Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 132

Figure 4.1 Cytoskeletal organization of Xenopus oocytes. (A) Oocyte bisected along the
animal-vegetal axis and immunostained against cytokeratin (mAb 1h5). Arrowheads
show the position of the equator. Cytokeratin is more densely organized in the vegetal
cortex. (B) Confocal stack of a cytokeratin-stained oocyte, vegetal view, showing the typ-
ical geodesic pattern at the vegetal pole. The inset shows a section of the cortex.
(C) Confocal stack of an -tubulin-stained oocyte, showing a dense reticular pattern.

GV, germinal vesicle; scale bar

a

¼ 30 m.

m

oocytes, although this remains to be shown functionally. Interestingly,
although the population of microtubules with plus ends toward the cortex
is minimal in the animal hemisphere ( < 5%), a greater number (

20%) of
cortical plus-end microtubules are oriented toward the vegetal cortex
( Messitt et al., 2008; Pfeiffer and Gard, 1999 ). This subpopulation has been
observed by both hook decoration and immunofluorescence ( Messitt et al.,
2008 ) and is likely responsible for the plus-end motor-dependent transport
of vegetally localized mRNAs and organelles (see Section 3 ).

2.1.3 Intermediate filaments
Intermediate filaments, consisting of 10-nm filament bundles, are one of

the major proteins of the cortex in animal cells. Unlike microtubules, these
are nonpolarized structures. In full-grown oocytes, the pattern of cytoplas-
mic keratin (cytokeratin) filaments in the cortex is strikingly polarized along
the animal-vegetal axis. In particular, the vegetal cortex has a much thicker
layer of cytokeratin, ordered in a geodesic mesh pattern ( Klymkowsky et al.,
1987 ; Fig. 4.1 ). By contrast, cytokeratin filaments in the animal hemisphere
are shorter and irregular, forming a finer mesh network. Cytokeratin is less
abundant in the deeper cytoplasm, present as shorter, less organized
filaments. The animal-vegetal organization of cytokeratin likely depends
on microtubules, since it is lost in oocytes treated with microtubule-
disrupting agents ( Gard et al., 1997 ).