Successful endometrial blastocyst adhesion involves modiication in expression of cellular adhesion molecules (CMs). The integrins-one of four families of CAMs-are cell-surface receptors that mediate cell adhesion to extracellular matrix proteins (Lessey, 2002). Endometrial integrins are hormonally regulated, and a specific set of integrins is expressed at implantation (Lessey, 1995). Recognition-site blockade of integrins needed for binding will prevent blastocyst attachment (Kaneko, 2013).
Human placental formation begins with the trophectoderm, which gives rise to a trophoblast cell layer encircling the blastocyst. From then until term, trophoblasts playia critical part at the fetal-maternal interface. Trophoblasts exhibit the most variable structure, function, and developmental pattern of all placental components. Their invasiveness promotes implantation, their nutritional role for the conceptus is relected in their name, and their endocrine organ function is essential to maternal physiological adaptations and to pregnancy maintenance.
By the eighth day postfertilization, after initial implantation, trophoblasts have diferentiated into an outer multinucleated syncytium-primitive syncytiotrophoblast, and an inner layer of primitive mononuclear cells-cytotrophoblasts. The latter are germinal cells for the syncytium. As cytotrophoblasts proliferate, their cell walls disappear, and the cells fuse to add to the expanding outer layer of syncytiotrophoblast. Each cytotrophoblast has a well-demarcated cell border, a single nucleus, and ability to undergo DNA synthesis and mitosis (Arnholdt, 1991). These are lacking in the syncytiotrophoblast, which provides transport functions of the placenta. It is so named because instead of individual cells, it has an amorphous cytoplasm without cell borders, nuclei that are multiple and diverse in size and shape, and a continuous syncytial lining. This coniguration aids transport.
After implantation is complete, trophoblasts further differentiate along two main pathways, giving rise to villous and extravillous trophoblasts. As shown in Figure 5-8, both have distinct functions (Loke, 1995). Villous trophoblasts generate chorionic villi, which primarily transport oxygen, nutrients, and other compounds between the fetus and mother. Extravillous trophoblasts migrate into the decidua and myometrium and also penetrate maternal vasculature, thus coming into contact with various maternal cell types (Pijnenborg, 1994). Extravillous trophoblasts are further classiied as interstitial trophoblasts and endovascular trophoblasts. he interstitial trophoblasts invade the decidua and eventually penetrate the myometrium to form placental-bed giant cells. These trophoblasts also surround spiral arteries. The endovascular trophoblasts penetrate the spiral artery lumens (Pijnenborg, 1983). hese are both discussed in greater detail in subsequent sections.
Early End of 1st pregnancy trimester
FIGURE 5-8 Extravillous trophoblasts are found outside the villus and can be subdivided into endovascular and interstitial categories. Endovascular trophoblasts invade and transform spiral arteries during pregnancy to create low-resistance blood flow that is characteristic of the placenta. Interstitial trophoblasts invade the decidua and surround spiral arteries.