Within the human placenta the villous trophoblast represents the epithelial covering of the chorionic villi. This layer of trophoblast actively participates in transport processes between mother and fetus and is the surface of the barrier separating maternal and fetal blood. It represents the main site of metabolic and endocrine activities of the placenta. It is comprised of an outer continuous layer without cell borders, the syncytiotrophoblast, beneath which resides a population of proliferating cytotrophoblast cells. Daughter cytotrophoblast cells fuse into the syncytium, while aged nuclei inside syncytial fragments are shed into the maternal circulation (Figure 1). Any process disturbing the balance between proliferation, syncytial fusion or shedding will alter the integrity and function of the layer as a whole, which in turn may impair gas and/or nutrient transfer to the fetus as well as endocrine activities. Trophoblast turnover is thus tightly regulated to ensure normal fetal development.
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