Mast-cell accumulation and skin morphology after local PMA treatment. Six weeks after treatment of ears with solvent (acetone) (A,C,E,G) or with PMA (B,D,F,H) ear sections were stained with toluidine blue to assess the overall skin morphology and to obtain mast cell counts. Strongly increased skin thickness is apparent in all PMA-treated ears (B-H). Arrows indicate metachromatically stained mast cells in PMA-treated ears. Mast cells accumulated exclusively in the ears of Kit^+/+ (B) and Kit^W/Wv (D) and R⁻γ⁻Kit^W/Wv (F) mice. Mice used were 8 weeks (Kit^+/+ [WB]), 11 weeks (Kit^W/Wv), 25 weeks (R⁻γ⁻Kit^W/Wv), and 13 weeks (Kit^W/W) of age. The skin of Kit^W/W mice that lack all Kit cell-surface expression was completely devoid of mast cells. This was true before (G) and after (H) PMA-induced inflammation. The scale bar (A) applies to all panels and corresponds to 40 μm.