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

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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)