The trophoblast is a structure made up of a group of cells that form the outer layer that surrounds a blastocyst, in early stages of embryonic development in mammals. The term comes from the Greek trophos, which means "to feed"; and blast, which refers to the embryonic germ cell.
During the early stages of placental mammalian pregnancy, trophoblast cells are the first to differentiate into an egg that has been fertilized. This set of cells is known as a trophoblast, but after gastrulation it is called a trophectoderm.
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.
Features
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 implantation of the blastocyst, new types of distinct trophoblastic cells are generated, called the villous and extravillous trophoblast. The former participates in exchanges between the fetus and the mother, and the latter joins the placental body to the wall of the uterus.
For its part, placentation is characterized by the invasion of the uterine spiral arteries by extravillous trophoblastic cells that arise from the anchorage of the villi. Due to this invasion, the arterial structure is replaced by amorphous fibrinoid material and endovascular trophoblastic cells.
This transformation establishes a low-capacity, high-capacity perfusion system 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 structural and functional changes in the villous and extravillous trophoblast.
This means that a disorder of these processes can lead to different types of complications of varying degrees of severity, including possible loss of pregnancy and fatal diseases.
The trophoblast, although it does not contribute directly 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 constitutes the outermost layer of the trophoblast, its cells do not have intercellular limits because their membranes (syncytium) have been lost. For this reason, the cells appear 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 uterine stromal cells spaces are created through which the blastocyst penetrates further into the endometrium.
Finally, the syncytiotrophoblast produces the hormone human chorionic gonadotropin (HCG) that is detected from the second week of gestation.
Cytotrophoblast
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 located directly below the syncytiotrophoblast and its development begins from the first week of gestation. The trophoblast facilitates embryonic 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 process that is highly regulated. However, the uncontrolled growth of these cells can lead to tumors, such as a choriocarcinoma.
Development
During the third week, the embryonic development process also includes the continuation of trophoblast development. Initially, the primary villi are formed by the inner cytotrophoblast surrounded by the outer layer of syncytiotrophoblast.
Later, the cells of the embryonic mesoderm migrate towards the nucleus of the primary villi and this occurs during the third week of gestation. At the end of this week, these mesodermal cells begin to single out to form blood vessel cells.
As this process of cell differentiation progresses, what is known as the hairy capillary system will form. At this point the placental villi is formed, which will be the final one.
The capillaries that form 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, at the moment in which 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 development, the cytotrophoblast penetrates further into the syncytiotrophoblast that covers the vellocity, until it reaches the maternal endometrium. They come into contact with hairy stems and form the outer cytotrophoblastic covering.
This layer surrounds the entire trophoblast and ends up 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 remains anchored to its trophoblastic covering by the fixation pedicle, a rather tight ligation structure. Later, the fixation pedicle will become the umbilical cord that will connect the placenta with the embryo.
References
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- Lunghi, L., Ferretti, ME, Medici, S., Biondi, C., & Vesce, F. (2007). Control of human trophoblast function. Reproductive Biology and Endocrinology, 5, 1–14.
- Pfeffer, PL, & Pearton, DJ (2012). Trophoblast development. Reproduction, 143 (3), 231–246.
- Red-Horse, K., Zhou, Y., Genbacev, O., Prakobphol, A., Foulk, R., McMaster, M., & Fisher, SJ (2004). Trophoblast differentiation during embryo implantation and formation of the maternal-fetal interface. Journal of Clinical Investigation, 114 (6), 744–754.
- Screen, M., Dean, W., Cross, JC, & Hemberger, M. (2008). Cathepsin proteases have distinct roles in trophoblast function and vascular remodeling. Development, 135 (19), 3311–3320.
- Staun-Ram, E., & Shalev, E. (2005). Human trophoblast function during the implantation process. Reproductive Biology and Endocrinology, 3 (Figure 2), 1–12.
- Velicky, P., Knöfler, M., & Pollheimer, J. (2016). Function and control of human invasive trophoblast subtypes: Intrinsic vs. maternal control. Cell Adhesion and Migration, 10 (1-2), 154–162.