β-Gal expressing tissue macrophage and microglia engraftment in B6/129F2 animals 1 year after transplantation. Cryostat sections of spleen, lung, liver, and brain from B6/129 F2 animals transplanted with ROSA 26 BM were histochemically stained blue to demonstrate the presence of β-gal expressing ROSA 26 origin cells followed by immunocytochemical staining red with the antibodies F4/80 and M1/70 directed against mouse macrophage markers. Double-positive cells (arrows) are shown in the spleen (A), lung (B), and perivascular areas of the brain (C). Only rare double-positive cells are found in the parenchyma of the brain (D). All double-positive cells in (B) and (D) are indicated by arrows. Some double-positive cells in (A) and (C) are highlighted by arrows.