Immunoelectron microscopy of mBMMC with mono- (a) and multi-lobular (b through e) nuclei. Most cells in the culture have numerous multi-vesicular structures in their granules (V). However, in terms of their electron density, some of the granules (D) in these cultured cells are nearly as mature as those in tissue mast cells. Frozen thin sections of the mBMMC were stained with immunogold anti–mMC-CPA Ig (c), anti–mMCP-6 Ig (d), or a control rabbit Ig (e). Ten-nanometer gold particles were used with the antibodies. N, nucleus.