Electron micrographs of multimerin-transfected AtT 20 cells, immunogold labeled with antibodies to ACTH (a) or multimerin (b). (a) Cells labeled with immunogold for ACTH contained gold particles within their secretion granules (sg), and the plasma membrane (pm) and mitochondria (m) were not labeled (original magnification × 60,000). (b) Cells labeled with immunogold for multimerin contained multimerin in secretion granules (sg). The mitochondria (m), as a control structure, and the plasma membrane (pm) displayed no labeling for multimerin (original magnification × 60,000).