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.