Abstract
The presence and origin of mast‐cell precursors fixed in the skin tissue of mice were investigated. Giant granules of beige (C57BL/6‐bg^j^/bg^j^, Chediak‐Higashi syndrome) mice were used to distinguish different populations of mast cells. Pieces of the skin were grafted from the intact WBB6F^1^ (WB × C57BL/6)F^1^−+/+ mice onto the back of the WBB6F^1^−+/+ mice which had been irradiated and injected with bone marrow cells of C57BL/6‐bg^j^/bj^j^ mice (bg^j^/bg^j^ −+/+ chimeras). Although the number of mast cells in the skin grafts decreased after the transplantation, the mast‐cell precursors circulating in the bloodstream of bg^j^/bg^j^ −+/+ chimeras (bg^j^/bg^j^ type) did not seem to enter into the skin grafts, because most of mast cells were of +/+ type after the recovery of mast‐cell number to pregrafting levels. As a considerable proportion of +/+‐type mast cells was labeled with ^3^H‐thymidine, the recovery of mast‐cell number in the grafts was attributed to the proliferation and differentiation of +/+‐type precursor cells fixed in the skin tissue of the donor. On the other hand, the skin of WBB6F^1^‐W/W^v^ mice seemed to be depleted of fixed precursors, because most of mast cells were of bg^j^/bg^j^ type in skin grafted from WBB6F^1^‐W/W^v^ mice to bg^j^/bg^j^−+/+ chimeras. Since the fixed precursor cells which proliferate and differentiate into mast cells after skin grafting may be transferred to WBB6F^1^‐W/W^v^ mice by bone marrow transplantation, such precursor cells seem to have been derived from the bone marrow.