Exocytotic Competence and Intergranular Fusion in Cord Blood−Derived Eosinophils During Differentiation

We studied degranulation of single cord blood−derived mononuclear cells differentiating to eosinophils in cultures containing recombinant human interleukin−5 (rhIL−5) and rhIL−3 by whole−cell patch−clamp capacitance measurements. As in mature cells, degranulation can be stimulated by intracellular application of guanosine−5'−O−(3−thiotriphosphate) (GTP) γS after 10 days in culture, simultaneously with the first morphological appearance of granules. These results demonstrate that the fusion machineries for exocytotic fusion are present and functional as soon as the granules are formed, presumably at the myeloblast stage. In the third week, the total amount of granules exocytosed upon stimulation is similar to that in mature eosinophils from peripheral blood. The capacitance step size distributions in promyelocytes and myelocytes confirm that mature large specific granules are formed by homotypic fusion of unit granules with similar size. Homotypic fusion is facilitated during early stages of differentiation associated with granulogenesis. Between day 10 and day 35 in culture the plasma membrane area of resting cells decreases from ≈700 µm2 to ≈400 µm2, approaching the value of mature cells from peripheral blood. The most prominent decrease occurs between day 25 and day 35 and is accompanied by the appearance of an exocytotic component due to small vesicles. This suggests that a class of small secretory vesicles is formed by endocytosis during a late phase in maturatio