Regulatory mechanisms to maintain epithelial barrier integrity in the placenta.

构成上皮屏障的细胞频繁更替。上皮屏障的维持取决于祖细胞的作用，祖细胞要么增殖以补充自身，要么分化成该组织功能所需的特化上皮。一个很好的例子是胎盘上皮，其中祖细胞滋养层细胞要么分裂以补充现有的祖细胞库，要么通过融合成覆盖的合胞体（合胞体滋养层）而分化。合胞体滋养层形成分离母体和胎儿组织的主要屏障，但在整个妊娠期间脱落到母体循环中，并且需要由下面的细胞滋养层细胞补充。控制细胞滋养层增殖向分化转变的调节机制尚不清楚，但这一过程是一个复杂的过程。

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