Prechordal Plate (Prochordal Plate) - Prechordal Mesoderm - Oropharyngeal membrane

In this video, Dr. Aizaz from MedicoVisual talks about; Prechordal plate [Prochordal plate]m it’s development and functions 00:00 What is Prechordal Plate 08:10 Functions of Prechordal Plate 09:55 Review 12:17 3D Tour of Prechordal plate Beginning the third week, at the cranial end of the embryo there is a circular thickening of cells present where only two germ layers i.e. Ectoderm and Endoderm are present with no mesoderm in between. This is the primordium of the future-mouth of the baby and thus is called the Oropharyngeal or buccopharyngeal membrane. The endodermal layer [lower layer] of this oropharyngeal membrane consists of thickened columnar hypoblast cells. This patch of columnar hypoblast cells is called the Prechordal plate. So, at this stage, we can say the Prechordal plate is forming the endodermal layer of the oropharyngeal membrane. While no such special name is given to the ectodermal layer of the oropharyngeal membrane. The prechordal plate is one of the most misunderstood structures of the embryo but I will try to untangle and simplify it as much as possible. We have been discussing the ingression of cells through primitive streak, but what about the primitive node. As it turns out, cells ingress through this node as well. Pertaining to its round outline, as the cell ingress, they too maintain a columnar shape. The few initial cells that ingressed through the primitive node travel all the way from the node and reach to the prechordal plate and here they displace the earlier prechordal plate that was formed by the hypoblast, laterally. So, now the prechordal plate is formed by the cells that came from the primitive node. Still, not much change in the structure as it is still two-layered. But later in the third week, an interesting thing happens. The so-called endodermal cells of the caudal most part of the prechordal plate proliferate, differentiate, and form a new layer between the caudal most part of the endoderm and caudal most part of the ectoderm of the oropharyngeal membrane. This layer is now called the prechordal mesoderm. Thus, some embryologists also like to call the cells of the prechordal plate as mesendodermal cells. [Mesoderm Endoderm], as it forms both endodermal as well as mesodermal [prechordal mesoderm] layers. This Prechordal mesoderm later forms the mesodermal derivatives of the head region. Later, this Prechordal mesoderm will be enriched by contributions from the neural crest cells. Neural crest cells may sound alien to you, but do not worry, we will discuss more about that in later lectures. It is interesting to note that as the caudal end became trilaminar now, after the formation of the prechordal mesoderm, the oropharyngeal membrane is now restricted to its cranial part that is still bilaminar. This bilaminar oropharyngeal membrane will later break down to form the opening of the mouth. Similarly, at the caudal end of the embryo, there is another bilaminar structure with no mesoderm in between, which is called a cloacal membrane. Actually, the prechordal plate is not a static structure. It’s a dynamic structure that continues to change. There are some stages of its development. At the end of the second and start of the third week, it’s a patch of thickened columnar cells with an overlying epiblast tightly stitched to it. Let’s call it “stage 1” Then during the third week, as the cells from the primitive node intercalates into the earlier stage 1 version of the prechordal plate. Now let’s call it the “stage 2” Prechordal plate and it too consists of overlying ectoderm of epiblast cells but the difference is that it is made of epiblast cells that came from the primitive node and not the hypoblast cells. The oropharyngeal membrane is still bilaminar. Finally, at stage 3, prechordal mesoderm also appears at the caudal end, and now cranially the prechordal plate has only a layer overlying it while caudally it has two layers i.e total of 3 layers. You get confused when in books you read that prechordal plate is a bilaminar structure and then suddenly, you see the prechordal mesoderm magically appearing in between this so-called “Bilaminar structure”. Some authors went miles-ahead to confuse my dear medical students by interchangeably using the two terms “Oropharyngeal membrane” and “Prechordal plate”. Please note that the Prechordal plate is the endodermal layer of the oropharyngeal membrane at stage 1 and stage 2 but at stage 3 this prechordal plate only forms the cranial part of the endoderm of the oropharyngeal membrane. At this stage, the caudal end of the prechordal plate has nothing to with the oropharyngeal membrane. It is the precursor of the head and along with its overlying mesoderm and ectoderm will form the head of the baby later on. Furthermore, it is an important structure in the induction of the forebrain in overlying neuroectoderm. Website: Email: draizaz@