SCL^−/Δ mice have increased mast-cell progenitors in the peritoneal cavity. (A) Numbers of toluidine blue⁺ and safranin red⁺ mast cells in connective tissue of the ears of SCL^+/Δ and SCL^−/Δ mice (n = 3 for each genotype). Results are the mean and standard error of the mean (SEM) of mast cells per 100× high-power field (HPF). (B) Wright-Giemsa stain of peritoneal fluid demonstrating the small, granular mast cells (→) seen only in the SCL^−/Δ mice. A normal, large granular mast cell from an SCL^+/Δ mouse is shown for comparison. The mean and SEM numbers of each type of mast cell were calculated by absolute peritoneal cell count × proportion of mast cells using flow cytometry. Data represent the mean and SD from 4 mice of each genotype. (C) Flow cytometric analysis of peritoneal cells from SCL^+/Δ and SCL^−/Δ mice stained for c-kit and Sca-1 expression. (D) Wright-Giemsa stain of sorted c-kit⁺Sca-1⁺ (K⁺S⁺) and c-kit⁺Sca-1⁻ (K⁺S⁻) peritoneal cells from an SCL^−/Δ mouse. (E) Expression of the high-affinity receptor for IgE (FcϵRI) on peritoneal mast- cell subsets from SCL^+/Δ and SCL^−/Δ mice. (F) Flow cytometric analysis of SCL^−/Δ K⁺S⁻ mast cells cultured for 3 weeks in media containing IL-3 and SCF.