Appearance and disappearance of acetycholine receptor during differentiation of chick skeletal muscle in vitro

During differentiation of embryonic chick skeletal muscle in culture, elaboration of acetylcholine receptor (AChR) and acetylcholinesterase occurs shortly after myoblast fusion. During further development, AChR was found to decrease markedly on the myotube surface, while acetylcholinesterase continued to increase. Surface distribution of AChR, as followed by autoradiography using ¹²⁵I-α-bungarotoxin, was homogeneous in newly fused myotubes. With further differentiation, clusters of AChR appeared on the surface of the myotubes, and their subsequent disappearance paralleled a decrease in overall AChR levels. Quantitative autoradiography showed a reduction of over 75% in the density of AChR on the surface of well differentiated, cross-striated myotubes. Thus the appearance of AChR on the cell surface, its condensation into clusters, and finally its depletion seem to be sequential events in the differentiation of skeletal muscle in culture in the absence of direct neuronal influence.