A pure population of AA4^-/BGD6⁺ mast cells can be isolated from the mast cells present in bone marrow. The AA4^-/BGD6⁺ mast cells are a homogeneous population of cells. Cells representative of the freshly isolated AA4^-/BGD6⁺ cell population are shown here. In this representative experiment from 12 experiments, the flow profiles show that the AA4⁺ cells constitute 2.58% of the cells in the bone marrow while the BGD6⁺ cells make up 2.51% of the cells in the bone marrow. R1 indicates gated area. (A) Bone marrow cells labeled with mouse IgG-FITC. (B) Bone marrow cells labeled with mAb-AA4–FITC. (C) Bone marrow cells labeled with mAb-BGD6–FITC. (D) These undifferentiated cells have large nuclei and little cytoplasm (arrows). Cells were stained with toluidine blue (arrowhead, mAb-BGD6–conjugated magnetic bead); magnification, × 2000. Image was collected with an Olympus BX50 microscope (Olympus America) equipped with a 60× UPLANFL objective (N.A. 1.25). (E) By electron microscopy, these cells (arrow) have a large concave nucleus (N), little cytoplasm, and no cytoplasmic granules (M indicates mitochondria; arrowhead, mAb-BGD6–conjugated magnetic bead); magnification, × 6600.