Macrophages in the urogenital ridge of the mid-gestation mouse fetus

Isolated urogenital ridges from mouse embryos 11 1/2-16 1/2 days post coitum were separated into genital ridge and mesonephric areas; the latter were then cut into small fragments and cultured as explants on glass or plastic. The explants soon became surrounded by flat outgrowths of cells, followed by a population of large round granular cells. The behaviour of these cells, first on the outgrowth and then on the glass is described. Evidence is presented that they are fetal macrophages.