Comprehensive Guide to Gastrulation: Formation of the Trilaminar Germ Disc

Hello! I am Dr Aizaz from medicovisual.com  and in this visual lecture we will talk   about gastrulation. Here is the embryo at  the start of third week of development.   If we cut open this embryo, if we divide  the embryo with a knife into two parts  

and if we remove this part of the embryo and  we observe this cross section of the embryo   what we will see is that it consists of two  types of cell forming the bilaminar germ disc   the upper layer of cell is called epiblast while  the lower layer of cells is called hypoblast.  

Amniotic cavity surrounds this embryo from above   while the secondary or definitive yolk  sac surrounds this embryo from below.   Then this yolk sac membrane or yolk sac  endoderm, it surrounds this yolk sac. Of course,  

then there is extra embryonic mesoderm and whole  this structure it is floating freely within this   chorionic cavity but it is still connected with  the connecting stalk and of course all this   structure is surrounded by cytotrophoblast cells  and of course outside this will be the syncytio-  

trophoblast which has not been shown in this  diagram. Let's focus on this bilaminar germ disc.   If we undo the cut here we will see that  it is a disc like structure consisting of   hypoblast as well as epiblast layers and this is  called bilaminar germ disc. Let's focus on this.  

So, here is this bi- laminar germ disc and let's  make a copy of this bilaminar germ disc and in   this copy version let's cut it from here. So, if  we cut it from here and we observe it from front   for example you are standing here and you are  observing it from front how it will look like?  

So, here is this bilaminar germ disc from front.  Again it consists of epiblast layer as well as   hypoblast layer. Now at this stage what  happens and of course we are talking about   third week of development so at the start of  third week of development what will happen  

is that this epiblast layer they will release some  special substances in between these two layers,   in between the hypo and epiblast layer. What  is this substance? It will release lots of   hyaluronic acid in between these two layers.  This hyaluronic acid is a hygroscopic compound  

it can bind with and it can pick up lots  of water molecules and thus a fluid filled   space will be formed in between hypo and epiblast  cell layer and in this way we will get a blotted   appearance of the embryo. Here we can see that  we have this blotted up appearance of the embryo  

then these midline epiblast cells they will  release some chemo attractants and what they and   what these chemo attractants will do is that they  will cause growth of epiblast in the periphery.   Now what these chemoattractants will do is that  they will cause proliferation of epiblast in the  

periphery and they will attract this epiblast  this newly formed epiblast towards the midline.   So, as these newly formed epiblast aggregate  in the midline what we will see is that in the   caudal midline there will be formation  of a streak of cells, it will start  

in the mid of caudal midline and it will extend  caudally as well as it will extend cranially   and at the cranial end it will form the swelling  or node called primitive node and this streak of   cell it is called primitive streak. So, here  is this structure called primitive streak on  

the cut section as well as while we view it from  the top and this node is called primitive node   or Hansen’s node. Remember that this structure  is formed only at caudal end. Caudal end means   tail end. Of course, we humans do not have  tail but still traditionally embryologists  

call it caudal end and this end as cranial end.  So, with the formation of primitive streak we can   clearly say that this end is caudal end  on which the primitive streak is formed   while the other end that is opposite  to primitive streak is the cranial end.  

Before the formation of primitive streak just by  looking at this bilaminar germ disc we cannot tell   that which end is the cranial and which end is the  caudal end. Along with that in the midline as this   primitive streak is formed we can say that this is  the right side of the embryo and this is the left  

side of the embryo. So, axis determination is one  of the important function of the primitive streak.   Now here an important question arises that why  primitive streak only forms at the caudal end?   What stops the formation of  primitive streak at the cranial end?  

Actually, we have these hypoblast cells and  these hypoblast cells they are like wise old man,   they give important instructions  to the overlying hypoblast cells,   the hypoblast cells of the cranial end  inhibit the formation of primitive streak  

at the cranial end of epiblast. As they do so  the epiblast in the caudal end they quickly   forms the primitive streak  as they are so keen to do so.   This cranial end hypoblast also helps in formation  of head structure so it is called anterior  

visceral endoderm. Anterior? *** the anterior  here? Of course, this is the cranial end this is   not the anterior and so why we call it anterior  visceral endoderm? I think it should be called   cranial visceral endoderm rather than anterior  visceral endoderm. Actually, this name is given  

to this structure by the mouse embryologist and  in mouse this structure is not just cranial but   it is also anterior. Well! let's see how? So, here  is a mouse and it has to pass through this door   the first structure of his body that will cross  the door is its head so it is called anterior  

