Pericytes of term human foeto-placental microvessels: ultrastructure and visualization

Placental microvessels examined by transmission electron microscopy showed many pericytes and pericyte foot processes included in the basement membrane. The foot processes were distinct from the endothelial cell extensions because of their lighter staining, their content of reticulum endoplasmic, their scarcity of microfilaments and their separation from endothelial cells by basement membrane material. Some of these features were held in common with early villous perivascular cells and pericytes from other microvascular beds. Denudation of the villi by dissection allowed the microvascular basement membrane to be reached and pericytes to be identified by scanning electron microscopy as large cells with extensions apposed to microvessels. Similarly, a two-step collagenase-dispase digestion with an intermediate Percoll gradient separation progressively removed the trophoblast and the stroma down to the microvessels. Pericyte-like cells were observed lying over the microvessels that exhibited some terminal dilatations where endothelial cells were still surrounded by the basement membrane. The origin and the potential functions of the pericytes (contractility, basement membrane secretion, angiogenesis and phagocytosis) in the placental vascular bed are discussed.