Trophoblast: Functions, Layers and Development

He trophoblast is a structure constituted by a set of cells that form the outer layer that surrounds a blastocyst, in early stages of the embryonic development of mammals. The term comes from the Greek trophies , Which means"feed"; and of blasto , which refers to the embryonic germ cell.

During the early stages of pregnancy of placental mammals, the trophoblast cells are the first to differentiate into an ovule that has been fertilized. This set of cells is known as trophoblast, but after gastrulation they are called trophoectoderm.

The trophoblast provides nutritive molecules to the developing embryo and facilitates its implantation to the uterine wall due to its ability to erode the tissues of the uterus. Thus, the blastocyst can join the cavity formed by the uterine wall, where it will absorb nutrients from the fluid coming from the mother.

Functions

The trophoblast plays a crucial role in implantation and placentation. Both processes occur correctly as a consequence of molecular communication between fetal and maternal tissues, mediated by hormones and membrane receptors.

During the implantation of the blastocyst new types of different trophoblastic cells are generated, called the villous and extravillous trophoblast. The former participates in the exchanges between the fetus and the mother, and the latter unites the placental body to the wall of the uterus.

On the other hand, the placentation is characterized by the invasion of the uterine spiral arteries by extravillus trophoblastic cells that arise from the anchoring of the villi. Due to this invasion, the arterial structure is replaced by amorphous fibrinoid material and endovascular trofoblastic cells.

This transformation establishes a perfusion system of low capacity and high capacity from the radial arteries to the intervillous space, in which the villous tree is embedded.

The physiology of pregnancy depends on the orderly progress of the structural and functional changes of the villous and extravillous trophoblast.

This means that a disorder of such processes can lead to different types of complications of varying degrees of severity, including the possible loss of pregnancy and deadly diseases.

The trophoblast, although it does not directly contribute to the formation of the embryo, is a precursor of the placenta whose function is to establish a connection with the maternal uterus to allow the nutrition of the developing embryo. The trophoblast is evident from day 6 in human embryos.

Layers

During implantation, the trophoblast multiplies, grows and differentiates into two layers:

Syncytiotrophoblast

The syncytiotrophoblast is the outermost layer of the trophoblast, its cells do not have intercellular limits because their membranes have been lost (syncytium). For this reason, the cells are observed as multinucleated and form cords that infiltrate the endometrium.

The syncytiotrophoblast cells come from the fusion of the cytotrophoblast cells and their growth causes the generation of chorionic villi. These serve to increase the surface area that allows the flow of nutrients from the mother to the fetus.

Through apoptosis (programmed cell death) of the cells of the uterine stroma spaces are created through which the blastocyst penetrates more into the endometrium.

Finally, the hormone human chorionic gonadotropin (HCG) is produced in the syncytiotrophoblast, which is detected after the second week of gestation.

Citotrophoblast

For its part, the cytotrophoblast forms the innermost layer of the trophoblast. Basically, it is an irregular layer of ovoid cells with a single nucleus and that is why they are called mononuclear cells.

The cytotrophoblast is directly below the syncytiotrophoblast and its development begins from the first week of gestation. The trophoblast facilitates embryo implantation through cytotrophoblast cells, which have the ability to differentiate into different tissues.

The proper development of cytotrophoblast cells is crucial for the successful implantation of the embryo to the uterine endometrium and is a highly regulated process. However, the uncontrolled growth of these cells can generate tumors, such as choriocarcinoma.

Development

During the third week, the embryonic development process also includes the continued development of the trophoblast. Initially, the primary villi are formed by the internal cytotrophoblast surrounded by the outer layer of syncytiotrophoblast.

Subsequently, the cells of the embryonic mesoderm migrate towards the primary villus nucleus and this occurs during the third week of gestation. At the end of this week, these mesodermal cells begin to singulate to form blood vessel cells.

As this process of cell differentiation progresses, what is known as the hair villous system will be formed. At this point the placental villi is formed, which will be the final one.

The capillaries that are formed from this process will subsequently come into contact with other capillaries that are forming simultaneously in the mesoderm of the chorionic plate and the fixation pedicle.

These newly formed vessels will come into contact with those of the intraembryonic circulatory system. Thus, the moment the heart begins to beat (this occurs in the fourth week of development) the villous system will be ready to supply the oxygen and nutrients necessary for its growth.

Continuing with the development, the cytotrophoblast penetrates even more in the syncytiotrophoblast that covers the hair, until reaching the maternal endometrium. They come in contact with hairy stems and form the external cytotrophoblastic covering.

This layer goes around the trophoblast and ends firmly joining the chorionic plate to the endometrial tissue at the end of the third week (days 19-20) of gestation.

While the chorionic cavity has enlarged, the embryo is anchored to its trophoblastic covering by the fixation pedicle, a rather narrow linkage structure. Subsequently, the fixation pedicle will become the umbilical cord that will connect the placenta with the embryo.

References

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