The effects of stem cell factor on the ultrastructure of Fc epsilon RI+ cells developing in IL-3-dependent murine bone marrow-derived cell cultures.

Stem cell factor (SCF) is known to alter the proteoglycans, proteases, and cytokines synthesized by mast cells and to activate basophils. To determine whether SCF could also effect the ultrastructural characteristics of basophils and mast cells, we examined the ultrastructure of these Fc epsilon RI+ cells over 42 days in IL-3-dependent murine bone marrow-derived cell cultures in the presence or absence of SCF. Initial experiments revealed that the addition of SCF to IL-3-dependent cells enhanced their proliferative rate without influencing the percentage of Fc epsilon RI+ or metachromatic cells. We next isolated the Fc epsilon RI+ cells using flow cytometry. Light microscopy of these cells revealed mixed cultures of both immature and mature mast cells and basophils with mature mast cells predominating by 3 wk. One hundred to 150 Fc epsilon RI+ cells were then photographed by electron microscopy at 3, 10, 21, and in some cases, 42 days of culture, and the ultrastructure of each cells was evaluated by morphometry. Each cell was scored as a mast cell or basophil using standard criteria. Analysis of this data revealed that SCF in the presence of IL-3 promoted the development of mast cells, although a significant number of basophils were noted at day 21 but were absent by day 42. When bone marrow cells cultured in IL-3 + SCF were compared with cells cultured in IL-3 alone, a significant decrease in cell and nuclear size and granule number and size was noted in both mast cells and basophils cultured in IL-3 + SCF, and basophils and mast cells under these conditions most resemble their in vivo counterparts. Thus, SCF in the presence of IL-3 increases the ratio of mast cells to basophils and alters the ultrastructural characteristics of mast cells and basophils toward a more mature phenotype.