[A 2-component cytoskeletal system as the basis of cortical contractility in clawed toad eggs]

The cortex of Xenopus laevis eggs includes as two components the plasma membrane with underlying microfilaments (external layer) and cytoplasmic matrix with embedded pigment granules (internal layer). The both components of the egg cortex are capable of contraction under the influence of calcium ions. The cortex of the fully grown oocyte is not capable of contraction but acquires it during progesterone-stimulated maturation, when the oocyte is transformed in the egg. It has been proposed on the basis of the data on the cortex cytoskeletal organization that in the oocytes the submembranous microfilaments form an anisotropic network, which is transformed in 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 formation of the actin contractile gel. The data are provided on the role of actin-associated proteins in changes of organization of the actin cortical cytoskeleton. Mechanisms underlying different sensitivity of microfilaments of the internal and external layers to cytochalasin B are discussed, as well as coordinated (in time) development of the contractility in these layers. A proposed model of development of the cortical contractility during oocyte maturation is based on the concept of two-component cytoskeletal system.