Part of “CHAPTER 107 – CONCEPTION, IMPLANTATION, AND EARLY DEVELOPMENT“

Development after implantation is rapid and complex. The embryo must establish both its placental compartment and its definitive fetal structures in a short time. The polar or embryonic trophectoderm (that overlying the inner cell mass) develops into an ectoplacental cone in the mouse, whereas in most primates, the trophoblast differentiates into syncytiotrophoblast and cytotrophoblast, the latter having a high mitotic rate. Rapid division produces a syncytial trophoblast surrounding the primate embryo, although the mural trophectoderm (that facing the uterine cavity) remains a single layer of cells. Lacunar spaces form within the syntrophoblast, which eventually becomes contiguous with the maternal capillary circulation, into which the chorionic villi will grow. The ectoplacental cone of the mouse and rat undergoes similar development, and the resulting placental structure is hemochorial, as in humans. The major placental classification among mammalian orders is derived from the number of tissue layers that separate the fetal and maternal circulations. There are six such potential barriers to exchange. Humans, like many other primates and murine species, have a hemochorial placenta in which three fetal tissues (endothelium, connective tissue, and chorionic epithelium) are bathed in maternal blood.65

As the process of placentation proceeds, definitive embryonic structures are developing. Immediately after implantation, a layer of cells appears at the blastocoele margin on the side of the inner cell mass. This layer is called the endoderm. The remaining cells of the inner cell mass are now called the epiblast or the primitive ectoderm. The endoderm proliferates rapidly and eventually surrounds the blastocoele. The epiblast cells (embryonic ectoderm) are now arranged in a columnar manner. Cells contiguous with the epiblast, called amnioblasts, appear; spaces between the amnioblasts develop (the proamnion) and eventually form the amnionic cavity. Although it is a matter of some debate, it seems possible that the amnioblast cells are the source of the amniotic fluid, which cushions and thereby protects the developing embryo. Apoptosis also plays a critical role in cavitation of the early embryo (not to be confused with cavitation of the blastocyst). A signal from the primitive endoderm acts over a short distance to induce apoptosis of the inner ectoderm cells, while survival of the outer ectodermal cells is mediated by interaction with the adjacent basement membrane that separates the ectoderm from the endoderm.66