Hello I am Dr Aizaz from medicovisual.com and today we will talk about vasculogenesis
and angiogenesis. During the third week of development of the embryo the blood vessels
begin to form. So, as we have discussed previously in the lecture of gastrulation
that from primitive streak the cell ingress downward and they convert into mesenchymal cells
and as you know that this endoderm it continues with the yolk sac
Endoderm and it is further outside plastered with the extra embryonic Splanchnopleuric
Mesoderm. Now what happens that these mesenchymal cells they come here into
this area the primitive Endodermal area and they displace these primitive endodermal cells to the
yolk sac endoderm. So, here they are displacing it to the yolk sac Endoderm and now this primitive
endoderm is almost completely replaced by those cells that came from the primitive streak and then
the more cells come into the space in between them and in as a result three germ layers are Formed.
Now what about this yolk sac endoderm? As we have discussed already that this yolk sac endoderm
through certain factors it releases certain factor that causes the formation of blood island Here.
What I think of it is that this yolk sac endoderm it is like a boss who orders the extra embryonic
mesoderm That: “hey! extra embryonic mesoderm I ordered you to start creating blood island
and it follows this extra embryonic mesoderm it follows the order of yolk Sac endoderm and
it creates these blood islands. Now, these blood islands they results in formation of blood vessels
as well as only or primitive blood cells. Now, what are these blood islands? Of Course, they are
not actual islands so what are they? Basically, these blood islands they are solid aggregates or
clusters of special type of cells so here you can see that this is a single blood island it is a,
it is a clump or cluster or Aggregate, solid aggregate of these small tiny cells
and what are these cells? These cells are called Hemangioblast. This term hemangioblast
it consists of three words; heme angio and blast. Heme means “blood” Angio mean “vessels” and blast
“to form”. so hemangioblast are named so because they are going to form
blood vessels as well as the primitive blood cells. So, these are the hemangioblasts and
aggregates of hemangioblast form a single blood island. But from where these hemangioblast Come?
Well! the answer is simple as you know that this extra embryonic mesoderm it consists of
irregularly shaped loosely arranged mesenchymal cells so this is the extra embryonic mesoderm it
consists of these mesenchymal cells and some of these undifferentiated mesenchymal cells
under the order of, under the induction of yolk sac endoderm, yolk sac endoderm
induces these mesenchymal cells it orders these mesenchymal cells to form the hemangioblast and
these hemangioblasts they come close together they clamp, they aggregate to form the blood islands.
During the third week of development at about day 17 or 18 this uh extra embryonic mesoderm
extra embryonic splanchnopleuric mesoderm that is overlying this yolk Sac so here is the yolk sac
and this extra embryonic splanchnopleuric mesoderm is plastering this this yolk sac and it is almost
completely covered with these aggregates these blood islands and later on such blood islands
also form within the intra-embryonic mesoderm but the difference between those blood islands and
these blood islands that are formed over this extra embryonic mesoderm is that these blood
cells of extra embryonic mesoderm they will form blood vessels as well as the primitive blood cells
but those blood islands that are formed within the intraembryonic mesoderm they are going to form
blood vessels only not the blood cells. Now, here is a single blood island that I have enlarged.
Now, what happens that some of the cells, some of the central cells of this blood island they
undergo the process of programmed cell death or apoptosis and so small spaces will form
within this blood island as more and more cells die these small spaces will become larger and
they will coalesce together and what happens that a central space will be formed but within that
some of the cells they remain alive. Now, these alive cells that are within this lumen
of the pot like structure, this vessel like structure that is now formed these cells they
are the precursor of blood cells. So, these are the blood cell precursors and these peripheral
cells that are arranged in the periphery they are going to form the endothelium of the blood vessel
they are called endothelial precursor Cells. They are also called endothelial precursor cells.
These endothelial precursor cells they will differentiate and they will flatten out to
form the endothelial cells and these blood cell precursor cells they will further differentiate
into the blood Cells. Now, at this point I really want you to have a crystal clear concept of this
structure so, let's have a 3D tour. so, let's suppose that this is a single blood island
and spaces have coalesced in between this blood island and a lumen has been formed at this Stage.
Now, if we cut a section here if we cut it from here and we see inside then what we will see
is that here is the this blood island and as you can see that these cells that Are,
that are in the periphery of this blood island they are going to form the
endothelium and they are called endothelial precursor cells
and these few cells that are within this within the lumen of the of this blood island they are
the blood cell precursor cells and they are going to form the blood cells. So, let's move forward.
Now, of course, this is not a single lonely blood island it has some neighbours and within its
neighbourhood there are some other blood islands as well. Now, what will happen that these cells,
these endothelial precursor cells they will rearrange themselves in such a way that these
blood island the lumen of these two blood islands they will coalesce together and a long vascular
cord will be formed. Of course, these are not usually two blood islands that are, that coalesce
there are numerous blood islands whose lumen coalesce, to form the long vascular Cords. Now,
what will happen with this vascular cord is that they are the precursor of mature blood vessel.
