Mast cells in the rat heart during normal growth and in cardiac hypertrophy

Mast cells in rat hearts were studied quantitatively during normal postnatal growth and in two types of cardiac hypertrophy. Normally, cardiac mast cell density in 11-12-day-old animals is very low, but increases markedly in the following 2-3 weeks to its highest values, with a subsequent decline toward adult values. At the peak of mast cell density, the percentage of mast cells in close proximity to capillaries is also highest. In adult animals, mast cell counts are significantly higher in the right ventricle than in the left. This relation is preserved even when the right ventricle is hypertrophic, as in rats born at simulated high altitude. Chronic hypertension and swimming have little effect on the mast cell density in rat hearts. Conspicuous changes in the mast cell density at the time of capillary proliferation seem to indicate a special role played by these cells in the formation of new vessels.