385other cell lineages. Those cells that receive signals from the Bone Morphogenic
Protein (BMP) family of growth factors originating from a proximal germ layer
ultimately specify which cells will enter the definitive germ line.
Regardless of whether germ cells are specified by the inherited or inductive mech-
anism, several general themes in vertebrate germ cell specification have emerged. (1)
Germ cell lineages are, in part, determined through a process of repressing programs
of somatic differentiation. A fast block to somatic gene programs occurs by globally
repressing mRNA transcription and thus preventing a response to any somatic dif-
ferentiation signals in the embryo. A slow, long-term repression occurs at the level of
chromatin modifications. (2) A reliance on posttranscriptional regulation of gene
expression, both repressing somatic determinants and promoting gene programs
required for germ cell characteristics such as migration, immortality and meiosis.
Many of the genes just downstream of pluripotency factors such as Oct3/4 are trans-
lational repressors like Nanos/Pumilio or activators like Dazl. (3) Germ line stem
cells are protected from expressing their potential for somatic differentiation within
the context of niches. Although it is possible to describe linear pathways that lead to
specific somatic fates such as pancreas or muscle, the germ cell lineage has eluded
such a characterization; there is no master switch. It appears that there is no one
pathway but several combinatorial pathways that may function independently, but all
are required. The molecular and genetic underpinnings of these general themes are
understood to varying degrees. Clearly, much remains to be learned.
8.2 Zebrafish
8.2.1 Background
Only relatively recently, in the 1980s, did the zebrafish (Danio rerio) emerge as a
genetic model organism for studies in vertebrate developmental biology. It offers
many advantages including transparent embryos, external development, and rapid
life cycle facilitating direct visualization of cellular processes and genetic
approaches. In the 1990s, thousands of mutant strains were obtained due to high
throughput mutagenesis, becoming one of the leading models to perform genetic
manipulations (Mullins et al. 1994 ; Kimmel 1989 ; Ingham 1997 ). At that time,
genetic approaches to study germ cells were based mainly in Drosophila melano-
gaster (Williamson and Lehmann 1996 ) and C. elegans (Kimble and White 1981 ;
Sulston et al. 1983 ; Ellis and Kimble 1995 ) putting zebrafish in the limelight as an
inexpensive vertebrate genetic model to study germ cell development (Haffter et al.
1996 ; Westerfield 2000 ). Originally, information regarding primordial germ cells
(PGCs) specification relied almost entirely on morphological identification of
PGCs. In 1997, Yoon and coworkers identified, for the first time, a specific germ
cell lineage marker in zebrafish (Yoon et al. 1997 ). They cloned the zebrafish homo-
logue of the Drosophila vasa gene. This discovery allowed the design of new tech-
niques for genetic manipulation and analysis in zebrafish germ cells, providing
opportunities for researchers to obtain and analyze embryonic development without
8 Mechanisms of Vertebrate Germ Cell Determination