The ultrastructural analysis of a variety of culture systems of human cord blood mononuclear cells (spanning a 10-year research effort) is reviewed. Human basophils, eosinophils and mast cells reliably developed from their agranular precursors that are present in human cord blood. Suspension cultures and cocultures with fibroblasts were used to examine the effects on differentiation and maturation of full (fibroblast), interleukin-2-depleted (human T cells), and murine inducer T cell culture supernatants, partially purified mouse fibroblast factor(s), recombinant human interleukins 3 and 5, and recombinant human and murine c-kit ligands (stem cell factor, mast cell growth factor). Together, these studies allowed us to define the differentiation and full maturation of the basophil and eosinophil lineages and provided evidence for the induction of a form of secretion (termed piecemeal degranulation) of the basophil and eosinophil lineages in interleukin-3- or-5-supplemented cultures. Mast cells were absent from interleukin-3- or-5-containing cultures. The development of fully mature mast cells occurred regularly in fibroblast-containing cocultures; partially mature mast cells developed in fibroblast culture supernatant-, partially purified mouse fibroblast factors(s)-, and either recombinant human or murine c-kit ligand-supplemented suspension cultures. Small numbers of basophils and eosinophils were present in the suspension cultures the received c-kit ligand in its recombinant or naturally occurring forms. Ultrastructural immunogold analyses confirmed that basophils and eosinophils contained the Charcot-Leyden crystal protein (in different subcellular locations) but that mast cells did not. In both cocultures and suspension cultures, the primary event recorded for mast cells was that of differentiation and maturation, with the ultrastructural correlates of synthetic activity and granule building prevailing. Spontaneous secretory events, recognizable by ultrastructural analysis, were not evident, in either mature or partially mature mast cells developing in these cultures.
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