Cells with many of the features of mast cells develop in cultures of normal mouse hematopoietic cells containing mast cell growth factor (1–8), an activity similar or identical to interleukin 3 (IL-3)"(9-11), histamine-producing cell–stimulating factor (9), or P cell-stimulating factor (12-14). Large numbers of these cells (> 10") can be generated as virtually homogeneous populations or as clones, thereby facilitating a wide variety of biochemical and functional studies (reviewed in 15). Yet, despite their popularity as an experimental model, the precise relationship of these cells to mast cell populations observed in vivo is controversial.

It has been clear from the beginning that the cultured cells differ in certain important respects from the best-characterized cell type available for detailed comparative studies: mature peritoneal mast cells (reviewed in 15). Cultured mast cells appear immature by ultrastructure (1, 2, 15), contain low levels of histamine (1, 2, 5, 12, 15–18), and express fewer surface receptors for IgE than do mature peritoneal mast cells (2). In addition, cultured mast cells incorporate Na," SO, into granule-associated chondroitin sulfates (2, 10, 17, 19), including