Mast cells in the skin of rats during development

: The number of mast cells in the skin of the rat increases from the 17th day of the embryonic period to the end of the first postnatal month in the dorsum of the trunk and the head, while in the overlip skin the peak values are reached at the end of the first postnatal week. In the adult animal, the normal values are about one third of the maximum values in all areas. The number of mast cells decreases during the three first days after incised skin injuries; later on, their number increases up to the tenth day, and subsequently decreases, reaching normal values at the end of the second week. These patterns parallel the increase in fibrillary proteins, the development of vascularization and growth of hair follicles in the different scar tissue in skin injuries, respectively. A trophic affect of mast cells may be postulated. This could be exerted by two pathways: an indirect one, related to the vasoactive action of histamine, an a direct one, showing the increased rate of cell division induced by histamine in cultures of epidermal cells. The addition of cimetidine inhibits this stimulating action of histamine; because of this, the stimulating effect of histamine appears to be mediated by H2 receptors.