Mononuclear phagocytes in the human placenta.

Hofbauer cells have been identified by morphological criteria in human placentae and are considered to be mononuclear phagocytes. An increasing recognition of the morphological heterogeneity of mononuclear phagocytes and of the considerable difficulty in their recognition in histological sections has suggested that they may be considerably more numerous in the placenta than has been appreciated. The use of specific markers for mononuclear phagocytes in the present study facilitated their identification and quantitation in the placenta. Approximately 65 per cent of the cells in cell suspensions from placentae at all stages of gestation exhibited avid Fc receptors and were morphologically indistinguishable from mononuclear phagocytes. Most of these cells also were phagocytic and expressed C3 receptors. Furthermore, when cells expressing alpha-naphthyl butyrate esterase positivity as well as high-affinity receptors for immune complexes were visualized in the stroma of chorionic villi, nearly all stromal cells were positive. The results suggest that Hofbauer cells represent the fully differentiated, morphologically-obvious end-stage members of a large mononuclear phagocyte population which is present in chorionic villi throughout gestation.