesoderm. So, this this caudal part of the prechordal plate.
Cells here, they proliferate and they form the prechordal mesoderm here. We have discussed all
these details in a previous lecture and here is the oropharyngeal membrane and by the way this is
the cloacal membrane and as you must be aware of the fact that this bluish thing it is the
embryonic ectoderm and this greenish thing it is the embryonic endoderm. Now what happens
that some of the cells they will ingress and they will go downward from this, from this part
and they will form a process and invagination or process in between the ecto and endoderm in the
central midline this process is called notochordal process. Now let's zoom this out and let's
cut a section from this part as well and here is the cut section here. So, we have,
we are looking from the side as well as we can look at from the front. So, two sections are here
and here is the notochordal process and here too is the notochordal process. If you can see here
that we have cut it almost from this part and here you can see that this is the notochordal process.
Then what happens that some of the central cells here they will undergo programmed cell death or
apoptosis and this, this notochordal process it will undergo what we call as canalization.
So, as it undergoes the process of canalization a canal is formed in between them and we call it
notochordal canal. So, this is a notochordal process and in between them there is a canal
in the centre of this notochordal process there is a canal that is called notochordal canal.
This notochordal process with its notochordal canal it continues to grow until it reaches
the prechordal plate then it further grows and then it intercalates into the
underlying embryonic endoderm. Here in this diagram you can also see
that this notochord process with the notochordal canal it became intercalated into this embryonic
endoderm. Here in this diagram you can also see that this notochord process
it intercalates into this embryonic endoderm and it slightly displaces it laterally in this part
then what happens that the floor of this notochordal process or floor of
this notochordal canal which is also the floor of this endoderm in the midline
it starts undergoing the process of apoptosis and ultimately the floor is lost.
Now it is not the notochordal process but it is a curved plate and we call it notochordal plate.
As notochordal plate forms there is now a free communication between this part and this part
of the embryo. As you know that in the central midline this ectoderm of the embryo it is going
to form the nervous system and here this endoderm it will form the GIT, gastrointestinal system.
So, here is the communication that connects the primordium of central nervous system to
the primordium of gastrointestinal system. So, this communication or canal is called
neurenteric canal. This neurenteric canal, it is connecting the two cavities temporarily. Actually,
it is a temporary connection later on we will see that as the notochord completely forms this
neurenteric canal is obliterated. So, it is temporarily connecting two cavities and what are
those two cavities you must be knowing that above this ectoderm is the amniotic cavity and below
this endoderm is the yolk sac cavity. So, it is a temporary connection between the amniotic cavity
as well as the yolk sac cavity. But what are the functions of this neurenteric canal? Sadly more
than a century has been passed since it was first discovered but still there is no properly known
function of this cavity but there are certain hypotheses to explain the functions of neurenteric
canal. what are those hypotheses? Number one is that it is there to equalize the pressure between
amniotic as well as yolk sac cavity. The other is that it may be an evolutionary vestige in
the humans and it may not have any functions in the human beings. The third hypothesis
is my own hypothesis and I propose that during this time period some substances may be required
to be exchanged between the amniotic as well as yolk sac cavity. So, this neurenteric canal it may
have some role in exchange of substances between these two cavities. I think some of you will still
be confused about the neurenteric canal so if want you to become really crystal clear about
this structure so let's have a 3D tour of this structure. So, that it becomes really clear to
you. So, here is the embryonic ectoderm and here is the embryonic endoderm and here you can see
that this notochordal plate it is intercalated in the midline of this embryonic endoderm and here
is the primitive node and by the way here is the above this is the amniotic cavity and below this
is the yolk sac cavity which is not shown here. So, here is the primitive node. Let's say you see
this beautiful hole and you want to rush inside this hole. Of course, my friend do not try to
enter each and every hole you ever see in your life. But for the sake of science let's enter into
this hole. So, here we are traversing into this hole. Yeah! Let's so, here is the primitive pit
and we are going inside the primitive pit and here is the rhis, what is this? notochordal plate
and now we are in between the endo and ectoderm and if we continue to traverse
oh my god I have fallen down. So, here we are and we have fallen into what cavity here this is the
yolk sac cavity. So, actually we started from the amniotic cavity and we've fallen into this yolk
sac cavity and we’ve, we fallen into this cavity through this neurenteric canal. And if we, so,
here you can see this is the embryonic endoderm, right? So, here is the embryonic endoderm and
similarly we can enter this hole from downward and if we try to enter into this hole from downward
here we will come out through the primitive node and here what we will come across is the
which cavity? Amniotic cavity. So, this is basically the neurenteric canal which is
connecting the amniotic cavity with the yolk sac cavity and it is connecting it
temporarily. Now let's cut a section of this embryonic ectoderm and let's see what happens?
