Cellular and extracellular changes following mast-cell secretion in avascular rat mesentery. An electron-microscopic study.

Mast cell (MC) secretion induces local cell proliferation lasting 48-72 h in fibroblasts and mesothelial cells in the almost avascular true mesentery of the rat. We studied this membranous tissue by transmission electron microscopy with regard to cellular and extracellular features occurring during the first 72 h following MC secretion. After MC secretion elicited by compound 48/80, apparently all individual tissue-bound cells (i.e. fibroblasts, mesothelial cells, and macrophages) show signs of accelerated metabolic activity. In fibroblasts, conspicuous increases in the volume of Golgi apparatus and rough endoplasmic reticulum and in the amount of plasmalemmal indentations suggest an increased production and secretion of the extracellular matrix. Released MC granules lying close to projections of nearby phagocytosing cells cause areas free from electron-dense material in the extracellular matrix. MC secretion therefore appears to produce a remodelling of extracellular matrix. Most of the activities initiated by MC secretion start to subside within (48-)72 h. The findings indicate a close functional relationship between the tissue MC and all its neighbouring cells and the surrounding extracellular matrix. The striking chain of events that it induces emphasizes strongly that the secreting MC plays a prominent although as yet in many respects enigmatic role in normal tissue.