Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 133

Intermediate filament function in the oocyte has been disrupted by injec-
tion of specific monoclonal antibodies (mAbs) against cytokeratin into the
cytoplasm. This treatment does not affect overall animal-vegetal polarity or
the organization of other cytoskeletal systems ( Gard et al., 1997 ). A role for
cytokeratin in anchoring of endoplasmic reticulum and localized mRNAs
to the vegetal cortex has been proposed ( Alarc´nandElinson,2001 ).
The intermediate filament vimentin, typically associated with mesen-
chymal cell types, has also been detected in the oocyte, using rabbit antisera
( Godsave et al., 1984; Torpey et al., 1990, 1992 ). Vimentin staining
predominated in the deeper cytoplasm and was specifically enriched in
the vegetal germ plasm islands, which persists during early embryonic
development. Other studies failed to find expression of vimentin in the
oocyte using mAbs against Xenopus and mammalian vimentin ( Dent
et al., 1989 ). The function of vimentin (or a related peptide) in the germ
plasm is not known.
Upon oocyte maturation, the cortical networks of microtubules and
intermediate filaments are rapidly disassembled, and the animal-vegetal
asymmetry in their organization is largely eliminated ( Gard, 1992;
Klymkowsky and Maynell, 1989 ). The cortical actin cytoskeleton remains
intact and gains contractility, probably in the form of actomyosin-dependent
motor activity. Filamentous actin also likely plays a role in organizing and
anchoring the meiotic spindle in the animal pole ( Gard, 1999 ).
2.2. Cell polarity signaling mechanisms in the oocyte
In addition to animal-vegetal polarity, regulation of cortical polarity in the
egg has important consequences for cell differentiation in the embryo. It
has long been appreciated that superficial epithelial cells of the amphibian
blastula inherit their outer apical membrane domain from the egg
( Holtfreter, 1943 ). Aside from providing an essential barrier function for
the early embryo, these superficial cells differ in fate from inner nonpolarized
cells. In particular, inner cells are competent to form primary neurons and
multiciliated cells, whereas superficial cells differentiate as secretory goblet
cells in the epidermis or secondary neural progenitors in the nervous system
( Chalmers et al., 2002; Deblandre et al., 1999; Drysdale and Elinson, 1992;
Hartenstein, 1989 ). It is thus important to understand the control of cell
polarity signaling in the oocyte and egg to understand the formation of
the cell layers of the blastula.