ACETYLCHOLINE-RECEPTOR FORMATION IN MOUSE-CHICK CHIMERA

This study investigated possible interactions between motoneurons and semitic-derived muscle cells in the formation of neuromuscular synapse s in the myotome. The peculiarities of the neuromuscular synaptic patt ern in chick and mouse embryos provided a model for studying the achie vement of synaptogenesis between chick motoneurons and mouse muscle ce lls. In chick embryo, initial AChR clustering occurs well before inner vation of the myotome, whereas in mouse embryo nerve axons invade the myotome extensively before the appearance of AChR clusters. Our approa ch was to replace somites from a chick host embryo with those derived from mouse donor embryos. We show that muscle cells from mouse myotome can differentiate in the chick embryo environment and form neuromuscu lar contacts with chick motor axons. Host axons invaded in ovo differe ntiating mouse myotome at a time when they had not yet reached the hos t myotome. This particular ingrowth of motor nerves was attributable t o the mouse transplant since use of a quail somite did not produce the same effect as the mouse somite, which suggests that developing mouse muscles specifically modify the time course of chick axogenesis. The synaptic areas formed between chick motor axons and mouse myotubes dev eloped according to the mouse pattern. Both the timing of their appear ance and their morphology correlated perfectly with events in mouse sy naptogenesis. These results indicate the important role played by post synaptic membrane in controlling the first steps of AChR formation. (C ) 1997 Academic Press.