Wild type muscle fibers cluster receptors when contacted by nic1 motoneurons. (A) Fluorescence illumination showing fluorescein-dextran-labeled donor wild-type muscle fibers (arrow) in a live, unlabeled nic1 host myotome. The remainder of the field of view contains unlabeled nic1 muscle fibers (two are marked with arrowheads). (B) View of same myotome showing R-BTX labeling. R-BTX-labeled AChRs (arrow) are clustered specifically on the wild-type fibers. Arrowheads indicate nic1 muscle fibers (also shown by arrowheads in A) which lack R-BTX labeling of AChRs although they are presumably innervated by the motoneuron contacting adjacent fibers. We made similar observations on 21 single muscle cells and three multiple cell patches in three embryos. In every case, the wild-type fiber developed clusters of AChRs and in no case could we assign a cluster to a mutant fiber. Trunk segment; 80 h; scale bar, 20 μm.

AChRs fail to cluster on nic1 muscle cells contacted by wild-type motoneurons. (A) The axon of a fluorescein-dextran-labeled wild-type motoneuron (arrowhead) contacts several labeled wild-type (arrow) and unlabeled nic1 muscle fibers in a live embryo. (B) R-BTX-labeled AChR patches (arrow) appear only on wild-type muscle cells. Arrowhead indicates the location of the axonal varicosity (indicated in A) overlying a nic1 muscle fiber. We obtained the same result with three transplanted wild-type motoneurons innervating nic1 muscle cells. Trunk segment; 80 h; scale bar, 20 μm.

nic1 muscle cells fail to cluster receptors in culture. Cells were cultured for 3 days. (A) Nomarski image of wild-type muscle cell. (B) R-BTX-labeled AChR clusters on same cell shown in A. (C) Nomarski image of nic1 muscle cell. (D) Same cell as in C labeled with R-BTX. Scale bar, 30 μm.