Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 141

For many years, the mouse was thought to be unique in lacking a mito-
chondrial cloud or Balbiani body, despite reports of this structure in oocytes
of other mammals, including humans ( Kloc et al., 2004b ). Recently, how-
ever, there is good evidence for the presence of Balbiani bodies in young
mouse oocytes ( Pepling et al., 2007 ). Histologically, mitochondrial aggre-
gates can be found in germline cysts, and clusters of mitochondria and Golgi
are found adjacent to the GV in oocytes within perinatal germline cysts and
early primordial follicles. Antibodies against Trailer Hitch, a Balbiani body
component in Drosophila , cross-react with mammalian RAP55 and label the
mouse structure, suggesting conservation at the molecular level ( Pepling
et al., 2007 ). Rather than fragmenting and translocating the cortex, how-
ever, the mouse Balbiani body disperses uniformly throughout the cyto-
plasm in growing oocytes ( Pepling et al., 2007 ), similar to the case for
other mammals ( Guraya, 1979 ). These data would be consistent with a lack
of localized maternal determinants in the mammalian oocyte, although there
may be subtle molecular localizations not apparent ultrastructurally. Data
have suggested that 4-cell mouse blastomeres have a nonrandom allocation
to different lineages ( Piotrowska-Nitsche et al., 2005; Tabansky et al., 2012 ),
but the extent that this fate choice in the blastocyst is driven by localized
determinants in the oocyte or egg is unknown.
Thus, the asymmetric establishment of a mitochondrial cloud/Balbiani
body in developing oocytes appears to be a fundamental feature of oogenesis
in vertebrates. In mammals, however, the maintenance of this structure and/
or concomitant polarity may have been lost. The Balbiani body may have
conserved roles in early oogenesis, including overall regulation of RNA
metabolism and organelle trafficking and function, although these potential
functions remain little understood.
3. PATHWAYS AND MECHANISMS OF RNA
LOCALIZATION
Recent transcriptome analysis has suggested that there are approxi-
mately 300 localized transcripts in the vegetal cortex of Xenopus oocytes
( Cuykendall and Houston, 2010 ), corresponding to about 2-3% of mater-
nally expressed genes. The first individual localized RNAs were identified
through molecular cloning and, consequently, few have been studied in
great detail. Comparisons of the in situ hybridization patterns of some of
the first localized RNAs identified two major pathways with overlapping