The fine structure of the cell surface of chromaffin cells in the rat adrenal medulla

Two types of chromaffin cells can be distinguished in the rat adrenal medulla. The plasma membrane of both cell types appears as a triple-layered structure, about 80 thick after fixation by glutaraldehyde perfusion and secondary fixation with osmium tetroxide. The membrane is usually symmetrical after double fixation and asymmetrical after fixation with glutaraldehyde or osmium tetroxide alone. At the cell boundaries the plasma membranes are either separated by a small space about 60–300 wide or they form canaliculus-like widenings. The plasma membranes exhibit expanded areas at the canaliculi and occasionally at the free cell surface. Round opaque particles 300–700 in diameter are present in the canaliculi and sometimes at the free cell surface, often along with the plasma membrane expansions. Vesicular structures and large opaque structures are also found in these areas. All the above-mentioned structures are sometimes observed inside the cells confined to large intracellular vacuoles. Similar vacuoles are constantly present in the medullary cells of heavily reserpinized animals.