Satellite cell proliferation in rabbit hindlimb muscle following immobilization and remobilization: an...

The time course and pattern of mitotic activity in the deep part of the vastus intermedius muscle (VIP) immobilized in a shortened position in 16 adult NZW rabbit hindlimbs for time periods ranging from 12 h to 56 days and in VIP muscles from 10 rabbit hindlimbs immobilized from 3 to 28 days and subsequently remobilized for 4 weeks were compared. Paraffin sections were stained immunohistochemically with monoclonal MIB 1 antibody raised against recombinant fragments of Ki-67 antigen, and a double staining with modified reticulin stain was carried out to facilitate differentiation between satellite cells and other proliferating cells outside the muscle. In samples taken 24 h after the onset of immobilization, the tissue was mitotically quiescent and at 3, 14, 28, and 56 days of immobilization 5.7%, 1.3%, 1.1% and 0.7%, respectively, of all counted nuclei were mitotically active. However, all of them were localized outside the basement membrane (BM) of the myofiber and were presumably proliferating fibroblasts, capillary endothelial cells or inflammatory cells. Satellite cell mitotic activity was first seen after 2 weeks of immobilization followed by 4 weeks of remobilization. In VIP muscle immobilized for 3 weeks and remobilized for 4 weeks the numbers of all proliferating cells as well as proliferating satellite cells reached a concomitant peak, and they were 7.5% and 3% respectively. The results indicate a late activation of satellite cell proliferation, which may relate to the initial lack of stretch and mechanical load during the cast immobilization followed by a late release of mitogenic factors during eccentric muscle work performed by the VIP muscle after removal of the cast.