Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 129

the vitelline membrane and establish gap junctions with the oocyte to help
regulate meiosis and yolk accumulation.
A full-grown oocyte is 1-1.3 mm in diameter ( 0.5 m l volume) and
shows the typical animal-vegetal differences in pigment and yolk accumula-
tion. Pigment granules are heavily distributed in the cortical layer of the animal
hemisphere. These accumulate to a much lesser extent in the vegetal hemi-
sphere and are often absent from an unpigmented equatorial band. The GV is
positioned eccentrically toward the animal pole. This structure is 100,000

times larger than a typical somatic cell nucleus and multiple extranucleoli, sites
of rDNA amplification and rRNA synthesis ( Rasar and Hammes, 2006 ). The
majority of yolk-free cytoplasm and most of the maternal mRNA and ribo-
somes are enriched animally. Yolk, in the form of crystalline membrane-
bound “platelets,” is enriched in the vegetal hemisphere, along with the germ
plasm material and many localized mRNAs. Surrounding the oocyte beneath
the plasma membrane is the cortex, a cytoskeleton- and organelle-dense layer
about 5 m m thick that gives the structure to the oocyte.
In Xenopus laevis , oogenesis begins about 3 weeks after fertilization (stage
52), as the somatic genital ridges and gonads undergo sexual differentiation
( Nieuwkoop and Faber, 1967 ). At this stage, the primordial germ cells
remain in the cortex of the gonadal tissue, differentiating into primary
oogonia and proliferating mitotically. Meiosis initiates at about 4 weeks after
fertilization (stage 56). The oocytes progress through early prophase stages
and become arrested in diplotene of meiosis I ( Nieuwkoop and Faber,
1967 ). The cells then enter a prolonged growth phase, during which the
oocyte increases in size 10,000-fold. During this phase, yolk accumulation

and other events allow the oocyte to become competent for maturation and
eventual ovulation. Oogonial stem cells continue to proliferate throughout
reproductive life, and oogenesis persists into adulthood. Dumont (1972)
established staging criteria for oogenesis in Xenopus , dividing the process into
six stages (I-VI) based on different morphological and physiological charac-
teristics. Oocyte growth takes 6-8 months, after which oocytes cease grow-
ing at stage VI (full grown) and then undergo atresia. The stages and some
relevant features in Xenopus and zebrafish are shown in Table 4.1 .

2.1. The cytoskeletal organization of the stage VI
Xenopus oocyte
The stereotyped animal-vegetal organization of the full-grown oocyte and
functional differences along this axis arise progressively during oogenesis and
are dependent on interactions between elements of the cytoskeleton. All