Ca2+-ionophore-induced microvilli and cortical contractions inXenopus eggs. Evidence for involvement of actomyosin

Summary Using scanning electron microscopy, we show that the calcium ionophore A23187 has a profound effect on the surface morphology ofXenopus laevis eggs. The response to ionophore can be interpreted with respect to animal/vegetal polarity and the presence of an asymmetrically organized actomyosin-based contractile system in the egg cortex. When incubated in ionophore, the egg cortex contracts, pigment granules move towards the animal pole, and microvilli increase dramatically in size. While at first overall microvilli density decreases, many additional microvilli appear later in the animal hemisphere but not in the vegetal hemisphere. Eggs incubated in high concentrations of A23187 undergo the same surface changes at a faster rate, and rupture due to a massive cortical contraction. Local application of ionophore to the egg surface results in increased microvilli size and density in that area, with the animal hemisphere showing the greatest response. Since the effects of ionophore are inhibited by the actomyosin probe, N-ethylmaleimide-modified heavy meromyosin, actomyosin is implicated in the ionophore-induced surface changes.