393maternal transcripts (Mishima et al. 2006 ). Conversely, germ cells are protected from
entering somatic fates by blocking somatic mRNA expression through promoting
their deadenylation and subsequent degradation. RNA binding proteins Nanos and
Pumilio play a key role in this process (Vasudevan et al. 2006 ; Collier et al. 2005 ).
Discovering the regulatory networks within germ plasm that are responsible for
specifying the germ cell lineage is a major goal of current research. Germ plasm
mRNAs like vasa, dnd and nanos1 exhibit an intricate and precise expression pattern
during embryonic development. They are present in small cortical aggregates that
become enriched in the cytokinetic ring (Braat et al. 1999b). Consequently, cytoskel-
eton components (microtubules and microfilaments) and germ plasm RNAs become
recruited to the forming cleavage furrow (Yoon et al. 1997 ; Pelegri et al. 1999 ).
Notably, zebrafish nebel mutant embryos, which have defects in furrow microtubule
array formation, exhibit defects in the peripheral aggregation of germ plasm mRNA. At
the 32-cell stage, four germ plasm aggregates ingress into four cells, where they
remain subcellularly localized (Yoon et al. 1997 ). Between 32-cell and 8k-cell stage
(cell cycle 13), these aggregates segregate asymmetrically during cell division, so that
only four descendant cells inherit the germ plasm (Braat et al. 1999c). In each of these
cells, germ plasm forms a cup-shaped structure that is often in apparent association
with one of the spindle poles (Knaut et al. 2000 ) (Fig. 8.2, inset A). This program of
asymmetric segregation changes at late blastula stage (8k-cell stage), when germ
plasm is distributed equally to both daughter cells, now called PGCs (Knaut et al.
2000 ). At this time, the germ plasm appears to disintegrate and spread throughout the
cytoplasm. Briefly before gastrulation (30 % epiboly), the germ plasm is entirely dis-
persed and fills the cytoplasm evenly in little patches (Knaut et al. 2000 ).
8.2.4 PGC Migration
In zebrafish, as in other vertebrates, PGC are specified before gastrulation in a differ-
ent position from where the gonads develop. PGCs have to acquire directional motil-
ity and migrate towards the site where PGCs associate with somatic gonadal cells to
subsequently differentiate into gametes (Raz and Reichman-Fried 2006 ). Zebrafish,
PGCs are specified before gastrulation in four different locations within four differ-
ent somatic environments. Therefore they have to traverse a complicated trail to
reach the developing gonads (Fig. 8.3). As in amphibians and mammals, somatic
tissues provide directional cues that help PGCs to reach their final destination.
The principal signaling system driving zebrafish PGC migration is Stromal-
derived factor 1 (SDF1A) and its receptor, the GPCr chemokine (CXC motif) recep-
tor 4b (CXCr4B), which is expressed in PGCs (Knaut et al. 2003 ). The PGCs
migratory pathway is closely associated with the somatic expression of SDF1A that
indicates the intermediate and final targets of migration (Reichman-Fried et al. 2004 ).
Ectopic expression of SDF1A is sufficient to cause PGCs to mis-migrate to those
positions in the embryo. In loss-of-function mutants of either SDF1A or CXCr4B,
PGCs randomly migrate through the embryo (Richardson and Lehmann 2010 ).
8 Mechanisms of Vertebrate Germ Cell Determination