Discontinuous capillary segments in the extensor digitorum longus muscle of aged BUF/Mna rats

We examined the structural changes of capillaries in the extensor digitorum longus muscle of aged (25 months) male BUF/Mna rats, which caused severe muscle weakness of hind legs during aging. The aged muscle mostly consisted of bundles of muscle fibres 15-35 microm in diameter (type 1). In some small areas, however, muscle bundles contained small muscle fibres mainly 15-25 microm in diameter (type 2), possibly indicating that these small fibres are in the course of regeneration after necrosis. Examination of serial ultrathin sections revealed some remarkable changes of capillaries in the type 2 muscle bundle. In some capillaries, the vascular lumen became discontinuous by several close contacts of opposed endothelial cells in their course, forming plural closed vascular segments. Moreover, a solitary endothelial cell was often observed within a scaffold of basal lamina, which remained after destruction of endothelial cells. The segmentation of capillaries and the occurrence of the scaffolds of basal laminae are considered to indicate the degenerative process of capillaries. In some instances, on the other hand, endothelial cells closely apposed each other with no vascular lumen for distances of up to 100 microm, and some capillaries had a narrow vascular lumen (1-3 microm diameter) for a long distance (approximately 300 microm), probably indicating that these structures are in the course of regenerating capillaries to remodel the capillary networks around the muscle fibres. Pericytic processes circularly arranged outside the endothelium at the slit-like and narrow vascular lumen, like hoops, possibly preventing the rupture of the newly-formed vascular lumen from the increased blood flow and/or blood pressure. In addition, the occasional occurrence of capillaries with fenestrations may participate to increase the supply of nutrients and oxygen to the regenerating muscle fibres. The present findings suggest that the capillary networks in the extensor digitorum longus muscle of aged BUF/Mna rats are remodelled following the regeneration of muscle fibres after necrosis, and that on occasion, neighbouring endothelial cells may closely contact with each other both in the degenerative and regenerative processes of capillaries.