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  

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.