cell mast cell A cell that is found in almost all tissues containing numerous basophilic granules and capable of releasing large amounts of histamine and heparin upon activation. Progenitors leave bone marrow and mature in connective and mucosal tissue. Mature mast cells are found in all tissues, except the bloodstream. Their phenotype is CD117-high, CD123-negative, CD193-positive, CD200R3-positive, and FceRI-high. Stem-cell factor (KIT-ligand; SCF) is the main controlling signal of their survival and development. mast cell