The AA4^-/BGD6⁺ mast cells differentiate into mature mast cells in vivo. Light microscopy of a paraffin section of the spleen from a lethally irradiated mouse, hemotoxylin and eosin stained (A), or a serial section stained with toluidine blue (B). No mast cells can be seen. Light microscopy of a paraffin section of the spleen from a lethally irradiated mouse reconstituted with AA4^-/BGD6⁺ mast cells stained with hemotoxylin and eosin (C) or toluidine blue (D). Mast cells with metachromatic granules (arrows) are preferentially present in the area adjacent to the capsule of the spleen. CAP indicates capsule; SEP, septum. Magnification, × 500. Image was collected with an Olympus BX50 microscope equipped with a 40× UPLANFL objective (N.A. 0.75).