Morphological changes of capillaries associated with muscle-fiber atrophy in the extensor digitorum longus muscle of aged rats

We examined the structural changes of continuous capillaries in the extensor digitorum longus muscle of 6, 12 and 24-month-old normal rats. Capillaries around each muscle fiber in the sixth month were of the continous type. In the 12th month, muscle fibers appeared almost normal except for widened intercellular spaces and degenerative changes in the subneural sarcoplasm. In the 24th month, many muscle fibers atrophied having greatly reduced diameter and irregular cross sectional profiles. Capillaries in the widened interstices, both in the 12th and 24th months, were embedded in an abundant amount of connective tissue components, including collagen fibrils and amorphous substances, and were surrounded by basal laminae arranged in concentric lamellae. Capillaries in the abundant connective tissues were mostly of the continuous type, while 10% in the 12th month, endothelial cells became partially disrupted and formed pores at the fenestrated portions. These findings confirm that intramuscular capillaries degenerate following the partial atrophy of muscle fibers, and suggest that the occurrence of fenestrations in the endothelial cells represent a first step in the degeneration of continuous capillaries.