To proliferate or to divide - to be or not to be

Placental growth depends on cytotrophoblast (CTB) proliferation. A portion of CTBs, which retain an undifferentiated phenotype throughout pregnancy, provide a reservoir of stem cells. The remaining CTBs give rise to the two major trophoblast subpopulations, syncytiotrophoblast (STB) and invasive CTBs. Some of the molecular mechanisms that govern human CTB differentiation and invasion are well understood. These include an upstream suites of transcription factors such as Mash-2, Hand-1 and Gem1, and a downstream set of effectors such as adhesion molecules, proteinases, and the trophoblast major histocompatibility antigen HLA-G. In comparison, much less is known about how CTB proliferation is coordinated with differentiation. We immunolocalized markers that are specifically expressed during all of the key transitions and phases of the cell cycle in tissue sections of the maternal-fetal interface. We mapped cell cycle progression in both populations with the goal of understanding the mechanisms that maintain a pool of CTB stem cells (villus CTBs) and govern CTB exit from the cell cycle during differentiation along the invasive pathway. The results showed that as CTBs differentiate/invade or differentiate/fuse, they down-regulate the expression of molecules that are associated with mitosis and up-regulate the expression of a number of inhibitors that engineer permanent withdrawal from the cell cycle. Multinucleate STB coexpressed an unusual repertoire of markers that are usually segregated to distinct portions of the cell cycle. One of the regulatory factors involved in the regulation of CTB proliferation during early pregnancy is oxygen. During much of the first trimester of pregnancy there is little endovascular invasion, so maternal blood flow to the placenta is at minimum. This creates hypoxic environment in which placental development occurs. During this period, placental mass increases much more rapidly than that of the embryo proper. Both in vivo and in vitro data suggest that oxygen tension might regulate cytotrophoblast proliferation and differentiation along the invasive pathway. The fact that hypoxia stimulates CTBs, but not other cells, to undergo mitosis could help account for the discrepancy in size between the embryo and the placenta, which continues well into the second trimester of pregnancy. As cytotrophoblast invade the uterine blood vessels, they encounter a steep, positive oxygen tension gradient and exit gradually from the cell cycle. Interesting gestation-related changes were also observed by the second trimester, many fewer CTB stem cells and cells in cell column expressed mitotic markers. There was a reciprocal increase in the number of column CTBs that expressed inhibitors. Together, these data suggest that CTB proliferation, like differentiation, is part of a developmental program that is timed to precede development of the embryo/fetus.