hello! I am Dr Aizaz from medicovisual.com and today we will talk about development of
chorionic villi during third week of Development. So, as we have discussed that at the end of second
week of development the embryo consists of two germ layers the epiblast and hypoblast
layer and this two germ layered embryo is converted to tri-laminar the three germ-layered
embryo during third week of development along with that it consists of two balloon-like structures
one is the amniotic cavity and the other one is yolk sac. Whole the structure this embryo
proper along with its two personal balloons is floating inside the chorionic cavity it is also
called extra embryonic cavity. So, this whole structure is floating in the chorionic cavity
except it is attached at this point that is called connecting stock
and this, this membrane this structure is called chorion and it consists of two layers the
inner layer this is the inner layer which is extra embryonic Somatopleuric mesoderm and the
outer Layer, this whole this layer, this outer layer it is it consists of cytotrophoblast and
of course outside this is the outside the cytotrophoblast is the syncytiotrophoblast
and you may be remembering from the lecture of second week of development that there are
lacunae present a liquid network is present within the syncytiotrophoblast and maternal blood flows
through this Lacunae network. Now, as we have discussed already that during the second week at
the end of second week of development the finger like projections arise from the Cytotrophoblast
these are the those finger like Projections, finger like processes that arise from this the
cytotrophoblast and we call it primary villi. Now, in today's lecture we will discuss that
what happens to these villi, that what stages of development these Villi undergo? These are also
called chorionic villi so let's enlarge a single primary villi and let's first see its structure
and then we will see what happens to that primary villi in subsequent stages of development.
So, here is a single primary villi and as you can see that it is a solid cord here in this 3D
diagram you can see that it is a solid cord made up of these cytotrophoblastic cells so
this is basically a cut section let's say we have cut here and we are looking at the top in
this 3D diagram so here is that cut section and here you can see that this primary Villi it is a
solid cord of cytotrophoblastic cells. Early on most of the primary villi are unbranched
but later on some of the primary villi they become branched. So, so these branched primary villi,
they are also similar in structure to the unbranched primary villi, the only
difference is that there is this kind of branching pattern and we call it brushed primary villi.
So, after the formation of primary villi during third week of development what happens to this
primary villi? So, what happened here is that this inner cell of this primary villi,
this inner cell they degenerated and here was the extra embryonic Somatopleuric mesoderm,
so what happens that as the inner cell degenerated in the cytotrophoblastic cells of this primary
villi degenerated the extra embryonic mesoderm it proliferated this mesenchyme it proliferated
And, and it went inside the core of this Villi. Now, at this stage this primary Villi,
these primary villi they are now called Secondary villi. Well villi is simply the
plural term and villus is the singular term so don't get confused by this terminology. So, this
is the Secondary villi. So, what is difference between primary and secondary villi? Primary villi
is the solid cord of cytotrophoblastic cells but the secondary villi it has an inner core,
it has an inner core of extra embryonic mesoderm. So, here in this 3D diagram you can see that
it has an outer periphery of Cytotrophoblast, this is the secondary villi, it has an
outer periphery of cytotrophoblast and inside is the mesenchymal core that is made up of
extension of this extra embryonic mesoderm into this villi. So, this is the secondary villi.
Then what happens to this secondary villi is that the mesenchyme of this extra embryonic
mesoderm leads to formation of blood vasculature inside the villi. So, as the blood vessels are
formed inside this villi now it is called tertiary villus. Now, now these types of villi in which the
blood vessels are present they are called tertiary villi. This capillary network of tertiary villi it
consists of an afferent and here is the afferent ends that brings the less oxygenated and nutrient
deficient blood to the Villi and then there is that efferent end that takes back the oxygenated
and nutritious blood back to the embryo this efferent end is also called arterial and and the
efferent end is also called the venous end in adults the veins contain deoxygenated
blood and arteries contains oxygenated blood but here the situation is opposite.
So, here in this 3D diagram you can also see the tertiary villus which has an arterial
or afferent end that contains less oxygenated blood and efferent or venous end that contains
highly oxygenated blood. So, this is the vascular network of this villus embedded inside the extra
embryonic mesodermal extension and outside both of these structure is the cytotrophoblastic
cells in the peripheries and of course outside this would be the Syncytiotrophoblast which is
not shown in this Diagram. So, in the last diagram we had seen the unbranched tertiary villus but
actually at this stage most of the villi they are branched. Now, how these tertiary Villi work? What
is their Function? What is their significance? We will see the details in the lecture of placenta
but here let's have a brief overview of the things that we will study in later lectures.
So, here is a single branch tertiary villus and here is its neighbouring branch
tertiary villus I’m just going to call it a tertiary villus. Of course,
outside this is the syncytio- trophoblast that contains these spaces, the lacunae
and the structure, this embryo it is embedded into the maternal endometrium.
As the embryo buries inside the maternal endometrium we have seen already that
it erodes the maternal tissue and here you can see that let's suppose it has eroded a
maternal blood vessel as it does so the maternal blood start flowing into this lacunar spaces as
these lacunar spaces become enlarged later on we will see that they are then named as
intervillous spaces. Because they become so large that they almost completely form the space between
the two neighbouring Villi. At this stage they are called lacunae but later they will form the
intervillous spaces. So, what happens here is that blood started flowing, the maternal blood started
flowing into Lacunae. Meanwhile what happened here is that the cytotrophoblast from this
villus branch as well as this villus branch it proliferated and it formed a layer or shell here
which we call as outer trophoblastic Shell. It penetrated through the Syncytiotrophoblast to
form this outer trophoblastic shell. here, you can see that especially this branch of tertiary villus
is helping in anchoring this, this structure, this future placenta to the maternal tissue. So, this
villi, this branch of villus is called anchoring branch of villi or simply anchoring villus. It
is also called stem villus. So, this is the stem villus or anchoring villus along with that
what happens that as we have discussed that blood starts flowing into these lacunae so these villi,
These, these free villi here they will, they will take up the nutrients and oxygen
from this lacunar blood and they will take up this nutrients and oxygen and bring it into These,
into this blood. So, in this way they absorb the nutrients, they help in absorption of nutrients.
So, they are also called absorbing villus. So, we have two types of villi some are helping to anchor
the placenta they are called anchoring villi, the other one the helps in absorbing the nutrition
and they are called absorbing villi. Please note that later on there will be some changes
in this apparatus such that it will become much more robust and efficient in absorbing the
nutrients from the maternal blood at that time we will call this apparatus as placenta and we will
discuss the details of placenta in later lectures. So, let's have a review of what we have Read.
So, primary villi which is a solid cord of cytotrophoblast,
the secondary villi which consists of extension of mesenchyme from extra embryonic mesoderm and
final stage is the formation of vasculature inside this mesenchymal extension from extra
embryonic mesoderm. So, three type of villi are formed at the end of second week. We have
primary Villi then subsequently in third week the development is up to the tertiary Villi.
And finally, here are these branching villi that consists of anchoring villi
as well as the absorbing villi. Thank you so much for watching this Video.