Laminins α4 and α5 exert an autocrine effect on postsynaptic maturation. (A) Myotubes from control and Lama4^−/−;Lama5^M/M mice were cultured on laminin 111 and stained for AChRs with Alexa 594–BTX. Micrographs show a complex AChR cluster on a control myotube and a simple AChR plaque on a Lama4^−/−;Lama5^M/M myotube. (B) Laminin α5 is associated with AChR clusters in control myotubes but was absent from clusters in Lama4^−/−;Lama5^M/M. Bars, 10 μm. (C and D) Quantification of AChR aggregate topology on myotubes from control, Lama4^−/−;Lama5^M/M, Lama5^M/M, Lama4^−/−, and Lamb2^−/− myotubes cultured on laminin 111 (C) or fibronectin (D). Myotubes from lama4^−/−;Lama5^M/M and lama5^M/M mice form fewer complex AChR clusters on both substrates than do control, Lama4^−/−, or Lamb2^−/− myotubes. For each experiment, at least 150 AChR clusters were examined. Counts show mean ± SEM from three animals per genotype. *, significant by one-way analysis of variance, P < 0.0001.