180
Houston and King 2000 ; Kloc et al. 2002 ). In the Xenopus embryo, the vegetal cells
that inherit the GP then adopt the PGC fate (Houston 2013 ; Whitington and Dixon
1975 ). In zebrafish it is more complex: some GP components localize to the oocyte
and egg vegetal pole (e.g., dazl RNA) (Kosaka et al. 2007 ), whereas vasa RNA, for
instance, is initially vegetally localized in the oocyte and then becomes radially
localized to the oocyte cortex, where it remains in the egg (Kosaka et al. 2007 ).
After egg activation in zebrafish, the GP components from distinct locations accu-
mulate at the cleavage furrows of the 2- and 4-cell stage embryo (Fig. 5.1c) (Braat
et al. 1999 ; Knaut et al. 2000 ; Riemer et al. 2015 ; Yoon et al. 1997 ). The difference
between frog and fish is likely due to the distinct architectures of these embryos. In
zebrafish the yolk lies at the vegetal pole of the egg separate from the blastomeres
that form in the animal half, whereas in the frog, the yolk and cytoplasm are not
segregated from each other, and blastomeres will comprise the entire AV extent of
the egg. So in the zebrafish, the GP must reaggregate at the cleavage furrows at the
yolk-cytoplasm interface at an animal-vegetal mid-region (Fig. 5.1b and c), whereas
in the frog, the vegetal-most blastomeres inherit directly the vegetal- oocyte local-
ized GP to become the PGCs.
In both frogs and fish, the germplasm components are first localized in stage
I oocytes to a structure called the Balbiani body (Bb) (Fig. 5.2b) (Chang et al.
2004 ; Heasman et al. 1984 ; Kloc and Etkin 1995 ; Kloc et al. 1996 , 1998 ;
Kosaka et al. 2007 ; Schnapp et al. 1997 ; Wilk et al. 2005 ). Here we will gener-
ally use the zebrafish oocyte staging convention (Selman et al. 1993 ), which is
slightly different to that in Xenopus. The Bb, also called the mitochondrial
cloud in Xenopus, is a highly conserved structure present from insects to
humans where specific RNAs, proteins, and organelles become localized
(reviewed in Kloc et al. 2004b). These RNAs and proteins, including GP com-
ponents, translocate via the Bb to the oocyte vegetal cortex (Fig. 5.2b) (Kloc
and Etkin 1995 ; Melton 1987 ; Wilk et al. 2005 ; Yisraeli et al. 1989 , 1990 ; Zhou
and King 1996b). In Sect. 5.4, we will discuss what is known about the RNA
localization mechanism via the Bb.
5.2.4 Fertilization: Oocyte Polarity Defines the Sperm
Entry Region
The AV axis defines the region of sperm entry in most vertebrates. Frog, zebrafish,
and mouse eggs are all fertilized predominantly in the animal half, although each
differs somewhat from the others (Grey et al. 1974 ; Hart et al. 1992 ; Piotrowska and
Zernicka-Goetz 2001 ). In the mouse, the animal 11 % of the egg lacks villi, reducing
the membrane surface area for sperm binding, and fertilization is excluded from this
region (Hiiragi and Solter 2004 ). However, fertilization occurs predominantly in the
remainder of the animal half of the mouse egg, largely due to the greater
M. Escobar-Aguirre et al.