Comparative morphometry of fibers and capillaries in soleus following weightlessness (SL-3) and suspension.

This work is a continuation of efforts to assess responses of rat skeletal muscle to weightlessness and earthside laboratory experiments with unloading of hind limbs. The soleus is a slow twitch, load bearing (antigravity) muscle. Both exposure to weightlessness (W) and to the hypokinesia/hypodynamia of whole body suspension (WBS) results in soleus atrophy. Cross sectional areas of both slow and fast twitch fibers decrease during 7 days of W, and 7 or 14 days of WBS. Density and area changes tended to reverse to control levels during 7 days of recovery (R) following WBS. Capillary density was increased with 7 days of W, and 7 or 14 days of WBS. During 7 days of R the capillary density returned toward control levels. In summary, the reduction in fiber cross sectional areas and increase in fiber and capillary densities support the hypothesis that in both forms of disuse, i.e., W and WBS, there is a loss in soleus muscle cell mass and not in fiber numbers.