Two populations of mast cells on fibroblast monolayers: correlation of quantitative microscopy and functional activity.

We have previously shown that a confluent layer of mature mast cells is obtained when lymph node cells are grown on embryonic fibroblast monolayers. Two populations of mast cells may be observed, depending on treatment of the mice from which the lymph node cells are derived. We report now on the morphometric evaluation of these two mast cell entities, and we correlate this with cellular biochemistry and secretory behaviour. The first type of mast cell is small (265 +/- 20 microns 3). It arises from the embryonic monolayer and the cells have feathers resembling those of connective tissue mast cells. These cultured cells are filled with about 1000 homogeneous electron-dense granules, which usually range in diameter from 0.05 to 0.2 microns. The second type of mast cell arises from precursors originating from the lymph node and they have feathers of mucosal mast cells. These cells are larger (480 +/- 40 microns 3) and contain about 300 heterogeneous granules, which range from 0.1 to 0.8 microns. Both cell entities contain about equal amounts of histamine, serotonin and chymase. Biologically, the two cell entities secrete soluble mediators (histamine and serotonin) at different rates compared to the rate at which they secrete chymase. We suggest that such a pattern of secretion exhibits a form of degranulation that permits the release of freely diffusible mediators that are loosely bound to granules, but only partially permits the secretion of insoluble mediators, which are stored in the granules. Alternatively, there might be a mechanism that rapidly inactivates or binds chymase so that only the vasoamines will be free.