Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 131

three of the main cytoskeletal systems are highly organized and complex in
the full-grown oocyte and have been described in detail at the level of both
electron microscopy and fluorescence/confocal microscopy. For excellent
reviews of the older literature, please see Gard (1999) , Sardet et al.
(2002) , and Wylie et al. (1985) .

2.1.1 Microfilaments
Filamentous actin is concentrated in the oocyte cortex, to a greater extent
animally ( Gard, 1999 ). Actin is highly abundant in the GV and also forms a
dense perinuclear network. Studies using cytochalasin to disrupt microfila-
ments suggest that actin organization links together and organizes both
microtubules and cytokeratin to the cortex ( Gard, 1999 ). The function of
nuclear actin is not clear, but evidence suggests roles in the regulation of
chromatin structures and of transcription. Injection of anti-actin antibodies
into the GV interfered with transcription from lampbrush chromosomes.
More recent data have indicated that nuclear actin polymerization is
required for chromatin remodeling and nuclear reprogramming in the
GV ( Miyamoto et al., 2011 ).

2.1.2 Microtubules
Cytoplasmic microtubules extend outward from the GV and terminate in
the subcortical regions ( Gard, 1991 ). Acetylated microtubules are also
enriched in the animal pole, indicating these are stable structures. Microtu-
bule arrays are also present in the cortex, typically forming clusters associated
with cortical endoplasmic reticulum ( Gard, 1991 ; Fig. 4.1 ). Disruption of
microtubules by cold or with colchicine-based drugs does not disrupt overall
animal-vegetal polarity (e.g., pigment, yolk distribution, etc.), although GV
positioning is altered and secretory pathways are inhibited ( Gard

et al., 1997 ).
Gamma-tubulin, an essential centrosomal component in somatic cells, is
found perinuclearly and shows a striking asymmetry in the cortex, being
enriched in numerous puncta throughout the vegetal cortical hemisphere
( Gard, 1994 ). These foci are likely to act as microtubule-organizing centers,
since the full-grown oocyte lacks visible and functional centrosomes.
In contrast to most somatic cells, the majority of cytoplasmic microtu-
bules in the Xenopus oocyte are oriented with their plus ends toward the
interior and the minus ends anchored near the cortex ( Pfeiffer and Gard,
1999 ). These data suggest that the cortical foci of gamma-tubulin are likely
responsible for maintaining the bulk of microtubule polarity in full-grown