The AA4^-/BGD6⁺ mast cells differentiate into mature mast cells in vitro. (A) Light microscopy of cells from cultures of AA4^-/BGD6⁺ mast cells. After 4 weeks in culture many of the cells now contain granules (arrows) when stained with toluidine blue; magnification, × 750. Additionally, an occasional granulated cell can be seen still attached to an mAb-BGD6–conjugated magnetic bead (inset, arrow-head; magnification, × 880). Image was collected with an Olympus BX50 microscope equipped with a 60× UPLANFL objective (N.A. 1.25). (B) Electron microscopy of cells from cultures of AA4^-/BGD6⁺ mast cells. By electron microscopy at 4 weeks, the cells from the culture of mast cell precursors have a morphology typical of cultured bone marrow–derived mast cells. Their surface is covered with thin microvilli (small arrow), and the cytoplasm contains a well-developed Golgi apparatus (G), numerous mitochondria (M), and cytoplasmic granules (large arrows) of varying density. N indicates nucleus. Magnification, × 4000.