Role of soluble myosin in cortical contractions of Xenopus eggs

The layer of cytoplasm underlying the plasmalemma of Xenopus eggs has contractile activity which is of vital importance in fertilization and early development, being involved in such processes as sperm engulfment, cortical granule exocytosis, development of the axes of embryonic symmetry and cleavage. In amphibian eggs this layer is also involved in wound healing and changes of cellular shape at gastrulation. Two kinds of contractile structures can be distinguished near the surface of Xenopus eggs. To characterize the mechanism and regulation of this contractile activity, we have experimentally induced cortical contractions in bisected living Xenopus eggs. We have shown previously that cortical contractions are induced by calcium ions in the bisected egg. Here we show that extraction of soluble cytoplasmic components prevents the calcium-induced contractions, but that addition of exogenous soluble myosin restores them. In oocytes, both soluble and insoluble components of the cortical cytoplasm are unable to support contraction. Thus, during meiotic maturation of oocytes into eggs, both of the components of the cortical cytoplasm must change so as to become competent for contraction.