Fellowship in Embryology by Medline Academics

In this article will talk about development of placenta. So, what is placenta? What is the definition of placenta? Placenta is a discoidal plate-like temporary but vital vascular ketomaternal organ that is responsible for metabolism and exchange of chemicals between mother and foetus or the embryo. Please note that we will use the term embryo and foetus interchangeably during this lecture but truly speaking the foetus is an embryo that has grown beyond the ninth week of development after fertilisation.

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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 contain nutritious egg yolk. The embryos of the bird develop inside these eggs while utilising the egg yolk. They utilise 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.

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The human mothers are much protective regarding their embryos. The human embryo grows inside the womb of his or her mother. Womb 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 foetus 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 fertilised egg called zygote it undergoes a series of mitotic division called cleavage to form a ball of cell called marula.

Marula consists of closely packed cells called blastomeres. They are tightly, closely packed with each other. At this marula stage these blastomeres they decide so as to say that these outer cells they will, they will arrange for the nutrition and these inner cells they will form the embryo proper. So, as this marula comes inside the uterine cavity here this zona pellucida 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 form the major part of the foetal placenta.

Actually, the placenta it has two parts the foetal 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 selectins 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 we 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. So, after loosely binding through the selectin molecules now this embryo will firmly bind with the help of fibronectin and integrins.

That 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 trophoblast 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 erode 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 cell cavity and whole this structure is surrounded by chorionic cavity it is also called extra embryonic coelom and here is the extra embryonic stomatologist mesoderm.

Remember that chorion consists of extra embryonic semitauonic 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 lacunar here the nutrients including oxygen and glucose they can diffuse into the chorionic cavity and from chorionic cavity nutrition’s 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 this stage it is secreted by these 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 a 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 decidua that is overlying the implanted conceptus is called decidua capsularis and this basal plate like decidua 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 partials.

So, we have three types of deciduae. 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 villus and let’s see what further happens to this primary villus. So, here is a single primary villus.

It consists of finger like projection of cytotrophoblast along with that here is the extra embryonic somatopluric isoderm. So, here is another 3d diagram of cut section of this primary villi. During the third week of development the underlying extra embryonic somatopluric mesoderm sends its extension into the core of this primary villi and as this mesenchymal core is formed inside the primary villi now this primary villi is converted to the secondary villus.

So, again in this 3d diagram you can see that there is the inner core, inner mesenchymal core which is the extension of extra embryonic somatopluric mesoderm. This mesenchyme then forms the blood vessels inside it and as the blood vessels form inside this villus 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 brown 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 a 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, these 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 fernosum 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 levi.

So, there are two faces of chorion the chorionic fernosum and chorionic levi. Please note that only the chorionic fernosum contributes towards the foetal 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 somatopluric mesoderm these chorionic vessels develop and these chorionic vessels they are connected with the vasculature of these 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 splanchnopluric 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 formed 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. So, 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 allentise 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 parts they are connected by this duct, by this connection called vitiline duct or umphaloenteric duct. As the embryo grow further this yolk sac is markedly diminished. Now, hold this structure this vitiline duct, this allentise and the connecting stalk along with these blood vessels it forms what we call as a primitive umbilical cord.

Later due to some unknown reason the amniotic cavity markedly grows in size. As it happens it completely surrounds and protect 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 the 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 decidua basalis and rest of the decidua is the decidua paritalis. As the embryo grows further the uterine cavity is obliterated and here you can see that this decidua capsularis it starts touching the decidua paritalis. 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 fifth 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.