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

Electron microscopic study of capillary network remodeling in the extensor digitorum longus muscle of normal adult rat

Capillary networks demonstrate structural changes during maturation, aging, vascular disease, and cancer. Their morphological structure and function have an important influence on each other. Understanding the process of morphological vascular changes in the capillary network with advancing age may help overcome fatal vascular diseases. Aging-related structural changes of the capillary segments may accompany degeneration and regeneration of muscle fibers and serve to remodel the capillary network as a means of adapting to the changing environment. However, difficulty in obtaining human samples has hampered clarification of these microstructural changes. Herein, we examined serial ultrathin sections of capillary segments in the extensor digitorum longus muscle of normal mature (12 months old) rats in an attempt to analyze their structural changes. After bifurcation, a minimum of one capillary segment was filled with erythrocytes and was found to have fenestrations and plural endothelial disruptions, or pores, at the fenestrated portions. Some of the stagnated erythrocytes demonstrated extended protrusions, and their processes appeared to penetrate the basal lamina through the pores. These findings can also show that capillary segments are involved in partial remodeling of the capillary network. A better understanding of age-related structural changes of the capillary networks will help in fine-tuning novel vascular therapy for not only several fatal vascular diseases but also malignant tumors.

Mapping 3-D functional capillary geometry in rat skeletal muscle in vivo

We have developed a novel mapping software package to reconstruct microvascular networks in three dimensions (3-D) from in vivo video images for use in blood flow and O2 transport modeling. An intravital optical imaging system was used to collect video sequences of blood flow in microvessels at different depths in the tissue. Functional images of vessels were produced from the video sequences and were processed using automated edge tracking software to yield location and geometry data for construction of the 3-D network. The same video sequences were analyzed for hemodynamic and O2 saturation data from individual capillaries in the network. Simple user-driven commands allowed the connection of vessel segments at bifurcations, and semiautomated registration enabled the tracking of vessels across multiple focal planes and fields of view. The reconstructed networks can be rotated and manipulated in 3-D to verify vessel connections and continuity. Hemodynamic and O2 saturation measurements made in vivo can be indexed to corresponding vessels and visualized using colorized maps of the vascular geometry. Vessels in each reconstruction are saved as text-based files that can be easily imported into flow or O2 transport models with complete geometry, hemodynamic, and O2 transport conditions. The results of digital morphometric analysis of seven microvascular networks showed mean capillary diameters and overall capillary density consistent with previous findings using histology and corrosion cast techniques. The described mapping software is a valuable tool for the quantification of in vivo microvascular geometry, hemodynamics, and oxygenation, thus providing rich data sets for experiment-based computational models.

Fine structural study of the regeneration of muscle fibers in the rat soleus muscle during aging

We examined the regeneration of muscle fibers in the soleus muscle of mature (12 months) and aged (24 and 27 months) rats by using electron microscopy. In both mature and aged muscles, regenerating muscle fibers were mainly formed within the scaffolds of basal laminae after necrosis. In the aged muscle, however, satellite cells within the scaffolds were occasionally destroyed, and immature muscle cells occurred in and around muscle bundles. These findings suggest that new muscle fibers formed in the interstitial spaces may contribute to the total number of regenerated muscle fibers. The origin of the immature muscle cells is briefly discussed.

Capillary changes with fenestrations in the contralateral soleus muscle of the rat following unilateral limb immobilization

Using electron microscopy, we examined structural changes in the capillaries of the immobilized and contralateral soleus muscles in the rat after the unilateral limb had been immobilized for 4 weeks. Individual contralateral muscle fibers showed a normal structure, compared with the structure in the immobilized muscle fibers. Most capillaries in the contralateral muscle were of the continuous type. However, about 5% of the examined capillaries in these muscles in each rat had fenestrations. The non-nuclear portions of the endothelial cells in these capillaries were extremely thin and were perforated by several fenestrations that were bridged by a single-layered diaphragm. The fenestration of capillaries in the contralateral muscle may have been caused by some the unilateral limb immobilization.