Abstract
1. The developmental time course of the appearance of ionic channels was studied with the voltage-clamp technique at 8 degrees C in ascidian embryos in which cleavage was arrested with cytochalasin B immediately after fertilization. The ontogeny of cell-surface antigens was also studied using monoclonal antibodies in both normal and cleavage-arrested embryos. The cleavage-arrested 1-cell embryo differentiates into a cell of epidermal type expressing Ca2+, anomalous rectifier and Ca2+-induced K+ channels, and cell-surface antigens against tunic. 2. The size of the Sr2+ current through egg-type Ca2+ channels decreased during the initial 15 h and disappeared. At about 45 h a Sr2+ current reappeared; the properties of these new channels were different from those of the egg type and were considered to be those of differentiated epidermal Ca2+ channels. 3. Na+ currents also decreased during the first 15 h, and then tended to increase, reaching a peak at about 35 h before decreasing again and finally disappearing. 4. The K+ current through anomalous rectifier channels gradually increased in amplitude, reached a peak at about 35 h and then slightly decreased to a minimum at 45 h. It then increased with further development. 5. The K+ current through the Ca+-induced K+ channels appeared at 50 h and then increased. 6. Input capacity started to increase at 15 h, attained a peak value of three times that of the egg at about 35 h, and then decreased. 7. Two anti-tunic monoclonal antibodies, C1 and 2C5, were obtained. C1 bound only to the tunic; 2C5 bound to the tunic and to the cytoplasm of epidermal cells. 8. C1 antigens first appeared on the surface of the epidermis of the normal embryos and on the surface of the cleavage-arrested 1-cell embryo at about 45 h, and then increased in amount. 9. In the normal embryo 2C5 antigen was first detected at about 40 h inside epidermal cells. It started to accumulate on the epidermal surface at about 45 h and then appeared also in the tunic. In the cleavage-arrested embryo the antigen was first detected at about 50 h, and became more intensely stained as development proceeded.(ABSTRACT TRUNCATED AT 400 WORDS)