Mitosis of the Hofbauer cell: possible implications for a fetal macrophage.

Human chorionic villi from placentae collected during the first half of pregnancy were examined by light microscopy and transmission electron microscopy. In serial semithin sections mitoses of Hofbauer cells, as well as those of other cellular components of the villous stroma, were generally easily identified. In some cases, when the Hofbauer cells showed very few vacuoles, thin sections were helpful in differentiating this cell type from the fixed stromal cells. Hofbauer cells in mitotic division were not uncommon, but were irregularly distributed. Mitosis of the Hofbauer cells could be a mechanism involved in maintaining the permanent presence in the chorionic villi of cell subpopulations with different origins and functions. Another explanation for the mitotic division of these cells could be that they are a self-renewing population independent of fetal monocytes which appear later in gestation in chorionic villi. In addition, mitosis of the Hofbauer cells could allow a rapid increase in their numbers when required by the local microenvironment.