Differentiation of membrane excitability in isolated cleavage‐arrested blastomeres from early ascidian embryos.

1. Differentiation of excitable cells was studied electrophysiologically and histochemically in cleavage‐arrested blastomeres isolated from early ascidian embryos. Blastomeres were isolated at the 4‐ or 8‐cell stage, and cultured in sea water containing cytochalasin B until the time of hatching of control larvae. Electrical responses, immunoreactivity to epidermis‐specific monoclonal antibody (2C5) and activity of muscle‐specific acetylcholinesterase were examined. 2. All cleavage‐arrested blastomeres isolated from an 8‐cell embryo differentiated to elicit either muscular‐ or epidermal‐type action potentials, but no neural‐type action potentials were observed in these blastomeres. The anterior‐animal and the posterior‐animal blastomeres developed only epidermal‐type action potentials, which involved expression of Ca2+ channels and immunoreactivity to 2C5. One‐third of anterior‐vegetal blastomeres developed epidermal‐type action potentials which are mediated by Ca2+ channels though the immunoreactivity to 2C5 was absent. A majority of remaining blastomeres showed action potentials composed of Ca2+ currents and TEA‐sensitive delayed K+ currents (type I response), and a few of them had fast transient K+ currents (A‐currents) in addition (type II response). One‐third of posterior‐vegetal blastomeres developed epidermal‐type action potentials without expression of the immunoreactivity to 2C5. The remainder differentiated into muscular‐type cells, which expressed Ca2+ currents, TEA‐sensitive and TEA‐insensitive delayed K+ currents, and showed acetylcholinesterase activity. 3. Cleavage‐arrested blastomeres isolated from a 4‐cell embryo also differentiated into epidermal‐ or muscular‐type cells, but not neural‐type cells. The anterior blastomere, which is the parent cell of anterior‐animal and anterior‐vegetal blastomeres of an 8‐cell embryo, developed epidermal‐type, type I or type II responses, as was the case in the anterior‐vegetal blastomere isolated from an 8‐cell embryo. The posterior blastomere, which was the parent cell of posterior‐animal and posterior‐vegetal blastomeres of an 8‐cell embryo, differentiated into either epidermal‐type or muscular‐type cells in terms of both membrane excitability and immunochemical reactivity. 4. Cleavage‐arrested 1‐cell embryos differentiated exclusively into epidermal‐type cells in terms of membrane excitability and 2C5 immunoreactivity, even when the cytochalasin B concentration was decreased below 0.1 microgram/ml.(ABSTRACT TRUNCATED AT 400 WORDS)