Hello! I am Dr Aizaz from medicovisual.com and today we will talk about development of placenta.
So, what is placenta? What is the definition of placenta? Placenta is a discoidal
temporary but vital vascular feto-maternal organ that is responsible for metabolism and
exchange of chemicals between mother and fetus or the embryo. Please note that I will use the
term embryo and fetus interchangeably during this lecture but truly speaking the fetus is
an embryo that has grown beyond the ninth week of development after fertilization.
If you want to learn more about such embryological terms please watch the first lecture of embryology
lecture series by medico- visual. Now that we have seen that what is placenta
let's first look at why there is the need of placenta for the human embryo?
As you know that birds lay their eggs outside their bodies their eggs contains nutritious
egg yolk the embryos of the bird develop inside these eggs while utilizing the egg
yolk. They utilize this egg yolk they eat up this egg yolk and develop inside the egg. This type of
development comes with a caveat that such eggs are vulnerable to harshness of external environment.
For example these eggs they may be destroyed by other animals, they may even be eaten up by humans
Hmm! the chick eggs are yummy I must say. The human mothers are much protective
regarding their embryos. The human embryo grows inside the womb of his or her mother. Warm,
nourished and well protected from the external environment, inside the bodies of their mothers.
So, as the human embryo develop inside the body of their mother they contain little to no egg yolk.
It means that they somehow need to fetch the nutrition from their mother.
Also they have to release the waste material into the maternal body and for this purpose
human and other mammals have evolved a special nutritious fetching device called placenta.
So, here is this plate-like device called placenta, plate-like organ called placenta with
the help of which the, the fetus or the embryo will fetch the nutrition from the maternal body
and release its waste materials into the maternal blood. So, this is the purpose of placenta.
Now let's see when does placenta start developing? Placenta start developing much earlier than you
think. So, as you know that a zygote, the fertilized egg called zygote it undergoes
a series of mitotic division called cleavage to form a ball of cell called morula.
Morula consists of closely packed cells called blastomeres. They are tightly,
closely packed with each other. At this morula stage these blastomeres they decide so as to say
that these outer cells they will for, they will arrange for the nutrition and these inner cells
they will form the embryo proper. So, as this morula comes inside the uterine cavity, here this
zona pelucida will rupture and a cavity will form inside this and now two types of cells are formed
these cells that will form the embryo proper they are called inner cell mass or
embryoblast and the other cells these cells that are outside this embryo proper they are
called outer cell mass or trophoblast. These trophoblasts they are the major contribution,
they forms the major part of the fetal placenta. Actually the placenta it has two parts the fetal
part as well as maternal part and we will discuss these details later. So, now this
inner cell mass and outer cell mass it has to implant into the endometrium of their mother.
Let's zoom in into this part and let's see what happens. So, here is the endometrium of maternal
uterus and here comes this embryo through selectin it will loosely bind with the uterine wall
as it happens these cells of the uterus they will undergo what we call as decidual reaction.
By decidual reaction I mean that they will accumulate
lots of glycogen and fat and they will become fat globular cells and we call them
decidual cells and this process of formation of decidual cells is called decidual reaction. As
this decidual reaction happens this outermost layer of the endometrium,
this luminal layer of the endometrium is now called decidua of endometrium of maternal uterus.
After loosely binding through the selectin molecules now this embryo will firmly bind
with the help of fibronectin and integrins. Then what happens that some of these trophoblast cells
at this part they will proliferate and they will lose their cell membranes and they will act as
a single structure called syncytium. Now the trophoblast has been divided into cytotrophoblast
that has clear-cut cellular boundaries and syncytiotrophoblast that does not have clear-cut
cell boundaries. The syncytiotrophoblast is highly invasive and it will invade the uterine wall.
Development up to this happens till the end of first week of development.
Now let's go to the mid of second week of development and let's see
what happens to trophoblasts during the mid of second week of development.
So, now we have fast forwarded towards the mid of second week of development. Here you can
see that this is the syncytiotrophoblast and here is the cytotrophoblast. These
are the two parts of trophoblast. Within the syncytiotrophoblast small but interconnected
network of spaces called lacunar network is formed. As the syncytiotrophoblast erodes the
maternal endometrium it also erodes the maternal blood vessels. Let's say it has eroded at one end
a maternal artery or arteriole and at the other end it has eroded the maternal venule. Actually
multiple arterioles and venules are eroded but for the sake of simplicity I have only shown the one.
So, with such erosions the arterial blood with high pressure they start flowing into the lacunar
network and then this blood goes back into the venule. So, as the maternal blood start flowing
into this lacunar network this is the formation of earliest form of the placenta and this is called
primitive uteroplacental circulation. Now if we unblur the embryonic part what we will see is that
this is the embryo proper and it consists of amniotic cavity and yolk sac cavity
and all this structure is surrounded by chorionic cavity it is also called extra embryonic coelom.
And here is the extra embryonic somatopleuric mesoderm. Remember that chorion consists of
extra embryonic somatopleuric mesoderm as well as the cytotrophoblast and according to some
authors the syncytiotrophoblast also contributes towards the chorionic membrane or chorion.
Now what happens here is that as the nutritious blood comes into the lacunae here the nutrients
including oxygen and glucose they can diffuse into the chorionic cavity and from chorionic cavity
nutrition can be picked up by yolk sac as well as the amniotic cavity and then this nutrition can
reach to these cells that are forming the bilaminar embryonic disc or bilaminar germ disc.
Similarly the waste material at the stage it is secreted by the cells into first yolk sac
as well as amniotic cavity and then these are released into the chorionic cavity and from
chorionic cavity they go back to the maternal blood and this maternal blood it will be then
released into the venules of the mother. So, at this stage there are no blood vessels here
simple diffusion is enough to meet the nutritious needs of the baby.
