Precursor of mast cells fixed in the skin of mice

The presence and origin of mast‐cell precursors fixed in the skin tissue of mice were investigated. Giant granules of beige (C57BL/6‐bgj/bgj, Chediak‐Higashi syndrome) mice were used to distinguish different populations of mast cells. Pieces of the skin were grafted from the intact WBB6F1 (WB × C57BL/6)F1−+/+ mice onto the back of the WBB6F1−+/+ mice which had been irradiated and injected with bone marrow cells of C57BL/6‐bgj/bjj mice (bgj/bgj −+/+ chimeras). Although the number of mast cells in the skin grafts decreased after the transplantation, the mast‐cell precursors circulating in the bloodstream of bgj/bgj −+/+ chimeras (bgj/bgj 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 3H‐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 WBB6F1‐W/Wv mice seemed to be depleted of fixed precursors, because most of mast cells were of bgj/bgj type in skin grafted from WBB6F1‐W/Wv mice to bgj/bgj−+/+ chimeras. Since the fixed precursor cells which proliferate and differentiate into mast cells after skin grafting may be transferred to WBB6F1‐W/Wv mice by bone marrow transplantation, such precursor cells seem to have been derived from the bone marrow.