So, here is the embryonic ectoderm which has been cut from here and here you can see that
this structure, this structure here is the notochordal plate. So, here is the
notochordal plate and this was the primitive pit and here is the neurenteric canal and now let's
cut the section from here from the front. So, now we have cut the section from front and this
is the diagram of what we call the prechordal not prechordal, notochordal plate and this is
what you are used to seeing in the textbooks and here again you can see the neurenteric canal here.
After this 3D tour of neurenteric canal let's move forward to the development of notochord.
So, right now this notochord is at the stage of notochordal plate now let's zoom into this diagram
and let's see what happens next? What happens is that this notochordal plate it rolls up inward
like a chicken roll without chicken inside it and as it rolls and it becomes closer to each other,
it’s ends, they become closer to each other these ends of the embryonic endoderm they also come
close to each other ultimately it rolls almost completely and it then moves upward and it further
grows and it becomes a flexible but solid rod. Here in this diagram you can also see that here
is that flexible or solid rod that's what we call as notochord. So, this structure is called the
notochord. Notochord is the structure due to which the phylum chordata is named so and by the way you
must be remembering the humans they are also the member of phylum chordata. What is the function of
notochord? Notochord is very important signalling structure for different surrounding structures.
It will signal the overlying ectoderm to form the neural tube and it will also signal in
formation of somites and some other structures we will discuss the details in later lectures.
In humans the notochord ultimately degenerates. However the remnant of notochord remains in
children's vertebral disc as nucleus pulposus. This is the structure called nucleus pulposus.
So, this was about the development of notochord. Let's have a review of this.
We can study the development of notochord in certain steps.
The first step is that this notochordal process develops and then this notochordal canal forms
as the central cells they undergo apoptosis the central nodchordal canal forms.
In the step three what happens that it further grows and it reaches towards the
prechordal plate then this notochordal process it intercalates into the embryonic endoderm
and then the cells forming the floor of notochordal canal they undergo apoptosis
and ultimately the floor of notochordal canal is lost and this temporary connection
between the amniotic cavity and yolk sac cavity called neurenteric canal
it develops. So, here is the neurenteric canal. Notochordal plate then rolls inward
and finally a flexible solid cord called notochord is formed. So, this was the development of
notochord. So, here is the complete review slide you can pause the video and you can
further see this diagram. Now let's have a 3D tour of notochord. So, here you can see,
you can now recognize the structure that is it is the embryonic ectoderm and here is the embryonic
endoderm. Now let's cut a section from here and let's see the notochord. So,
here is the section of this embryo and here you can see that this structure is the notochord.
So, here is the notochord and as you can see that it start from the what is the structure?
The primitive bit and it reaches up to this prechordal plate and specifically the prechordal
mesoderm. Now let's cut the section from here. First let's uncut this and then we will cut
a section from the front. So, here is the front cut section and here you can see that this is the
notochord. so, this is the notochord and you can see that it is made up of
cells and these cells are arranged in such a fashion that it becomes a solid
yet flexible cord and this is the diagram that you are used to seeing in the textbook and all around
is here is the mesoderm which is an embryonic mesenchyme. So, this was the 3D tour of notochord.