Mast cells of the Mongolian gerbil (Meriones unguiculatus). Morphology, histamine content and role of calcium in the histamine release process.

The fine structure, histamine conten;, and role of calcium in the histamine release process were studied in peritoneal mast cells of the Mongolian gerbil (Meriones unguiculatus). Stereological methods were applied to obtain quantitative data on their structure. The findings were compared with results obtained from the same type of cells in the rat. The gerbil mast cells were smaller in size (mean volume 242 micrometer3 vs 684 micrometer3 in the rat), and the nuclei were also smaller (55 micrometer3 vs 102 micrometer3). There were fewer granules in the gerbil mast cells and their diameter averaged 0.54 micrometer as compared with 0.78 micrometer in the rat). Only 20% of the cytoplasm of the gerbil mast cell was occupied by granules. This figure is approximately one third of that obtained in rat mast cells. The mean total histamine content per cell was 9 pg as compared to an estimated 30 pg/cell in rats. Calculated molar concentration of histamine in the mast granules, however, was higher in the gerbil than in the rat (2.3 M vs. 0.9 M). The mast cells of the gerbil were much more sensitive to the histamine-releasing agent compound 48/80 and in contrast to rat mast cells they were entirely dependent on calcium for their amine release. The fine cellular structure of both species showed multitudinous plasma membrane folds on their surfaces. In addition gerbil mast cells showed extensive surface invaginations. Apart from this were no major differences at the ultrastructural level between unstimulated cells of the two species. During histamine release, however, the mast cells of the gerbil showed a much greater tendency to form large, intracytoplasmic vacuoles and a decreased propensity for fusion of perigranular and plasma membranes (exocytosis) as compared with the corresponding cells in the rat.