Ultrastructural morphology of human mast cell progenitors in sequential cocultures of cord blood cells and fibroblasts.

The ontogeny of human mast cells (HMCs) is not known, but agranular precursors committed to HMC development are present in cord blood mononuclear cells. Individual cord bloods were cocultured with murine fibroblasts and prepared for light microscopy, immunofluorescent determination of tryptase and electron microscopy after sequential times in culture. Mast cells and their progenitors were identified and quantitated during their evolution from agranular precursors in human cord blood. Each of these methods revealed similar numbers of HMC progenitors. The earliest, visible granule contents were particulate; later, scrolls superimposed on particulate materials were noted. HMC progenitors did not contain crystal granules, but fully mature HMCs present in long-term cocultures did. The developmental sequence for the evolution of the HMC lineage in vitro, made available by this new culture system, is identical to that described for the development of HMCs in vivo in sequentially examined samples of human fetal tissues.