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.