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with it to the oocyte cortex, where the Bb disassembles and the transcripts become
anchored to the cortex. Many of the RNAs localized through the early pathway
will constitute the germplasm during early development but also include develop-
mental patterning factors (Forristall et al. 1995 ; Kloc and Etkin 1995 ; Yisraeli
et al. 1990 ). In the late pathway, RNAs like Vg1 in frog (Cote et al. 1999 ), and
bruno-like and mago nashi in zebrafish (Kosaka et al. 2007 ), localize to the veg-
etal pole in stage III of oogenesis.
5.4.1 The Early Pathway: Rue Balbiani
The delivery of the early pathway mRNAs to a restricted region of the oocyte cortex
specifies this region as the vegetal pole, and their localization is executed by the
Balbiani body (Bb). The mature Bb is first evident at a perinuclear position during
mid-diplotene stages of oogenesis. At this stage the Bb is loaded with its specific
mRNAs and aggregated mitochondria. The Bb ultimately docks at the oocyte
plasma membrane, where it disassembles and its content is unloaded (Fig. 5.2b).
The unloaded RNPs are then anchored to this region of the cell membrane, defining
it as the oocyte vegetal pole.
The Bb is a universally conserved structure found in oocytes from insects (Cox
and Spradling 2003 ; Jaglarz et al. 2003 ), arthropods (Jedrzejowska and Kubrakiewicz
2007 ), fish (Marlow and Mullins 2008 ), frogs (Dumont 1978 ), birds (Carlson et al.
1996 ; Rodler and Sinowatz 2013 ; Ukeshima and Fujimoto 1991 ), rodents (Pepling
et al. 2007 ; Weakley 1967 ), and primates (Barton and Hertig 1972 ) to humans
(Albamonte et al. 2013 ; Hertig 1968 ). In all these species, two common themes of
the Bb are apparent: (1) the aggregation of mitochondria and electron-dense mate-
rial, presumably corresponding to mRNA, and (2) the localization of the aggregate
at a perinuclear position. In fact, in the original discovery of the Bb in 1845 (Hertig
1968 ), it was described as “a yolk nucleus.” In mammals, the Bb is present in early
oocytes, though it is not known if it polarizes the early mammalian oocyte or if it is
related to the asymmetric formation of the antral cavity or to the asymmetries that
are established during oocyte maturation (discussed above).
Among vertebrates, the Bb has been most studied in Xenopus and zebrafish.
Only a few components of the Bb have been identified. These include germline-
specific mRNAs (like dazl, vasa, and nanos (Kosaka et al. 2007 ; Mosquera et al.
1993 ; Zhou and King 1996a)) and mRNAs encoding dorsal determinants and its
localizing machinery (like wnt8, grip2a, and syntabulin) (Colozza and De Robertis
2014 ; Ge et al. 2014 ; Kirilenko et al. 2008 ; Lu et al. 2011 ; Nojima et al. 2010 ;
Tarbashevich et al. 2007 ), as well as mRNA-binding proteins (like Rbpms2 (Hermes)
(Kosaka et al. 2007 ; Song et al. 2007 )) and RNP scaffold proteins (like GasZ
(Marlow and Mullins 2008 ; Yan et al. 2004 ; Yano et al. 2004 )). Zebrafish females
mutant for the Bb-localized transcripts grip2a (hecate) (Ge et al. 2014 ) or syntabu-
lin (tokkaebi) (Colozza and De Robertis 2014 ; Nojima et al. 2010 ) show normal Bb
formation and overall AV polarity, although they display a maternal-effect mutant
ventralization defect in their embryos.
M. Escobar-Aguirre et al.