Kinetics of tissue macrophage and microglial engraftment. Cryostat sections of spleen, lung, liver, and brain from B6/129 F2 animals transplanted with ROSA 26 BM were histochemically stained to show the presence of β-gal expressing ROSA 26 origin cells followed by immunocytochemical staining with the antibodies F4/80 and M1/70 directed against mouse macrophage markers. A percent engraftment with ROSA 26 origin cells was calculated using the formula: % Engraftment = β-Gal⁺ Macrophages/Total Macrophages. Macrophages were enumerated on three representative sections each from three transplanted animals at every time point. (▧), 1 month, (▪), 2 months; (□), 6 months; (▧), 1 year.