The Cell Cycle Governs the Onset of Spherulation of Xenopus Eggs Fused by an Electric Field

The mechanism of electric field-induced fusion has been studied in detail (Zimmermann, Biochim. Biophys. Acta. 694 (1982) 227), but little is known about the process by which the two fused cells become a single entity, a process we term spherulation. We observe a clear difference between activated and unactivated Xenopus eggs in the time after electic field (EF) application when spherulation starts, and in the time required for spherulation to be completed. In unactivated eggs, spherulation started 7 min after EF application and was completed within 5 1/2 min. In activated eggs, the lag between EF application and the start of spherulation increased with the cell cycle. At the end of the first cell cycle spherulation started 78 min after EF application and was competed 30 min later. The lag period is not due to delayed fusion, for electric coupling between activated eggs can be recorded before the start of spherulation. The morphology of the contact zone between paired eggs, as observed by light and electron microscopy, is also described. We suggest that the difference in the timing of spherulation reflects a difference in the lability of the cytoskeleton through the cell cycle.