Stereological measurement of cellular and subcellular hypertrophy and hyperplasia in the papillary muscle of adult rat.

Abstract Structural changes in the papillary muscle were examined following an induced growth of 51% by integrating tissue, cellular and subcellular morphometry. Comparison of the adaptive response in different cell populations by morphometric nuclear enumeration demonstrated 48% hyperplasia of endothelial cells, 35% for connective tissue cells and no change in the number of myocyte nuclei. Increases in the cell volume per nucleus, a measure of cellular hypertrophy, were 35% for endothelial cells, 64% for connective tissue cells and 53% for myocytes. No significant changes were found in the lengths of the papillary muscle, the average myocyte or the capillary network. Total capillary volume and the luminal and abluminal endothelial surfaces were significantly increased by 59, 38 and 43%, respectively. Each cell population in the papillary muscle showed varying increases in the volumes, surface areas and number of cytoplasmic organelles accompanying cellular hyperplasia and/or hypertrophy. Micropinocytotic vesicles in endothelial cells increased 73%; RER in fibroblasts: approximately 150%; SER and T-system in myocytes: each approximately 100%; and the numbers of mitochondria in endothelium, fibroblasts and myocytes: 54, 78 and 65%. As a result of a lateral duplication of normal sized components, the myofibrillar network in myocytes increased 67% in volume, 55% in surface area, 82% in the length of myofibril branches, and 86% in the overall number of its sarcomere units. These results provide a morphological basis for functional studies of normal and hypertrophied papillary muscle.

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