Electron microscopic studies of most cells of human fetal skins

Electron microscopic study of human fetal skins revealed thick-type and elongated-type mast cells which seem similar to those in postpartum skin. Four types of mast cell granules were distinguished in these mast cells: granules presumed to be in mid-stage of maturation, with internal structures not contained inside a membrane-limited vacuole; granules of an undetermined nature, with internal structures contained inside a membrane-limited vacuole; electron dense granules presumed to be fully mature; compound-form granules presumably representing lysosomal structures. Among these granules, structures probably representing an earlier stage of granule formation were found. They consisted of membranous components, coarse granular components, and fine granular components. Based on the morphology of these granules and structures, a sequence in granule formation was postulated. Ultrastructural comparison between mast cells and dermal melanocytes in the same fetal skins revealed that these two mesenchymal cells were similar but not identical in the process of specific granule formation.