183Riemer et al. 2015 ). It has now been shown in zebrafish that the first stages of Bb
formation and oocyte polarization occur much earlier, at the onset of meiosis
(Elkouby et al. 2016 ). In the premeiotic oogonial stages, Bb precursor components,
such as Bucky ball, GasZ, and mitochondria, are distributed symmetrically in the
perinuclear cytoplasm (Fig. 5.2a). However, this symmetry is broken when these
precursors initiate their specific and asymmetric localization at the early meiotic
zygotene bouquet stage. At these stages, oocyte polarity aligns to a nuclear polarity
that is a characteristic feature of the zygotene stage (Fig. 5.2a). During zygotene, the
telomeres of the chromosomes become tethered to the nuclear envelope and move
and rotate actively in the nucleus, finally orienting their telomeres so they are clus-
tered to one pole of the nuclear envelope, a configuration called the chromosomal
bouquet (Scherthan 2001 ; Shibuya et al. 2014b). This dynamic process is mediated
by microtubules and the centrosome and facilitates chromosomal pairing and mei-
otic recombination (Ding et al. 2007 ; Link et al. 2014 ; Sato et al. 2009 ; Scherthan
2001 ; Shibuya et al. 2014a, b).
Remarkably, Bb precursors initially localize to the cytoplasmic region that
apposes the bouquet telomere cluster and contains the centrosome (Elkouby et al.
2016 ). Significantly, disruption of microtubules causes a loss of telomere clustering
of the bouquet and a parallel loss of mitochondrial enrichment to form the Bb pre-
cursor aggregate, indicating that these processes are coordinately regulated (Elkouby
et al. 2016 ). Furthermore, epistasis experiments showed that initial oocyte
polarization was normal in buc−/− oocytes, indicating that these early symmetry-
breaking events lie functionally upstream to the Bucky ball protein.
Subsequent to the bouquet stage, the Bb precursor components localize as mul-
tiple aggregates within a nuclear indentation positioned at the location of the centro-
some and the presumptive former telomere cluster of the bouquet (Fig. 5.2b). We
have termed this structure a nuclear cleft. The nuclear cleft then gradually rounds
out, and the Bb aggregate becomes more condensed and spherical, giving rise to the
typical mature Bb of mid-diplotene stages. Therefore, the centrosome and telomere
cluster association at zygotene stages marks the future vegetal pole of the oocyte
and is proposed to form a meiotic-vegetal center that couples oocyte patterning to
meiotic genetic events (Elkouby et al. 2016 ).
5.3.3 Potential Upstream Regulation of Oocyte Symmetry
Breaking
The question arises if further regulation upstream of the bouquet may be important in
setting up the axis of polarity. The tissue organization in cysts could be important for
polarization. It has been suggested that the cytoplasmic bridges that form due to
incomplete cytokinesis of oogonial cell divisions in the cyst may be the sites of Bb
precursor aggregation. Clusters of mitochondria have been observed in frog oogonia
close to the cytoplasmic bridges (Kloc et al. 2004a). However, whether this aggrega-
tion of mitochondria is linked to the mitochondria of the mature Bb is not known.
5 Localization in Oogenesis of Maternal Regulators of Embryonic Development