As you know that these blood islands are in the extra embryonic mesoderm so
within the extra embryonic mesoderm you know that there are the mesenchymal cells
and these mesenchymal cells they will come and they will surround this
vascular cord and then they will differentiate into the connective tissue of this blood vessel as
well as the smooth muscle of this blood vessel and now a beautiful new blood vessel has been formed
and this process is called Vasculogenesis. So, vasculogenesis means de novo or new synthesis of
blood vessel from the embryonic mesenchyme. Most of the major blood vessels of the
embryo are initially laid down by the process of Vasculogenesis. vasculogenesis is an embryological
process and it occurs in the prenatal life and it rarely occurs Postnataly. After the baby is born
then angiogenesis can occur but vasculogenesis is almost an exclusive embryologic process
so, this was the process of Vasculogenesis. Now, there is another way to form the blood vessels.
What happens that certain factor for example VEGF, vascular endothelial growth factors
they are released this vascular endothelial growth factor is like a beautiful girl it
stimulates or excites one of the cells, one of the endothelial cells of this blood vessel and as this
endothelial cell become excited it is converted into what we call tip cell and this tip cell it
starts to move towards this beautiful girl and as it does so these cells they will proliferate
behind this and they will as a result form new blood vessel. so, a whole new blood vessel is
formed at an angle by the movement of the tip cell towards the vascular endothelial growth Factor.
Of course, this is the tip of blood vessels don't think of something else. This process is called
angiogenesis and specifically the sprouting angiogenesis. The vessel sprouts towards the
vascular endothelial growth factor and this is called the sprouting Angiogenesis. Please note
that only a single cell will turn into tip cell if all of these endothelial cells they convert
into tip cells the structure of this blood vessel will fall apart. Fortunately that does not happen.
So, angiogenesis is the formation of blood vessels from existing blood vessels and this process
that we have just talked about is sprouting angiogenesis and then there is another type of
angiogenesis as well that is called non-sprouting angiogenesis that we will just talk about.
The angiogenesis is not an exclusive embryological process it happens in the embryonic life as well
as it also occurs during the postnatal life. It plays an important role in physiological and
pathological conditions. For example you develop a wound in your finger or any part of your body
and at that injured part the blood vessels may get damaged and to repair those blood vessels
new blood vessels will arise from the existing blood vessels and that happens due to the process
of Angiogenesis. Similarly, the tumors get extra nutrition by stimulating the process
of angiogenesis Within the Tumor. So, new blood vessels are formed within the tumor by the process
of Angiogenesis. How can you differentiate the angiogenesis from Vasculogenesis?
To remember this you can think that angiogenesis this angiogenesis is anglogenesis
what I mean by this is that here you can see that this blood vessel it is forming at an angle this
new blood vessel it is forming at an angle so when a blood vessel forms at an angle that is
angiogenesis and if a new blood vessel forms from, from the mesenchymal tissue that is called
Vasculogenesis simple, isn't it? But remember that this term angiogenesis it does not mean angle this
term angiogenesis what it means is that the term angio means it came from the greek word called
angion which means small vessel and genesis means to form to produce production or synthesis. So,
angiogenesis literally means formation of small Vessels. So, major vessels they are formed by
the process of vasculogenesis but the smaller vessel they sprout, they arise from those major
vessels by the process of Angiogenesis. So, as I have told you that this is sprouting angiogenesis
then what about the non-supporting Angiogenesis? Well, it's a simple process so here is a single
blood vessel now what happens to this blood vessel is that this blood vessel it will split
into two so here are sort of pillar forms and then it is converted into two blood vessels
so this process is called non-sprouting Angiogenesis. It is also called splitting
Angiogenesis. And it is a process in which one vessel literally splits into two blood vessels. I
will not discuss the detailed molecular mechanism of this but the basic idea is that what I think
of it is that it is like a mitosis of the blood vessels as in mitosis one cell splits into two
in this splitting angiogenesis the blood vessel literally splits into two simple as that. So,
now let's review what we have studied so embryonic blood vessel formation consists of
two types the Vasculogenesis that consists of formation of blood vessels from the blood island
and blood islands they came from the mesenchymal tissue and then there is angiogenesis and
angiogenesis consists of two types the sprouting angiogenesis in which the tip cells are formed and
it is stimulated by the VEGF, vascular endothelial growth factor and the other is the non-sprouting
or splitting angiogenesis in which the one blood vessel literally splits into two. Please note
that whether it is the process of angiogenesis or vasculogenesis both of these processes are
followed by a process called vascular maturation and during vascular maturation the smooth muscle
and vascular connective tissue is formed around the newly formed blood vessels. So, this was
about the vasculogenesis and angiogenesis. Thank you so much for watching this video.