I mean the head of embryo is anterior, anterior  means something that comes before the other part   and the last part of his body to cross that this  door is his tail so his caudal end or his tail is   also the posterior end. So, in mouse anterior  as well as the cranial both of these terms are  

synonymous. Similarly, the term caudal as well  as posterior both of these terms are synonymous.   But this does not happen with humans.   Because let's say if this human has to cross this  door both the head as well as, of course humans  

do not have tail, both the head as well as his  buttocks or hips both of them will cross the door   at the same time so we cannot demarcate the head  and hips in terms of anterior and posterior.   So, in humans the head is cranial but  it is not necessarily anterior [Music]  

but still conventionally we call it anterior  visceral endoderm and you have to do the same.   Right, epiblast cells just like any other type of  epithelial cells they are closely bound with each   other kind of like holding hands of each other and  that is due to special proteins called e-cadherin  

proteins that keep them formed together with each  other. Now what happens that as these primitive   streak cells come into the midline, in the centre  these midline epiblast cells what they do is that   they don't regulate the e-cadherin thus they  lose their hands and they are no more bond  

with each other. So, what happens here in this  strange land of primitive seek these epiblasts,   these midline epiblast cells what they do is that  first they invite these guests into this area   and then they remove their hands and of course,  now what will happen that they will not be able to  

stay together. So, let's suppose that this midline  epiblast cell it loses its e-cadherin molecule so   it will tend to fall into this space and as  it turns so it loses its particular shape and   it forms a bottle like shape a flask like shape  and now it is called bottle cell or flask cell.  

So, ultimately it will fall into this space  and now we call it mesenchymal cell. This is   the epithelial cell and this is a mesenchymal  cell. The difference between epithelial cell   and mesenchymal cells is that number one the  mesenchymal cell it has an irregular shape  

but epithelial cell it has a particular shape.  For example, this epiblast cell these are, these   epiblast cells they are columnar in shape but  this mesenchymal cell it has some irregular shape.   Secondly, the epithelial cells they are tightly  bound together. They are closely packed together  

with each other but that is not true for  mesenchymal cells. So, what we can say is   that as this epiblast it invaginated inward into  this space what happened with this epiblast cell   is that it underwent epithelium to mesenchymal  transition. So, as more and more cell invaginate  

inward from this point here we will see that a  depression, a small depression is formed here.   Here you can see there is a depression the  name of this depression is primitive groove.   So, more and more cells will come into  this space and here the structure that  

is formed is mesenchyme. Mesenchyme is  basically the embryonic connective tissue.   It consists of ground substance that  is filled up with hyaluronic acid and   other substances and within this ground  substance mesenchymal cells are loosely arranged.  

On this 3D diagram you can see that this  was the primitive streak and as the cell   invaginated inward a groove is formed in  the, in the centre of this primitive streak   so this groove is named as primitive groove.  Here on the cut section you can see this  

primitive groove and on the 3D diagram  you can also see the primitive groove.   This primitive groove it extends caudally into an  other groove, a shallow depression that is called   primitive pit. So, that  there is that primitive pit.  

Now how this primitive pit forms? What happens  that here also some cells invaginate inward,   right, some cells invaginate inward and  they move forward in between the space,   between epi and hypoblast and in this way this  rod-like structure will be formed here below  

this epiblast layer this rod like structure is  called notochord. We will discuss that how it form   and what is its function in the next lecture.  Right, and regarding the primitive groove as   you know that how it forms? The cell invaginate  here from this groove the cell invaginates inward  

and this leads to formation of this  depression called primitive groove. Right?   So, now we have three layers this is the epiblast  layer, this is the hypoblast layer and in between   them there is another layer of mesenchymal  cells. And now we have this three layers. Is  

this the process called gastrulation? Well! No,  it is not that simple unfortunately. What happens   here is that as the embryo is growing in size the  simple diffusion process is not simply enough to   meet the nutritious need of the embryo. So, this  is the right time that blood cells should form.  

As you know that here we have this yolk  sac endoderm or yolk sac membrane and   outside this yolk sac endoderm is extra  embryonic splanchnopleuric mesoderm.   Now these mesenchymal cells they leave this space  

and they come here into the place of hypoblast and  they displace these hypoblast into the yolk sac   endoderm. So, hypoblasts are displaced to yolk sac  endoderm and here these mesenchymal cells come.   So, these mesenchymal cell will almost completely  replace the hypoblast and those hypoblasts they  

will then be displaced to the yolk sac endoderm.  These mesenchymal cells as they come here they   will not remain mesenchymal they will then again  undergo mesenchymal two epithelial transition   and here they will form the definitive  endoderm. They will acquire the hypoblast  

