The surface contraction waves of Xenopus eggs reflect the metachronous cell-cycle state of the cytoplasm

Activated Xenopus laevis eggs undergo a series of surface contractions in response to cell-cycle progression but fall to cleave unless the sperm centrosome is present. These surface contraction waves (SCWs) begin at the animal pole and progress around the egg, occur every cell cycle and precede cleavage [1] [2] [3]. The SCWs are biphasic, comprising a relaxation phase (SCWa) and a contraction phase (SCWb). To investigate how these events are linked to the underlying cell cycle, we studied the temporal and spatial relationship between the SCWs and previously characterized biochemical markers of cell-cycle progression. We found that the relaxation phase was a response to activated maturation-promoting factor (MPF). In contrast, the contraction phase required inactivation of MPF and was blocked when MPF activity was maintained at elevated levels. We also found that a wave of MPF activity traveled within the cell from the animal to the vegetal hemisphere. Taken together, these experiments suggest that the SCWs are a local response to a wave of MPF activation and inactivation. The egg cytoplasm, therefore, is metachronous in terms of cell-cycle progression; multiple cell-cycle states are present and spatially distinct within the egg at the same time.