Neonatal Satellite Cells Form Small Myotubes in Vitro

Although palatal muscle reconstruction in patients with cleft palate takes place during early childhood, normal speech development is often not achieved. We hypothesized that the intrinsic properties of head satellite cells (SCs) and the young age of these patients contribute to the poor muscle regeneration after surgery. First, we studied the fiber type distribution and the expression of SC markers in ex vivo muscle tissue from head (branchiomeric) and limb (somite-derived) muscles from neonatal (2-wk-old) and young (9-wk-old) rats. Next, we cultured SCs isolated from these muscles for 5, 7, and 9 d, and investigated the in vitro expression of SC markers, as well as changes in proliferation, early differentiation, and fusion index (myotube formation) in these cells. In our ex vivo samples, we found that virtually all myofibers in both the masseter (Mass) and the levator veli palatini (LVP) muscles contained fast myosin heavy chain (MyHC), and a small percentage of digastric (Dig) and extensor digitorum longus myofibers also contained slow MyHC. This was independent of age. More SCs were found in muscles from neonatal rats as compared with young rats [17.6 (3.8%) v. 2.3 (1.6%); P <0.0001]. In vitro, young branchiomeric head muscle (BrHM) SCs proliferated longer and differentiated later than limb muscle SCs. No differences were found between SC cultures from the different BrHMs. SC cultures from neonatal muscles showed a much higher proliferation index than those from young animals at 5 d (0.8 v. 0.2; P <0.001). In contrast, the fusion index in neonate SCs was about twice as low as that in SCs from young muscles at 9 d [27.6 (1.4) v. 62.8 (10.2), P