A two-component cytoskeletal system of Xenopus laevis egg cortex: concept of its contractility

The cortex of Xenopus laeviseggs comprises two components: the plasma membrane with underlying microfilaments (external layer) and the cytoplasmic matrix with embedded pigment granules (internal layer). Both components of the egg cortex are capable of contracting under the influence of calcium ions. The cortex of the fully grown oocyte does not have the ability to contract, but acquires it during progesterone-stimulated maturation, when the oocyte is transformed into an egg. It has been proposed, on the basis of the data on the cortex cytoskeletal organization, that the submembranous microfilamentsform an anisotropic network in the oocytes, which is transformed into an isotropic, randomly organized network in the egg. The latter is capable of contractile acts. Reorganization of the cytoskeleton in the internal cortex layer leads to the formation of the actin contractile gel. Data are provided on the role of actin-associated proteins in changes of organization of the actin cortical cytoskeleton. Mechanisms underlying the different sensitivity of microfilaments of the internal and external layers to cytochalasin B, as well as the coordinated (in time) development of the contractility in these layers, are discussed. The model proposed for development of the cortical contractility during oocyte maturation (Ryabova et al., 1994a) is considered on the basis of a two-component cytoskeletal system.