In this lecture we will discuss gastrulation,
an incredibly important event in development
that brings us from a bilaminar or two layered stage to the trilaminar
or three layered stage.
In this process, we create a series of three specific germ cell layers.
What´s gonna happen here is that the epiblast of the bilaminar embryo
which can create any structure in the human body
subdivides into three layers: the ectoderm, the endoderm, and the mesoderm.
And in that point, these layers have distinctive fates.
Things that form from ectoderm
cannot form from endoderm or mesoderm and vice versa.
So these germ cell layers are gonna be the first step
in very, very specialized structural formation
that we´re gonna follow through the rest of these lectures.
Now, essentially, the bilaminar embryo begins the process of gastrulation by becoming polarized.
This occurs when a series of cells called the prechordal plate
become distinct and mark the eventual location of the head.
On the opposite side of the bilaminar embryo,
we develop something called the primitive streak.
So on the more caudal side of the developing bilaminar embryo,
a series of cells fold inward creating a streak
that moves closer and closer to the prechordal plate.
At the very tip of this streak is a structure called the primitive pit
and the cells that are located on the primitive pit
are known as the primitive node.
As we move into this process further and further,
cells are going to start folding from the epiblast down through the primitive streak
and into the space underneath it.
In this process, they will create a layer between the hypoblast and the epiblast
that will become mesoderm
and as they continue to involute through the primitive streak
and move alongside it and cranially along that space,
they will utterly replace the hypoblast cells with a brand new layer called the endoderm.
The epiblast cells are then gonna change their name, same cells
but different name, to become the ectoderm.
So when all is said and done as the process of gastrulation completes,
we have ectoderm on the top or dorsal surface of the embryo,
mesoderm in the middle, and endoderm lining the underside.
As we continue, a specialized set of cells
are going to move through the primitive streak and primitive node,
and go straight towards the prechordal plate.
This specialized group of cells that are migrating directly anteriorly
are called the notochordal process
and they´re moving close to the prechordal plate but not quite all the way to it.
As development continues,
the notochordal process moves closer and closer to the prechordal plate
which is also going to change names to become the oropharyngeal membrane
and without too much imagination,
you can probably forecast that the oropharyngeal membrane
will become the mouth.
On the opposite side just past the primitive streak,
we have the development of the cloacal membrane
which is going to form the primordia of the anus
and the urogenital systems´ openings to the outside.
We have now taken a sagittal cut through the developing trilaminar embryo.
We can see the ectoderm colored in blue,
the endoderm in yellow, and the mesoderm in between in red.
You can see that extending from the primitive streak closer to the prechordal plate,
we have thenotorchord and notochordal process.
As the norochordal process extends,
a hollow tube forms in the center of it called the notochordal canal.
As in extends closer and closer to the prechordal plate,
you´ll notice it never quite makes it there
and the reason for that is the prechordal plate
is going to become the oropharyngeal membrane
and the ectoderm and endoderm are firmly adhered to each other
and will not allow mesoderm to pass between them.
The same thing is happening posteriorly at the cloacal membrane.
So the firm adherence of ectoderm and endoderm marks the eventual openings of
and exit from the urogenital and gastrointestinal systems.
We now have a coronal view through the trilaminar embryo.
We can see that the notochordal plate
is migrating in between the endoderm below and the ectoderm above.
As it´s going to form a tube,
the two ends of the notochordal plate will pinch together within the mesoderm
and once that happens, it´s gonna pull the underlying endoderm completely together
to form a continuous sheet below the notochord
just as the ectoderm forms a continuous sheet
above or dorsal to the notochord.
At this point, the notochord is going to be a major factor in driving further development.
Now, one quick preview is that the notochord only becomes the central area
of our intervertebral disks
and doesn´t contribute much structurally to the adult human being
but during this very early stage of development,
the signals it releases drive the majority of important developmental sequences
that are gonna form our central nervous system and musculoskeletal system.
In particular, the ectoderm above the notochord will thicken and on either side,
there will be distinctive neural folds that develop
and a neural groove in between the two.
Now, as mesoderm is forming
and ectoderm is pulling itself in between the two layers,
we´re gonna get thickening
and subdivision of the different regions of the mesoderm.
The portion that migrates alongside the notochord, not on the exact midline,
but just a little bit lateral to it will flank the oropharyngeal membrane
and this is the cardiogenic mesoderm
and as the name implies, it will be the mesoderm
that helps form the early heart.
Thereafter, mesoderm that migrates a little bit more laterally
will form specialized structures called the paraxial mesoderm,
the intermediate mesoderm, and lastly, the lateral plate mesoderm.
After the lateral plate mesoderm,
we´ve reached a point where we´ve met the extraembryonic mesoderm
which does not contribute any tissue to the embryo itself
but helps keep us connected to the umbilical cord and placenta.
Now, things that can go wrong in the process of gastrulation are many
and most of them are incompatible with further life.
One set of structures that can create problems that make it to term
are referred to as teratomas or monster tumors.
These are tumors that arise from the primitive streak
and because they are coming from the primitive streak,
these overgrowths of cells, these tumors can form any structure in the human body.
Because the epiblast makes any structure in the body,
tumors arising from the epiblast can form anything
and it´s not uncommon to find partially formed limbs,
hair, teeth, muscle, other structures inside these teratomas.
Most commonly, they are going to occur near the sacrococcygeal region
and they´re the most common neonatal tumor.
They affect females more frequently than males in a roughly four to one ratio
and they are thankfully readily treated surgically
and can be removed and cause no further problems.
Another very strange problem that can affect the body at this early stage of gastrulation
is called primary ciliary dyskinesia
and it can result in the organs of the body
being completely flipped into a mere image of their normal position.
So instead of having my liver on the right side of my abdomen, it could be on my left.
And I can have my appendix on the left side
with my heart directed to the right side of my body.
This happens because as the process of gastrulation occurs,
our body is deciding which side is left and which side is right
and the reason it´s able to decide that is because of a group of cells
on the primitive node right on the midline
where involution is occurring have cilia that beat to the left.
And as they beat to the left,
they´re driving a factor released by those nodal cells called nodal.
So the primitive node releases the signaling molecule, nodal,
and the cilia beat that signaling molecule to the left
which influences the cells migrating onto the left side to form left sided structures
and if those cilia are immobile
and they do not push nodal to the left,
then, both sides of the body receive equal amounts of it
and the body has to flip a coin
and determine what side is left and what side is right
and in half the people with ciliary dyskinesia,
you will find situs inversus where the organs are flip flopped
because the body flipped a coin
and took the opposite route than normal.
Thank you for your attention and I´ll see you for our next talk.