Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 138

During subsequent growth to stage I, one of the preclouds preferentially
accumulates germ plasmmaterial and certain localized mRNAs and acquires
the structural and functional properties of the definitive mitochondrial cloud
( Heasman et al., 1984; Kloc et al., 2004a ). The cloud begins to accumulate a
cytokeratin “basket” and becomes resistant to disruption following
microtubule-disrupting drugs, whereas other peri-GV mitochondrial aggre-
gates are affected. Also, live imaging showed that in contrast to the smaller
aggregates, the primary cloud is immobile and anchored in place ( Heasman
et al., 1984 ), suggesting that polarity has been reestablished. By late stage I (or
in oocytes > 250 m m), the cloud begins to disperse toward the prospective
vegetal hemisphere. Thus, by stage I, the position of the mitochondrial
cloud identifies the future animal-vegetal axis of the oocyte. By stage II,
vegetal cortical identity is established by localization and anchoring of the
mitochondrial cloud material. In stage III oocytes, visible and cytoskeletal
polarization is not yet evident, although by stage IV the oocytes have
acquired animal-vegetal differences

typical of full-grown oocytes

( Gard, 1999 ).

2.4. Molecular regulation of animal vegetal polarity

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2.4.1 Polarity in the germline cyst
In Drosophila , polarity throughout the cyst divisions is maintained by the
fusome, a vesicular structure likely derived from mitotic spindle material
( Lin et al., 1994 ). The fusome associates with the centrosome and is also
thought to anchor one end of subsequent spindle poles, such that later divi-
sions occur invariantly, leading to cysts with mirror-image symmetry.
Fusome material penetrates the ring canals and connects with the structures
from previous divisions, forming a branched organelle contiguous in all
cystocytes ( Lin et al., 1994 ). The fusome is thought to originate from the
spectrosome, a biochemically similar cytoplasmic organelle in germline stem
cells. Both structures contain small vesicles linked by proteins normally
found in the cortical submembrane skeleton, including Spectrins and

Adducin-like proteins.
Fusome proteins are found in similar distributions in Xenopus germline
cells, and Spectrin is found in mitochondrial aggregates in the cystoblasts,
suggesting that this structure may function analogously to the spectrosome
( Kloc et al., 2004a ). In Drosophila , fusomes are proposed to regulate transport
of oocyte determinants between cystocytes ( Lin et al., 1994 ), making it
problematic to dissociate roles in transport from direct roles for fusome
material in oocyte specification. In Xenopus , all 16 cells become oocytes,