Interleukin 3 (IL3), dependent cells were obtained from bone marrow (9/10 experiments) and spleen cells (4/5 experiments), but not from the thymus. These cells were similar to mucosal mast cell toluidine blue staining and electron microscopy. They had heterogenous metachromatic granules, and some had large scroll-like structures. They also contained histamine (200–800 ng/106 cells) for the first 2–5 weeks, whose level diminished to less than 30 ng/106 cells by 10 weeks of culture. They also generated leukotriene (LT) C4/D4 (10–40 ng/106 cells) and LTB4 (2–5 ng/106 cells) for over 100 days of culture. In one experiment, bone marrow-derived mast cells after 150 days of culture began to produce an IL3-like substance and proliferated exponentially without exogen
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