Ultrastructural identification of human mast cells resembling skin mast cells stimulated to develop in long-term human cord blood mononuclear cells cultured with 3T3 murine skin fibroblasts.

Human mast cells developed in vitro when cord blood mononuclear cells were cocultured for 3 months with 3T3 embryonic mouse skin fibroblasts. The metachromatic cells that arose in these cultures contained histamine, a functional Fc epsilon receptor and granule proteases (tryptase, chymase), and they were definitively identified by the ultrastructural demonstration of crystal granules. We present a detailed ultrastructural analysis of this newly available system for the reliable development of human mast cells in vitro and provide criteria for definitive identification of the mast cell and basophil lineages in humans.