At the end of second week of development the embryo is completely implanted into the decidua
of endometrium of their mother. Topographically we can divide the decidua into three types
this capsule-like deciduous that is overlying the implanted conceptus is called deciduous capsularis
and this basal plate-like deciduous that is underlying the conceptus is called
decidua basalis and all the remaining part of decidua all this part of the decidua
which is not involved in this process it is named as decidua parietalis.
So, we have three types of decidua. Please note that decidua basalis is the only part
of decidua that contributes towards the formation of maternal part of placenta.
So, now let's see further development of placenta. So, here we are near the end of
second week of development at this stage the finger like processes primary villi
arise from the cytotrophoblast and they penetrate into the syncytiotrophoblast.
Let's zoom into the one primary villi and let's see what further happens to this primary villi.
So, here is a single primary villi. It consists of finger-like projection
of cytotrophoblast along with that here is the extra embryonic somatopleuric mesoderm.
So, here is another 3D diagram of cut section of this primary villi.
During the third week of development the underlying extra embryonic somatopleuric mesoderm
sends its extension into the core of this primary villus and as this mesenchymal core is formed
inside the primary villus. Now this primary villus is converted to the secondary villus.
So, again in this 3D diagram you can see that there is the inner core in a mesenchymal core
which is the extension of extra embryonic somatopleuric mesoderm.
This mesenchyme then forms the blood vessels inside it and as the blood vessels form inside
this Villi. Now it is termed as tertiary villus. The blood vessels has an arteriolar end as well
as the venous end. Interestingly the arterial end here it is shown in blue colour because it
is oxygen deficient and the venous end is shown in red colour because it is oxygen rich because the
nutrient from the maternal blood comes inside it and it becomes oxygen rich. Usually the tertiary
villi are branched but the structure of each of its branch is similar to the main stem villus.
So, here is a trophoblast with that number of branched tertiary villi.
Initially the chorionic villi surrounds the embryo from all sides but later on perhaps due to
decreased blood supply at the luminal side these villi, this chorionic villi here they degenerate
and the chorionic villi are mainly present on that side of trophoblast that is facing the endometrium
of mother. So, this part of chorion or this part of trophoblast it has lots of villi and it is
rough in structure it is leaf-like in structure we call it chorionic frondosum but this side,
this luminal side of trophoblast or chorion it has little to no chorionic villi. So, it has a
smooth structure and we call it chorionic laeve. So, there are two faces of chorion the chorionic
frondosum and chorionic laeve. Please note that only the chorionic frondosum contributes towards
the fetal part of placenta. So, here are the maternal endometrial spiral arteries that open up
into these lacunae and then within this lacunae the exchange of material occurs
and then this blood is taken up by the endometrial veins back into the maternal circulatory system.
So, here you can see that within the extra embryonic somatopleuric mesoderm these chorionic
vessels develop and these chorionic vessels they are connected with the vasculature of
the tertiary villi. At the end of third week of development the primordial heart is also formed
and primordial blood vessels also begin to form within the embryo as well as in the wall of yolk
sac and this wall is formed by the extra embryonic splanchnopleuric mesoderm. This primordial heart
it starts pumping the blood. Now as the embryo has grown enough the simple diffusion from this
lacunae into these cavities is simply not enough to meet the nutritious needs of the baby. So, this
circulatory system is required to help in exchange of material. So, now you can see that blood
vessels are found within the embryo and blood vessels are formed within this primordial placenta
but still there is no connection between these blood vessels. So, how the connection will be
established? Well! the answer is simple we have this connecting stalk. The blood vessels also
form within the connecting stalk and these are called the umbilical vessels. This connecting
stalk is the primordial of umbilical cord. Now let's see how umbilical cord forms.
First of all you know that an extension of yolk sac into the connecting stalk called allantois
forms. Now as the embryo undergoes the embryonic folding, this will be the structure of conceptus
a part of yolk sac is incorporated into the embryo and the remaining part of yolk sac is outside the
embryo but both of these part they are connected by this duct by this connection called vitelline
duct or omphaloenteric duct. As the embryo grow further this yolk sac is markedly diminished.
Now the whole this structure, this vitelline duct, this allantois
and the connecting stalk along with these blood vessels it forms what we call as
primitive umbilical cord later due to some unknown reason the amniotic cavity markedly grows in size
as it happens it completely surrounds and protects this umbilical cord from all sides.
As it happens the chorionic cavity is obliterated. Now there is no more chorionic cavity and this
amnion it is connected with chorion and these two layers they are called Amniochorion. So, as
you know that this embryo along with these extra embryonic membranes and this primitive placenta
it is implanted into the decidua of his or her mother and here is the decidua capsularis
and here is the deciduous basalis and rest of the decidua is the decidua parietalis,
as the embryo grows further the uterine cavity is obliterated and here you can see that this decidua
capsularis it starts touching the deciduous parietalis. As the embryo grows further the blood
supply of decidua capsularis is also compromised this results in degeneration of decidua
capsularis. Ultimately the decidua capsularis is also lost. This degeneration of decidua capsularis
happens somewhere between 5th and sixth month of development. So, what we can say is that initially
the embryo starts growing within the decidua but later on it is growing within the uterine cavity.
Remember that initially the embryo is not growing within the uterine cavity initially it is growing
within the uterus but later on as the decidua capsularis is lost now it is growing within
the uterine cavity and it is growing so much so that uterine cavity is even obliterated.
Thank you so much for watching this video. In the next lecture we will see the structure of placenta
in a bit more detail we will also learn more about the physiology and functioning of the placenta.