©Polish Histochemical et Cytochemical Society

Human placental development The human placenta is a villous placenta; the structur-al and functional unit of the human placenta is the chorionic villous which becomes apparent in its defin-itive structure as early as day 21 of pregnancy [1] (Fig. 1). After nidation, the trophoblast differentiates in two ways: the villous and the extra-villous trophoblast. In the villous phenotype, the cytotrophoblastic cells (CT) of the floating villi (in the intervillous space) remain attached to the villous basement membrane, forming a monolayer of epithelial cells. These cells proliferate and differentiate, by fusion, to form a syncytiotro-phoblast that covers the entire surface of the villus (Fig. 1). This multinucleated syncytiotrophoblast is regenerated along pregnancy by a continuous turnover process including proliferation of mononuclear CT fol-lowed by the induction of early stages of apoptosis and fusion of these CT into ST. Apoptotic progression in the ST leads to the accumulation of condensed nuclei into syncytial knots and the shedding of these aggre-gates into the intervillous spaces. This membrane fusion process is complex and involves different fac-tors [see for review 2]. Among them endogenous retro-viral envelope proteins (syncytins), are specifically expressed in the trophoblast; syncytin 1, the envelo