Formation of cytoplasmic granules in human eosinophilic myelocytes: an electron microscope autoradiographic study.

T HE CYTOPLASMIC GRANULES of eosinophils may be considered a form of lysosome, for they are membrane bounded structures containing, among other substances, hydrolytic enzymes.’ These granules appear in the immature cell as large membrane bounded structures with a homogeneous electron dense content; as the cell matures, a crystalloid inclusion commonly develops within the granule.2.3 Electron microscope autoradiography has been used to study cytoplasmic granule formation in a number of cell types including pancreatic acinar cells and more recently two types of leukocytes-macrophages and heterophilic myelocytes.46 In both secretory cells and leukocytes, the labeled amino acid flowed through the Golgi complex before the newly synthesized protein was incorporated into cytoplasmic granules. The studies reported here were designed to determine whether the human eosinophilic myelocyte was similar to the rabbit heterophiic myelocyte in its mode of cytoplasmic granule formation. Small tissue blocks aspirated from the bone marrow of a patient with eosinophiia but morphologically normal cells were studied by the in vitro methods developed previously to study granule formation in the heterophilic myelocyte.