Blood mast cell progenitors divide slowly. (A) Single Lin⁻ CD34^hi CD117^int/hi FcεRI⁺ or Lin⁻ CD34^hi CD117^int/hi FcεRI⁺ CD45RA⁻ CRTH2⁻ integrin β7⁺ mast cell progenitors (MCp) were sorted into individual wells and cultured in a myeloerythroid cytokine cocktail. The number of cells in each well was counted after 7 days. (B) Cytocentrifuged and May-Grünwald Giemsa–stained cells from the wells in panel A were characterized based on morphology. At least 1 cell was identified from 25 of the wells in panel A. The Lin⁻ CD34^hi CD117^int/hi FcεRI⁺ and the Lin⁻ CD34^hi CD117^int/hi FcεRI⁺ CD45RA⁻ CRTH2⁻ integrin β7⁺ cells were sorted in 1 experiment each, and the results from each experiment were pooled.