Development and differentiation of dermal cells in man.

Development and differentiation of the single free cells of mesenchyme and dermis of human embryos and fetuses from week 6 to term is described. From week 6 to week 14, three cell types are present: stellate general mesenchymal cells with long processes, phagocytic macrophages of probable yolk-sac origin, and a granule-secretory type of cell, which could be either a melanoblast or a mast stem cell. From week 14 to week 21, fibroblasts are numerous and active, and perineurial cells, pericytes, melanoblasts, mast cells, and Merkel cells can be individually identified. There is also present another cell type, possible of bone marrow origin, that may be ancestral to the Langerhans cell and that may be carried over into postanal dermis as the "histiocyte" or fixed dermal macrophage. From week 24 to term there is little change apart from the development of fat cells in the deeper dermis. Neither lymphocyte nor plasma cell was observed at any stage of development. These observations were used in the specification and identification of cells of fully developed postnatal dermis.