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8.1 Introduction
Successful formation of the germ line in each individual is required for genetic infor-
mation to proceed into future generations. During development, the precursors to the
gametes, the primordial germ cells (PGCs) segregate from somatic cell lineages. At
the proper moment, PGCs migrate a significant distance to enter the developing
gonads. Here they divide, acquire their sexual identity, enter into meiosis, and fully
differentiate into haploid eggs or sperm. At fertilization, the genetic information from
each gamete recombines and a genetically unique individual is formed. Continuity of
the germ line from generation to generation is thus established. It follows that two
hallmarks of the germ line is its ability to avoid death, the inevitable fate of somatic
cells, and to retain full developmental potential. A major goal of research on the germ
line is understanding how this unique lineage comes to be specified within the context
of a developing embryo that allows it to both differentiate yet retain full potential.
Germ cell specification proceeds through basically two different mechanisms that
have been broadly characterized as inherited or inductive. In this chapter, we discuss
how germ plasm driven germ cell specification (inherited) occurs in both zebrafish
and the frog Xenopus. To provide an evolutionary context, we discuss both the seg-
regation of germ cells during embryonic development of solitary and colonial ascid-
ians, as well as the regeneration of germ cells in colonial species, since these
organisms provide salient examples of both inherited and inductive specification
patterns. Ascidians are marine chordates closely related to vertebrates that also con-
tain germ plasm. Finally, we examine the mechanism of germ cell specification via
induction, as exemplified by mouse and axolotl. Although these model systems are
highlighted because more is known about them, it is important to point out that there
are important differences between mouse and human germ cell specification.
Inherited mechanisms operate by assembling, during oogenesis, a molecular
“tool kit” of proteins and RNAs into a specialized subcellular domain called germ
plasm. Cells that inherit germ plasm will enter the germ cell lineage and if they
survive, will give rise to the gametes. No other lineage can do this in vertebrates.
Germ plasm, although not membrane bound, retains a subset of proteins and RNAs
that are both required and sufficient to confer germ line identity. The known compo-
nents of germ plasm have been conserved across a wide variety of species and
include endoplasmic reticulum, germinal granules, mitochondria, and lipid droplets.
Such similarity is especially interesting when one considers that germ plasm has
been reinvented repeatedly during evolution. The inheritance mechanism of germ
cell specification apparently derived from the basal inductive mechanism (Johnson
et al. 2001 , 2003a; Extavour and Akam 2003 ). In both solitary and colonial species
of ascidians, maternally specified cells containing germ plasm segregate early in
development and, in colonials, give rise to gonads in asexually developing bodies.
In some colonial species, however, segregation of germ line and soma is not abso-
lute, as germ cells can be generated de novo at postembryonic stages. The inductive
mechanism for germ cell formation in both mammals and axolotls involves the
selection from a population of pluripotent cells formed at the time of gastrulation.
Not all of these pluripotent cells will give rise to PGCs, but have the potential for
T. Aguero et al.