Morphological Studies on Skeletal Muscle Capillaries Under Conditions of High Altitude Training*

Abstract

The skeletal muscle capillaries adapt to high altitude training by changing their pattern. The capillaries become tortuous and many capillary cross-bridges are developed, hence enlarging the endothelial surface. Even a restitution period of double the training period does not suffice to reverse these changes. The results are discussed with regard to other investigations, and it is suggested that higher capillary counts on cross sections are mostly derived from the altered capillary pattern but not actually from new capillaries. The mechanism of capillary “hypertrophy” is investigated by means of electron microscopy. There is no evidence for the development of new capillaries. The changes of the capillary pattern, however, are shown by morphometric examination to depend on structural changes of the endothelial cells: the overlap of adjoining endothelial cells is shortened in the experimental groups, and the amount of endothelial processes into the capillary lumen is reduced. By this mechanism, the capillary vessel can elongate and increase its diameter. Some conceptions on the mechanisms of capillary “sprouting” as a result of physical training are discussed.