Human eosinophils in vitro. An ultrastructural morphology primer

An ultrastructural morphological primer of human eosinophils is presented. Mature and immature eosinophils, obtained from peripheral blood and bone marrow, as well as activated tissue eosinophils are all used to illustrate the various morphologies assumed by eosinophils in vivo. The various ultrastructural changes expressed by this cell lineage in vivo reflect the impact of differentiation, maturation, activation, secretion, and cell injury on morphology. Nearly all of the changes described in vivo are also evident in eosinophils arising in in vitro systems. We review published studies of these culture systems, which have been supplemented with various conditioned media containing naturally occurring growth factor(s) that are permissive (or not permissive) for eosinophils or with the recombinant growth factors, IL-5 or IL-3. These studies were helpful in the recognition of eosinophil-promoting, -sustaining and -activating properties of human IL-3 and IL-5. Moreover, mature and immature eosinophils were shown to release a granule matrix protein--eosinophil peroxidase (EPO)--by its transport in small cytoplasmic vesicles, a process termed piecemeal degranulation (PMD), accounting for the gradual emptying of granule contents in the absence of granule fusions to the plasma membrane. Also presented are eosinophil morphologies that occur in vitro in suspension cultures of human cord blood supplemented with the c-kit ligand from various sources. The wide variety of eosinophil subcellular changes in the c-kit ligand-supplemented cultures, like the changes of which eosinophils are capable in vivo, reflects the processes of differentiation, maturation, activation, secretion and cell injury. Presentation of this ultrastructural morphological primer of human eosinophils in vitro should enable investigators to recognize eosinophils in all of their diverse morphologic forms in cultures that contain differentiating and functioning members of other lineages, also present in c-kit ligand-supplemented cultures. These lineages include mast cells, basophils, neutrophils, monocytes, macrophages, megakaryocytes, and endothelial cells.