Microtubules (MTs) are hollow cylinders, 25 nm in diameter,  assembled from a 100 kDa heterodimer composed of the 50 kDa proteins, a- and b-tubulin. In most animal cells, interphase MTs are organized into a radial array from a central microtubule-organizing center, or centrosome. MTs are also the major structural elements of meiotic and mitotic spindles.

Tubulins are two of the most abundant proteins of Xenopus oocytes and eggs, constituting nearly 3% of the total cytoplasmic protein. Indeed, one fully grown oocyte or egg contains sufficient tubulin to assemble 1.5-2 kilometers of MT. Biochemical estimates suggest that 15-20% of the tubulin pool is present as polymer, corresponding to 300 meters of MT per oocyte! These results are consistent with those obtained by confocal immunofluorescence microscopy, which suggest that a single stage VI oocyte contains 0.5-1 million individual MTs. MTs play important roles in establishing and maintaining the animal-vegetal polarity of oocytes during stages IV-VI of oogenesis, including the transport of developmentally important maternal RNA and positioning of the germinal vesicle  (the oocyte nucleus) in the animal hemisphere.

Microfilaments (F-actin) (6-9 nm in diameter) are assembled from non-muscle or cytoplasmic isoforms of actin (40 kD), the major structural component of thin filaments in muscle. In non-muscle cells, actin filaments microfilaments ( F-actin) are commonly found in the cell cortex, as well as in specialized assemblies such as microvilli, adherens junctions, and stress fibers (in cultured cells).

Xenopus oocytes and eggs contain an abundance of maternally-supplied actin, with cytoplasmic concentrations exceeding 4 mg/ml. Confocal microscopy revealed that oocytes contain a substantial cytoplasmic network of actin cables extending throughout their cortex and cytoplasm. Actin is also a major component of the germinal vesicle, or nucleus, of Xenopus oocytes, constituting nearly 6% of the nuclear protein in stage VI oocytes. Cytoplasmic and nuclear F-actin appear to play a major role in maintaining the shape and position of the GV during oogenesis.

In addition to actin, several actin-binding proteins have been identified in or isolated from Xenopus oocytes and eggs, including spectrin, the focal adhesion proteins vinculin and talin, the disassembly factors gelsolin and cofilin (ADF), and myosins (including at least two isoforms of type II myosin). These, and other actin-binding proteins, may play important roles in regulating the assembly, organization, and function of F-actin during oogenesis and early development.

In addition to nuclear lamins, Xenopus oocytes contain intermediate filaments assembled from three cytokeratin proteins: two type I (acidic) cytokeratins and a single type II (neutral/basic) cytokeratin that is most closely related to human CK-8. During oogenesis, keratin filaments (KFs) are assembled into a complex network that extends throughout the oocyte cytoplasm. The presence of an additional network of vimentin filaments is controversial. Our rresults (Gard et al., 1997) suggest that the organization and A-V polarity of the cortical CK network of stage VI oocytes is dependent upon both F-actin and MTs.