Hello! I am Dr Aizaz from medicovisual.com and
in this visual lecture we will talk about what is prechordal plate? how this structure develops?
And what is the function of this prechordal plate? Prechordal plate is one of the most
misunderstood structures of the embryo but I’m gonna untangle it and simplify it
for you. So, let's start the video. So, this is the embryo at the start of third week of
development and you are looking it from the top and what you will see is that here is the
epiblast layer or ectodermal layer of the embryo and here you will see that this is the primitive
node. This swelling is called primitive node and there is a circular depression within the
centre of primitive node called primitive pit. And here you can see that this is the primitive
streak and as the cell ingresses downward from its midline there will be, there will be this
depression form that is called primitive groove and as you know from the lecture of gastrulation
that it continues cranially with the primitive pit we have discussed all these details in the
lecture of gastrulation. Now let's focus our attention to this special structure here. Here you
can see that in the ectoderm there is a circular depression here and this is the primordium of
future mouth of the baby and that is why it is called oropharyngeal membrane or according to some
author the buccopharyngeal membrane. So, now I am gonna rotate this embryo and now let's see what
is there and at the underside of this embryo. So, here is the embryo from the underside and here again you can see that this is the
oropharyngeal membrane and this whole, this layer is the hypoblast layer or endodermal
layer. Now let's look it from the front. So, here is the embryo from front and now
let's cut a section from this and remove this front side. So, here you can see that this is the
primitive node with the primitive pit and this is the primitive streak with a primitive groove
here and here is the oropharyngeal membrane. Now you should note an interesting thing here
at this place the epiblast layer or ectodermal layer is tightly adherent, it is tightly
stitched with the underlying a hypoblast layer. So, this is actually the oropharyngeal
or buccopharyngeal membrane and this structure is prechordal plate.
So, what is prechordal plate? So, in this diagram you can see that what is prechordal
plate, it is the hypoblastic part of the oropharyngeal membrane. It is the part of
hypoblast that underlies that, that makes up the oropharyngeal membrane or you can also say that it
is the primitive endodermal part of oropharyngeal membrane. As you know that this hypoblast
it is also called the primitive endoderm. So, during the third week of development as
the process of gastrulation starts the ground substance is laid down between these two layers
the hypo and epiblast layer but interestingly at the, at the place of oropharyngeal membrane
as these two layers are fused together they are tightly sticked together so there will be
very little almost no ground substance here and thus there will be no mesoderm between these
two layers at the place of oropharyngeal membrane. Now what happens that from this primitive pit
some of the epiblast cells they proliferate and they migrate from this place and they move
towards this prechordal plate. Right, so, more and more cells will come and they will follow and they
will reach the prechordal plate and here they will intercalate into this this prechordal plate and
they will displace the original hypoblast which were forming the prechordal plate and now this
prechordal plate is no more formed by hypoblast but now it is formed by the cells the epiblast
cells that came all the way from primitive pit to this prechordal plate and they have displaced
the original hypoblast. Literally till there is not much change in the structure of oropharyngeal
membrane it is just the cells that are displaced. The oropharyngeal membrane as a whole is still a
bilaminar structure. As the gastrulation proceeds all of this hypoblast will be replaced by
the cells that came from the epiblast. Now here something interesting happens the prechordal
plate cells that are lying caudally what they do is that they proliferate and they differentiate
into another layer in between these two cells. So, what is that layer? They will form another
layer here and we call it prechordal mesoderm. So, now the interesting thing to note here is that at
the caudal end this structure is not bilaminar now at the caudal end it, it has become a trilaminar
structure but at cranial end it is still a bilaminar structure because this prechordal plate
cells they also form the prechordal mesoderm. We call the prechordal plate as a mesendrodermal
structure. This is an interesting name that is given by Larson. Later when the neural crest cells
develop what happens that this prechordal mesoderm it is also enriched by neural crest cells. What
are neural crest cells? I know this neural crest cell it sounds an alien concept to you don't worry
we will discuss it in detail in later lectures. But for now you can see that here is the neural
tube and this is the neural crest cells here or they are formed from where they came we
will discuss the details in later lecture. So, in short what happened that the cells at the
