Ultrastructural characterization and morphometric analysis of human eosinophil degranulation.

Human eosinophil degranulation induced by the calcium ionophore A23187 was examined by transmission and scanning electron microscopy, and morphometric analysis. After incubation with A23187, eosinophil degranulation was characterized by granule movement to the cell periphery and pentalaminar membrane fusion between perigranular and plasma membranes. As adjacent granules fused they became swollen and vesiculated; their contents were released into large cytoplasmic vacuoles, which communicated extracellulary through surface pores. Extracellular release of eosinophil peroxidase without release of lactate dehydrogenase occurred after treatment with the ionophore. Morphometric analysis of the transmission electron micrographs indicated a significant reduction of cytoplasmic granules in the degranulated cells. There was a loss primarily of larger granules and alternatively an increase in the smaller-sized granules (less than 0.1 micron2), suggesting the possibility that exocytosis of mature granules is accompanied by new cytoplasmic granule formation.