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Mammalian placentation is a highly regulated process and is dependent on the proper development of specific trophoblast cell lineages. The two major types of trophoblast, villous and extravillous, show mitotic arrest during differentiation. In mice, the transcription factor, glial cell missing-1 (Gcm1), blocks mitosis and is required for syncytiotrophoblast formation and morphogenesis of the labyrinth, the murine equivalent of the villous placenta. The human homolog GCM1 has an analogous expression pattern, but its function is presently unknown. We studied GCM1 function in the human-derived BeWo choriocarcinoma cell line and in first trimester human placental villous and extravillous explants. The GCM1 expression was either inhibited by siRNA and antisense oligonucleotides methods or upregulated by forskolin treatment. Inhibition of GCM1 resulted in an increased rate of proliferation, but prevented de novo syncytiotrophoblast formation in syncytially denuded floating villous explants. GCM1 inhibition prevented extravillous differentiation along the invasive pathway in extravillous explants on matrigel. By contrast, forskolin-induced expression of GCM1 reduced the rate of proliferation and increased the rate of syncytialization in the floating villous explant model. Our studies show that GCM1 has a distinct role in the maintenance, development and turnover of the human trophoblast. Alterations in GCM1 expression or regulation may explain several aspects of two divergent severe placental insufficiency syndromes, namely preeclampsia and intrauterine growth restriction, which cause extreme preterm birth. Successful mammalian pregnancy demands two key steps in placental development, both of which are regulated by differentiation of the epithelial trophoblast lineage. 1 First, maternal blood flow to the implantation site must increase exponentially to serve the demands of the rapidly growing fetus. This is achieved through the invasion of extravillous cytotrophoblasts (EV-CTs) from the base of the placenta into the maternal uterine stroma, where they surround and dilate the distal portions of the utero-placental arteries. 2 Second, the increased delivery of maternal blood to the placenta must be matched by an exchanger organ to transfer nutrients, remove waste products and respiratory gases between the mother and the developing fetus. This requirement is achieved through the expansion and differentiation of the chorionic villous trees of the placenta. These villi are covered by a villous trophoblast compartment comprising of villous cytotrophoblasts (V-CTs) beneath a continuous multinucleated layer of syncytiotrophoblast (SCT), which is in direct contact with maternal blood. 3 In the human placenta, EV-CTs of the proximal portion of the anchoring villus cell column are proliferative, but more distal cells differentiate, as they invade and disperse into the uterine stroma. Here these cells surround and replace the smooth muscle cells of maternal uterine arteries, converting them into high-flow dilated sinusoids. The ar...

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