Satellite cells on isolated myofibers from normal and denervated adult ratmuscle

Satellite cells (SCs) in normal adult muscle are quiescent. They can enter the mitotic program when stimulated with growth factors such as basic FGF. Short-term denervation stimulates SC to enter the mitotic cycle in vivo, wh ereas long-term denervation depletes the SC pool. The molecular basis for t he neural influence on SCs has not been established. We studied the phenoty pe and the proliferative capacity of SCs from muscle that had been denervat ed before being cultured in vitro. The expression of PCNA, myogenin, and mu scle (M)-cadherin in SCs of normal and denervated muscle fibers was examine d at the single-cell level by immunolabeling in a culture system of isolate d rat muscle fibers with attached SCs. Immediately after plating (Day 0), n either PCNA nor myogenin was present on normal muscle fibers, but we detect ed an average of 0.5 M-cadherin(+) SCs per muscle fiber. The number of thes e M-cadherin(+) cells (which are negative for PCNA and myogenin) increased over the time course examined. A larger fraction of cells negative for M-ca dherin underwent mitosis and expressed PCNA, followed by myogenin. The kine tics of SCs from muscle fibers denervated for 4 days before culturing were similar to those of normal controls. Denervation from 1 to 32 weeks before plating, however, suppressed PCNA and myogenin expression almost completely . The fraction of M-cadherin(+) (PCNA(-)/myogenin(-)) SCs was decreased aft er 1 week of denervation, increased above normal after denervation for 4 or 8 weeks, and decreased again after denervation for 16 or 32 weeks. We sugg est that the M-cadherin(+) cells are nondividing SCs because they co-expres s neither PCNA or myogenin, whereas the cells positive for PCNA or myogenin land negative for M-cadherin) have entered the mitotic cycle. SCs from den ervated muscle were different from normal controls when denervated for 1 we ek or longer. The effect of denervation on the phenotypic modulation of SCs includes resistance to recruitment into the mitotic cycle under the condit ions studied here and a robust extension of the nonproliferative compartmen t. These characteristics of SCs deprived of neural influence may account fo r the failure of denervated muscle to fully regenerate.