Ultrastructure of human basophils developing in vitro. Evidence for the acquisition of peroxidase by basophils and for different effects of human and murine growth factors on human basophil and eosinophil maturation.

We examined the ultrastructure of human fetal cord blood mononuclear cells cultured in medium supplemented with supernatants of human T lymphocytes (basophil growth factor, BGF), in medium supplemented with supernatants of cloned mouse leukocytes (mast cell growth factor, MCGF) or in MCGF followed by BGF. These cultures provided the first opportunity to perform detailed morphologic and cytochemical studies of human basophilopoiesis in a nonleukemic setting. Cultures of human fetal cord blood mononuclear cells in BGF contained numerous mature basophils, which underwent a sequence of maturation similar in ultrastructure to that previously described in guinea pigs and mice. The cultures also contained variable but often substantial numbers of eosinophils. Eosinophils exhibited peroxidase activity in secretory structures (Golgi, rough endoplasmic reticulum) and cytoplasmic granules, and peroxidase-positive eosinophil granules were released into the medium by dead eosinophils. By contrast, basophils never exhibited peroxidase-positive Golgi structures or rough endoplasmic reticulum but frequently contained variable numbers of peroxidase-positive granules and occasional small cytoplasmic vesicles. Taken together, these findings suggest that basophils do not synthesize peroxidase but acquire the enzyme by the vesicular uptake of peroxidase released from eosinophils. In medium supplemented with MCGF, cells in the basophil lineage proliferated, but their maturation generally did not progress beyond the early basophilic myelocyte stage. The replacement of MCGF by BGF permitted the basophils (and eosinophils) to complete maturation, as judged by a reduction in overall cell size, increased nuclear segmentation and chromatin condensation, and cytoplasmic granule maturation. Mouse MCGF did not promote the development of mast cells in cultures of human fetal cord blood mononuclear cells, when used either alone or in series with BGF. Indeed, no mast cells were observed in any of the cultures examined in this study. Nor did we find cells with ultrastructural features intermediate between those of basophils and mast cells or with features intermediate between those of basophils and eosinophils.