caudal end of prechordal plate they proliferated and they differentiated into another layer
here and this is called prechordal mesoderm. This prechordal mesoderm, it will contribute
to formation of mesodermal derivatives of head of the baby as the caudal end of
prospective oropharyngeal membrane became our trilaminar structure. Now the actual
oropharyngeal membrane is not all of this structure now oropharyngeal membrane is only
restricted to the cranial bilaminar end. So, here is the oropharyngeal membrane. Previously whole
this structure was the oropharyngeal membrane but as the caudal end became trilminar this
oropharyngeal membrane it is now restricted to the cranial part only. Because by definition the
oropharyngeal membrane is a bilaminar structure. Later on you will learn that this oropharyngeal
membrane will rupture to form the mouth opening. Now what is the function of prechordal
plate? One of the function that you can see here that this prechordal plate, the
cranial part of this prechordal plate it is forming the endodermal layer, it is
forming the endodermal part of oropharyngeal membrane. The other function of this prechordal
plate is that it is an important structure in induction of forebrain later on in the neural
ectoderm. We will learn those details later. And the third function is that this prechordal
plate is going to form this prechordal mesoderm which is an important structure to form the
mesodermal derivative of the head of the baby. So, as the notochord is formed you can see that what
is the exact position of this prechordal plate? It is lying cranial to the notochord and just
at the cranial tip of a notochord you can see that there is the pre-chordal mesoderm.
This structure is actually the notochord. It is very important to mention here that we have
been just focusing on the oropharyngeal membrane but there is an other membrane as well that is
the cloacal membrane and cloacal membrane is also a bilaminar structure just like the
oropharyngeal membrane. Here also the ecto and endodermal layer are tightly stitched together
with no interspersed mesoderm in between them. Cloacal membrane lies at the caudal end of
the embryo and it will form the opening of urogenital system as well as anus of the baby.
So, let's review what we have read. So, here at the start of third week of development you can
see that this is the oropharyngeal membrane and here the prechordal plate is the structure that
is forming the endodermal layer of or primitive endodermal layer of this oropharyngeal membrane.
Now an interesting point to note here is that it consists, this prechordal plate it consists of
tall columnar cells, this tall columnar hypoblast. Rest of the hypoblast you can see or rest of the
primitive endodermal cells you can see that these are cuboidal cells but here at this place, at the
place of prechordal plate these hypoblast cells, these are specially thickened in such a
way that they are tall columnar cells. Here in stage 2 what happens that as the gastrulation
process proceeds and this hypoblast it is displaced laterally to the yolk sac endoderm.
Here new cells came from the primitive streak and this pre-chordal plate cells they are also
displaced laterally and now the prechordal plate it is formed from the cells that ingressed
downward from the primitive pit still there is not much change in the structure of oropharyngeal
membrane it is still bilaminar structure and in stage 3 what happens that at the
cells at the caudal end of prechordal plate they proliferated and they differentiated to
form the prechordal mesoderm and now only this cranial part is the definitive oropharyngeal
membrane. You know why you people get confused? What you read in textbook is that this
prechordal plate is a bilaminar structure and then you suddenly see that this prechordal
mesoderm it suddenly appears in between this supposedly bilaminar structure and you
are like how come this bilaminar structure has prechordal mesoderm? Here some authors go
miles ahead to confuse my dear medical students by interchangeably using the terms
prechordal plate and oropharyngeal membrane. So, here is the final review. Now let's
go for a 3D tour of the structure. So, here is the embryo from the top and this is the
epiblast layer and this structure this is the, what is this structure? This is oropharyngeal
membrane and this is the cloacal membrane here and this is the primitive streak with the
primitive groove going here in the midline and this, this swelling is the primitive node with
the primitive pit here the central depression here and this is the epiblast or
primitive ectodermal layer and this is the hypoblast or primitive
endodermal layer and here again this is the cloacal membrane and this is the what is
this? Oropharyngeal membrane. Is that right? Now let's cut a section from here and let's
see what's inside it. So, here you can see that this structure here this, this structure is
the notochord and here is the prechordal mesoderm and this greenish structure this is what is this?
This is the prechordal plate. This cranial part is the oropharyngeal membrane where it is where
these two layers are tightly stitched together and here you can see that these are the, these
cells, these are the mesodermal or mesenchymal cells that are forming the mesoderm and
all this is the ground substance here.