Spatial distribution of acetylcholine receptors at developing chick neuromuscular junctions.

The development of high-density clusters of acetylcholine receptors (AChRs) and the relationship of these clusters to nerve contacts on embryonic chick wing muscle fibres has been studied. Fluorescent labelling of AChRs with rhodamine-conjugated alpha-bungarotoxin (R-Bgt) revealed the presence of irregularly shaped AChR clusters in wing buds at 4 1/2-5 days of incubation. This is within a day of when myotubes first appear in the wing bud, and close to the time when functional innervation becomes established. At 10 days of incubation AChR clusters present on muscle cells in anterior and posterior latissimus dorsi appear as round or oval, uniformly labelled plaques. At about the time of hatching, however, these plaques break into numerous smaller clusters. Similar changes in the morphology of AChR clusters have been observed previously in mammalian skeletal muscle during development. Using horseradish peroxidase labelled alpha-bungarotoxin (HRP-Bgt), the relationship between AChR clusters and motor nerve terminals was studied at the ultrastructural level. At all stages of development nerve-muscle contacts were labelled with HRP-Bgt. In wing buds, however, the majority (90%) of labelled clusters observed were not in contact with a motor nerve terminal. The incidence of AChR clusters with axon contacts increased sharply during development such that by 10 days more than 50% and by hatching more than 90% of all sections through labelled AChR clusters contained nerve terminal profiles. At all times studied nerve-contacted receptor clusters were longer (about 5 micron) than non-contacted clusters (about 2 micron).