Ultrastructural characteristics of developing eosinophil leukocytes in human bone marrow during acute leukemia: evidence for extracellular granule release from human eosinophils.

Developing eosinophils from the bone marrow of a patient with acute "eosinophilic" leukemia were characterized by electron microscopy. It was suggested that the first sequential step in granule formation occurred at the level of the endoplasmic reticulum without actual participation of the Golgi complex. Progressive densification of the former profiles, presumably mediated by Golgi vesicles, resulted in the formation of dense immature granules. Ultrastructural observations of the "leukemic" eosinophils which were generally arrested at an intermediate stage of maturation revealed also large vacuoles containing sequestered immature granules, without any indication of phagocytic activity. Morphological evidence that has been accumulated indicates that the membrane of these vacuoles fused with the cell membrane, thus being in contact with the extracellular space. These profiles strongly suggested that granules and/or granule-associated material were secreted by developing bone marrow eosinophils.