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This structure properly positions germ plasm within the protective environment of
the future endoderm. Endoderm also provides the proper location from which PGCs
can correctly migrate and reach the presumptive gonads (Gupta et al. 2010 ). The
majority of the mRNA present in the Bb corresponds to germ plasm components,
nevertheless some mRNA involved in zygotic patterning including wnt11 and
syntabulin (Makita et al. 1998 ; Nojima et al. 2010 ), also utilize the early Bb-mediated
vegetal pathway. It has been proposed that the Bb has a basal role protecting RNAs
from degradation as well as preventing any premature translation. The fact that
maternal mRNA and translational regulators, required for germ line specification, are
components of the Bb supports this idea (Collier et al. 2005 ; Marlow 2010 ).
Fig. 8.1 Germ plasm localization during Zebrafish oogenesis and early embryogenesis: During
oogenesis (top row), the Balbiani body (Bb) or mitochondrial cloud, shown in light brown, embed-
ded with mitochondria, shown in green, marks the vegetal pole of the oocyte (stage I) and indicates
the future vegetal pole of the egg and embryo (zygote). In stage I oocytes, newly synthesized RNAs
enter the Bb as either a component of nuage or later by a diffusion/entrapment mechanism. In stage
Ib and II oocytes, the Bb moves to the vegetal cortex and begins to disperse. Bb continues to disperse
during the next two stages. Germinal granules (red balls) and germ plasm RNAs form a disc at the
vegetal cortex. Other RNAs are localized at the animal pole (light green disc). Bucky ball originally
localizes to the vegetal pole, but then translocates to the animal pole during St. III. After egg activa-
tion (bottom row), germ plasm mRNAs undergo a redistribution. Cytoplasm segregates from the
yolk and starts to migrate towards the animal pole to form the blastoderm. At that time, vasa, dnd,
and nos1 mRNAs translocate towards the animal pole (blue and red circles). Arrows indicate direc-
tion of germ plasm movement. After reaching the blastoderm, the rearrangement of the cytoskeleton
(F-actin and microtubules) recruit germ plasm RNAs to two domains of the first cleavage furrow. In
this way, the germ plasm becomes concentrated in a cytokinetic ring. dazl mRNA is recruited
slightly later to the germ plasm suggesting a separate route of RNA transport (pink circles)
T. Aguero et al.