Lineage-specific development of calcium currents during embryogenesis.

The development of electrophysiological properties of isolated, identified ascidian blastomeres was followed from the fertilized egg to the neurula, and the stage at which cells of different lineages first express different functional ion channel populations was determined. Little has been known about such events because of the difficulties of making voltage-clamp recordings from small embryonic cells and of identifying their developmental fates in dissociated preparations. The problem of small cell size was circumvented by using the whole-cell patch clamp, and identification was facilitated by the use of a species of ascidian, Boltenia villosa, in which endogenous pigment marks cells of specific developmental fates. Within approximately 3 hours after gastrulation, muscle-lineage blastomeres in these embryos developed a voltage-dependent calcium current while surrounding blastomeres of other lineages did not. At about the same time, all cells developed delayed outward potassium currents and lost the inwardly rectifying potassium currents present at earlier stages.