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.

Development of cortical contractility in the Xenopus laevis oocyte mediated by reorganisation of the cortical cytoskeleton: a model.

As the amphibian oocyte becomes the fertilisation-competent egg an actin-myosin network assembles in the cortex which provides for the cortical contraction that accompanies fertilisation. A number of recent investigations provide data for development of a model detailing the structural changes which should accompany the development of this contractile network as well as the signalling mechanisms which regular assembly and contraction.

Distribution of prosome proteins and their relationship with the cytoskeleton in oogenesis of Xenopus laevis.

The presence of prosome proteins (p25K and p27K) was shown and their distribution was studied in oogenesis of Xenopus laevis using immunoblotting and immunofluorescence. These proteins form numerous granular clusters of variable size all over the cell. At previtellogenic stages, the prosome antibodies homogeneously stain the oocyte nucleus and the evenly distributed relatively large clusters in the cytoplasm. As the oocyte grows, the pattern of distribution of the prosome proteins undergoes changes: animal-vegetal and cortical gradients appear in the cytoplasm. In the course of oocyte maturation the size of clusters diminishes. Artificial activation of the egg leads to a dorso-ventral gradient in distribution of the prosome proteins. In this way, specific localization of prosome proteins is first visualized during formation of the dorso-ventral polarity. Co-localization of prosome proteins and actin and myosin was found in the oocyte by double staining. Small clusters of prosomes dispersed in the cytoplasm acquire capability of movement (after artificial activation) due, in all likelihood, to persisting connection with the acto-myosin complex of the egg.

Contractile proteins and nonerythroid spectrin in oogenesis of Xenopus laevis.

The distribution of contractile proteins, actin and myosin, and an actin-binding protein, spectrin, was studied in oogenesis of Xenopus laevis. These proteins are present in oocytes already at the previtellogenic stages, which are characterized by their diffuse distribution. The localization of proteins changed with the beginning of vitellogenesis. At all vitellogenic stages, including the fully grown oocyte, animal-vegetal differences were noted in localization of actin and myosin: in the animal hemisphere they appear as fibrillar-like structures, while in the vegetal one they are localized around the yolk platelets. By the end of the oocyte's growth, a cortical gradient appeared: predominant localization of actin and myosin in the cortical area. As the oocyte maturation proceeded, the distribution of actin and myosin again became diffuse and nonuniform, so that a cortical gradient appears. At the beginning of vitellogenesis spectrin is distributed as a network all over the ooplasm, while in the fully grown oocyte it is localized mostly in the subcortical area of the animal hemisphere and, as individual inclusions, in other regions of the oocyte. No spectrin is found by the end of maturation. Actin, myosin, and spectrin are also present in the oocyte's nuclei. Changes in the distribution of contractile proteins and spectrin during oocyte maturation are discussed with respect to the development of cortical contractility, as well as to the changes in spatial distribution of yolk platelets and regional sensitivity of the maturing oocyte to cytochalasin B.

Influence of cytochalasin B on oocyte maturation in Xenopus laevis.

Cytochalasin B (CB) exerts an inhibiting effect on the formation, migration and anchoring in the cortex of the meiotic spindle in maturing Xenopus laevis oocytes. Regional sensitivity to CB (CB-sensitive zones) has been found in the oocytes which varies with reference to the stage of oocyte maturation at which CB is applied. Light and electron microscopy has shown that in these CB-sensitive zones the yolk and pigment granules, unlike the cortical ones, are displaced into the cytoplasm centripetally under the influence of CB.

Maturation of Rana temporaria and Acipenser stellatus oocytes induced by the cytoplasm of embryos at different cell cycle phases and at different stages of cleavage and blastulation.

Experiments involving injections of cytoplasm from the cleaving embryos of Rana temporaria and Acipenser stellatus into fully grown oocytes of the same species and of Xenopus laevis, show that at all mitotic phases, and throughout the period of synchronous cleavage divisions, the cytoplasm is characterized by high activity of the germinal vesicle breakdown factor. This activity decreases during nuclear division desynchronization and drops sharply after the mitotic index fall upon blastulation. Aside from germinal vesicle breakdown in the oocytes, the A. stellatus embryos' cytoplasm also induces development of a cortical reaction capacity.

The cytostatic effect of the cytoplasm of mature, non-activated and cleaving eggs of Rana temporaria, Acipenser stellatus and Xenopus laevis.

Corroborated here is the fact, earlier established by Chulitskaya and Felgengauer (1977), that the cytoplasm of mature non-activated eggs of Rana temporaria and Acipenser stellatus, unlike that of Rana pipiens, exerts no cytostatic effect on the nuclei of the cleaving embryo, but acquires such a capacity after being treated with EGTA. EGTA treatment imparts cytostatic properties also to the cytoplasm of cleaving embryos. Revealed is the dependence of the cytostatic effect and death of injected embryos on the dose of EGTA introduced into the egg. No mitotic figures have been detected in embryos with a cytostatic effect. Upon reciprocal transplantation of the cytoplasm between Rana temporaria and Xenopus laevis, only the latter's cytoplasm possessed a cytostatic effect, while the arrest at a metaphase was found only in a few arrested blastomeres.