Atrophic effects of proximal tendon transection with and without denervation on mouse soleus muscles.

The response of mouse soleus muscles to transection of the proximal tendon was examined at both gross and microscopic levels. Changes in length and mass of the muscle, and in the dimensions, number, and ATPase staining characteristics of the muscle fibers, were determined at various times to 4 to 6 weeks after tenotomy. Muscles shortened by more than 50% and lost about 60% of their mass. Fiber cross-sectional area increased, and then decreased and stabilized below control after 7 days, and degenerative lesions appeared in some fibers. Fiber numbers decreased by nearly 20% and then remained constant, apparently by regeneration of new fibers replacing atrophied ones. The proportion of muscle fibers of the slow-twitch type was reduced. No structural or functional changes in motor nerve terminals could be detected, except for detachment from severely atrophied fibers. Concurrent denervation decreased both shortening and the loss of mass after tenotomy. Fiber diameter decreased but no significant degenerative lesions or loss of fibers could be detected. Fiber-type composition was unchanged. These experiments provide quantitative information about the degenerative changes in tenotomized muscles without the variable complications arising from tendon reattachment. Comparison of the responses with those that follow transection of the Achilles tendon shows that the degree of atrophy is independent of the extent of active muscle shortening after tenotomy.