like shape. Of course, they are not hypoblast.  They will acquire hypoblast like shape and they   will form the definitive endoderm. What about  the hypoblast that were displaced here into   the yolk sac endoderm? As I have told you the  hypoblast they are like wise old men. They have  

a real bossy nature. What they will do is that  they will order the overlying extra embryonic   mesoderm that “Heyy! extra embryonic mesoderm  start creating blood island.” And of course,   it will do so. The blood island will be formed  in the extra embryonic splanchnopleuric mesoderm  

on the order of yolk sac endoderm. In books what  they say is that the underlying yolk sac endoderm   induces the formation of blood island into  the extra embryonic splanchnopleuric mesoderm.   What is the function of blood island? For now just  remember that it helps in formation of blood cells  

we will discuss the details of  blood island in upcoming lectures.   So, as these mesenchymal cells that were  lying here they displaced the hypoblast and   came here and differentiated into the definitive  endodermal cell this space is now empty so more  

epiblast cells will proliferate and invaginate  inward. They will migrate towards the midline and   invaginate inward through the primitive groove  and again they will come into this space   to form the definitive mesoderm. The  definitive mesenchyme will be formed here.  

So, now ultimately we have three germ layers due  to the faithful work done by the primitive streak.   As the primitive streak has  performed its function nicely   during the fourth week of development  primitive streak usually disappears  

but unfortunately sometimes the primitive  streak failed to disappear and the remnants   of primitive streak remains and it forms  what we call as sacrococcygeal teratoma.   here is a new born with a sacrococcygeal  teratoma in the hip region here this is  

called Teratoma. Teratoma means monstrous  tumor because it looks like a monster.   As you know that these epiblast cells and  this primitive streak it is the precursor   of all different types of cells it forms  the three germ layers that are ultimately  

going to form the complete embryo. So, this tumor  it consists of all different varieties of cell.   There may be nail, there may be teeth and  hair and all those types of adult cells. So,   it may look like a monster so that is why  we call it teratoma, the monstrous tumor.  

So, now here we have three germ layers the outer  layer is called ectoderm, the inner layer is   endoderm and in between them is the mesoderm. Meso  means in the middle, that comes in the middle.   And it consists of mesenchymal tissue. This is  trilaminar germ disc and the origin of all these  

germ layers is epiblast cells. This process of  formation of trilaminar germ disc is termed as   gastrulation. Gastrulation? There is no formation  of stomach here, there is no formation of gut   tube here why the hell we call it gastrulation?  Well! this is confusing so let me explain why it  

is called gastrulation? Actually, this process was  first discovered in small animals for example, sea   urchins and this primitive organism their blastula  it consists of a hollow spherical ball of cell   and during the process of gastrulation, their  process of gastrulation is very simple it looks  

like that someone has pinched here with a thumb.  Right, so someone has pinched here with the thumb.   So, here they will invaginate inward as you can  see here that here it has invaginated inward and   in this way what happens is that here this part  here, here we have formation of primitive gut or  

Archenteron. The water and food material it enters  first into this primitive gut and it is released   outside through this blastopore. So, during  gastrulation what is happening here is that there   is formation of primitive gut during the process  of gastrulation but along with that we have also  

the formation of germ layers. Here is the endoderm  that has formed along with that there is ectoderm   and in some organism in between them the  mesoderm is also formed. So, three germ layers   are formed here along with the formation of three  germ layers the primitive gut is also formed.  

So, scientists initially thought that  the formation of primitive gut that is   gastrulation is the most important process here  so they simply ignored the trilamination. So,   rather than calling this process as trilamination  they simply call it gastrulation. Why? Because in  

small organism primitive gut is formed here but  no such process occur in human. In human, during   the gastrulation process no gut forms but still  somehow it is called gastrulation. Why so? By   now you must have understood this. Yes! You got it  right. Conventions my friend conventions. Right?  

So, finally let's review it. So,  first of all this is the epiblast   layer what happens that it releases chemo  attractants that causes proliferation   and migration of epiblast from the  periphery into the caudal midline  

and then it down regulates the e-cadherin thus  these epiblasts they tend to fall into the space.   Firstly forming the bottle cells and then  they come here forming the mesenchymal cell.   Some of these mesenchymal cells they will  displace the hypoblast to yolk sac endoderm and  

here they will differentiate into the definitive  endoderm. The more epiblast will invaginate inward   and they will form the mesenchymal cells  here that will form the definitive mesoderm   and the cells that remain behind they will form  the ectoderm. So, ultimately through this process  

of gastrulation we have three germ layers  the ectoderm, the mesoderm and the endoderm.   So, that was about gastrulation. Thank  you so much for watching this video.