Regulatory mechanisms to maintain epithelial barrier integrity in the placenta.

构成上皮屏障的细胞频繁翻转。上皮屏障的维持取决于祖细胞的行为，祖细胞要么繁殖以补充自己，要么分化为组织功能所需的特化上皮。胎盘上皮是一个很好的例子，在胎盘上，祖细胞滋养层细胞要么分裂以补充现有的祖细胞池，要么通过融合成覆盖的合胞体(合体滋养层细胞)而分化。合体滋养层细胞构成分隔母体和胎儿组织的主要屏障，但在整个妊娠过程中进入母体循环，需要底层细胞滋养层细胞的补充。从细胞滋养层细胞增殖到分化的调控机制还不是很清楚，但这个过程是

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