The extravillous and villous trophoblasts are the embryonic-fetal tissues of the anatomical interface of the placental arm; the avascular fetal membranes—the amnion and chorion laeve—are the fetal tissues of the anatomical interface of the paracrine arm of this system.
The placental arm of this system links the mother and fetus as follows: maternal blood (spurting out of the uteroplacental vessels) directly bathes the syncytiotrophoblast, the outer surface of the trophoblastic villi; fetal blood is contained within fetal capillaries, which traverse within the intravillous spaces of the villi. This is a hemochorioendothelial type of placenta. The paracrine arm of this system links the mother and fetus through the anatomical and biochemical juxtaposition of (extraembryonic) chorion laeve and (maternal uterine) decidua parietalis tissue.
[...] The embryonic period extends until the end of the seventh week, at which time the major structures are present. Fetus: After the embryonic period, the developing conceptus is referred to as the fetus. Conceptus: This term is used to refer to all tissue products of conception—embryo (fetus), fetal membranes, and placenta. The conceptus includes all tissues, both embryonic and extraembryonic, that develop from the zygote. FERTILIZATION OF THE OVUM AND CLEAVAGE OF THE ZYGOTE Few, if any, naturally occurring phenomena are of greater importance to humankind than the union of egg and sperm. [...]
[...] Desmosomes develop between the cells; and as the cytotrophoblasts fuse, the expression of E-cadherin diminishes. The cytotrophoblast is the germinal cell; the syncytium, or the secretory component, is derived from cytotrophoblasts. Therefore, the cytotrophoblasts are the cellular progenitors of the syncytiotrophoblast. Well-demarcated borders and a single, distinct nucleus characterize each cytotrophoblast; and there are frequent mitoses among the cytotrophoblasts. These characteristics are lacking, however, in the syncytium, in which the cytoplasm is amorphous, without cell borders, and the nuclei are multiple and diverse in size and shape. [...]
[...] The gradual accumulation of fluid between blastomeres within the morula results in the formation of the blastocyst. The compact mass of cells at one pole of the blastocyst, called the inner cell mass, is destined to be the embryo. The outer mass of cells is destined to be the trophoblasts. THE EARLY HUMAN ZYGOTE. Hertig and co-workers (1954) found in the two-cell zygote that the blastomeres and the polar body are free in the perivitelline fluid and are surrounded by a thick zona pellucida. [...]
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