Regulatory mechanisms to maintain epithelial barrier integrity in the placenta.

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Cells that comprise epithelial barriers turnover frequently. Maintenance of epithelial barriers is contingent on the actions of progenitor cells, which either propagate to replenish themselves, or differentiate into the specialized epithelium required for that tissue’s function. An excellent example of this is the placental epithelium, where progenitor cytotrophoblast cells either divide to restock the existing pool of progenitor cells, or differentiate by fusing into an overlying syncytium (syncytiotrophoblast). Syncytiotrophoblast forms the primary barrier separating maternal and fetal tissue, but is shed into maternal circulation throughout pregnancy and needs to be replenished by underlying cytotrophoblast cells. The regulatory mechanisms that govern the transition from cytotrophoblast propagation to differentiation is not well known, but this process is