Steady state and inflammation-induced mast cell numbers in the skin of Kit^+/+, Kit^W/Wv, R⁻γ⁻, R⁻γ⁻Kit^W/Wv, and Kit^W/W mice. After 6 weeks of PMA-induced chronic dermatitis, numbers of metachromatic mast cells were determined on toluidine blue–stained sections of ear skin. Closed symbols represent control (acetone)–treated ears, and open symbols represent PMA-treated ears. The figure summarizes data from 2 independent experiments with 3 mice per genotype and experiment (total number of mice = 6). Horizontal lines indicate the mean cell number for each group of mice (A-E). Numbers in healthy skin and the inflammation-induced increase were comparable in Kit^+/+ (A) and R⁻γ⁻ (C) mice. Numbers in healthy skin were very low in untreated Kit^W/Wv (B) and R⁻γ⁻Kit^W/Wv (D) skin. In PMA-treated skin, the increase in mast cell numbers was approximately 10-fold larger in Kit^W/Wv compared with R⁻γ⁻Kit^W/Wv mice. In the skin from Kit^W/W mice, mast cells were undetectable and remained absent after PMA-induced chronic dermatitis (E). The differences in mast cell counts comparing untreated (closed symbols) and PMA-treated (open symbols) skin samples had significant one-sided P values (< .05) (Mann-Whitney test) in panels A through C. The P value (.09) in panel D was not significant. Data from mice of 2 independent experiments were pooled in this figure. Mice used in these experiments were 13 to 21 weeks (Kit^+/+ [3 × C57BL/6; 3 × WB]), 8 to 19 weeks (Kit^W/Wv), 10 to 19 weeks (R⁻γ⁻), 9 to 16 weeks (R⁻γ⁻Kit^W/Wv), or 9 to 36 weeks (Kit^W/W) of age.