The ontogenesis of the villous and vascular arborescence in normal pregnancy is reviewed. The emergence of the villi and the fetal vessels is described from early pregnancy to term together with the specializations of the villi, vessels and capillaries observed in the last trimester. The expression, localization and role of the angiogenic growth factors (FGF, VEGF, PLGF, HGF) are described and discussed. Pathological pregnancies with hyper- and hypocapillarization are related to altered oxygenation. The potential roles of growth factors are presented and hypotheses proposed which may provide a molecular basis for the development of the most frequent placental pathologies.

The phenotype of perivascular placental cells has previously been studied using tissue sections from the fetal villi. The examination of these cells in culture by scanning electron microscopy gives us the opportunity to observe their three-dimensional phenotypes and associations outside their normal constraints. Human umbilical endothelial cells, which have a phenotype comparable to that observed in other studies, seem more flattened in culture than in their usual environment. Microvascular endothelial cells did not attain an epithelioid phenotype with close contacts between cells but formed a network of branched, elongated cells with phagocytotic activity. Some circular associations were observed when using a gelatinized matrix. Microvascular pericytes were large, flattened cells with an irregular border that pushed up nodular associations on a gelatin matrix. Chorioplacental myocytes adopted a network template comparable to that developed by microvascular endothelial cells. However, these elongated cells were thicker, without microvilli, and superficial filaments could be observed. In culture, confluent endothelial cells from the umbilical cord or microvascular pericytes associated as nodules reached a cell phenotype close to their in vivo counter-parts. This attainment of an in vivo phenotype remains questionable for chorioplacental myocytes. Microvascular endothelial cells, however, though there was sparse formation of circular associations, remained far from their in vivo phenotype.

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