Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 139

suggesting that the relative amount of fusome material is not important for
specifying the oocyte. Fusomes are aligned with centrosomes and mitochon-
drial aggregates in early meiotic oocytes in the cyst ( Kloc et al., 2004a ).
These data support the idea that the fusome helps maintain inheritance of
polarity in developing oocytes, through interdependent interactions with
centrosomes and cellular organelles ( Kloc et al., 2004a ). Thus, at the end
of the cyst divisions, the oocyte has likely inherited polarity from early oogo-
nial stem cells, but the relationship of this oogonial polarity to final animal-
vegetal polarity is unclear, since early stage I oocytes lack a continuity of
visible polarity.

2.4.2 Establishment of animal vegetal polarity in early oocytes

In Drosophila , a repolarization event is proposed to occur at the analogous
stage, mediated by reciprocal oocyte-follicle cell signaling ( Huynh and
St Johnston, 2004 ). Fusome components, mitochondria and centrosomes,
become relocalized from the anterior to the posterior pole of the fly oocyte,
and this process requires polarization of the oocyte into complementary cor-
tical domains through antagonism of cortical cell polarity proteins, Prkci and
Pard1 ( Huynh and St Johnston, 2004 ). The mechanisms regulating
cell polarity in this case are unknown, but follicle cells are implicated
since they invade and surround the oocyte at this time. Additionally,
oocyte Dystroglycan, a receptor for extracellular matrix, is required for
anteroposterior polarity ( Deng et al., 2003 ), further suggesting an external
signal is required.
It is unclear if similar processes are at work in vertebrates. Follicle cells
invade and visible polarity is lost, but there is no obvious relocalization of
fusome components and organelles, which are uniformly distributed
( Kloc et al., 2004a ). Prkci is not asymmetrically distributed in immature
Xenopus oocytes ( Nakaya et al., 2000 ), but early prestage I oocytes were
not specifically examined in that study. Recent maternal genetic studies
in zebrafish however have generated important new insight into the forma-
tion of animal-vegetal polarity in vertebrate oocytes.
Among the mutants identified in a series of forward genetic screen for
developmental maternal-effect mutations ( Dosch et al., 2004; Wagner
et al., 2004 ), buckyball ( buc ) showed striking defects in the animal-vegetal
polarity of the egg—including loss of vegetal RNA localization, failure of
blastodisc formation of the animal pole after egg activation, and formation
of ectopic micropyles ( Dosch et al., 2004; Marlow and Mullins, 2008 ). The
micropyle is a pore-like structure at the animal pole derived from follicle

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