Capillaries with fenestrae in the rat soleus muscle after experimental limb immobilization.

Capillaries in the rat soleus muscle were examined by electron microscopy 4 weeks after limb immobilization and compared to those in untreated muscle. Immobilized muscles showed degenerative changes such as dissolution of sarcomeres, vacuoles, and a small increase in connective tissues, including collagen fibers, around muscle fibers. Eight percent of the capillaries in these muscles were found to have fenestrae. The non-nuclear portions of the endothelial cells were extremely thin and were perforated by several fenestrae which were bridged by a single-layered diaphragm. The untreated muscles contained only continuous capillaries. These findings suggest that the occurrence of capillaries with fenestrae in the immobilized muscle may be a result of structural changes in the muscle.