Structural changes of microvessels in the extensor digitorum longus muscle of the aged rat.

We examined the structural changes of microvessels (arterioles, capillaries, and venules) in the extensor digitorum longus muscle of the aged (27 months) rat. Muscle bundles in the aged muscle almost consisted of medium-sized muscle fibers which were peculiar in the aged EDL muscle. However, microvessels in and around these muscle bundles varied in shape. Degenerating capillaries and scaffolds of basal laminae remaining after necrosis of preexisting capillaries were frequently observed around these medium-sized muscle fibers. In addition, the vascular lumen was often very narrow or irregular slit-like. In terminal and precapillary arterioles, the endothelium and smooth muscle cells showed a constricted appearance and their vascular lumen was often irregular slit-like, probably playing an important role in intercepting the blood flow into the disrupted capillaries. Moreover, some venules had the slit-like vascular lumen, sawtooth-like endothelium and thick or multilayered basal laminae, and occasional erythrocytes were found outside the endothelium, probably indicating that these venules are in the course of regeneration. These findings suggest that in addition to the frequent destruction of capillaries, the structural changes of arterioles and venules may be involved in remodeling the microvasculature of the muscle bundles after maturation of regenerating muscle fibers.