Cleavage-arrested ectodermal blastomeres isolated from the 8-cell stage ascidian embryo differentiate into neural cells after cell-contact with a vegetal blatomere over a critical period. In this two-cell system, neural differentiation can be exemplified by the appearance of Na-dependent action potentials and the transcription of a Na
+ channel gene, TuNaI. This system enables us to precisely study gap-junctional communication between early developing blastomeres by microinjection of tracer molecules and by double voltage clamp method. Gap junctions are likely to determine timing of functional expression of Na
+ channels and K
+ channels.
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