31.2.1 Establishing the Germline
In multicellular organisms, the cells of the germline are specialized to pass on the
genetic information to the progeny. As opposed to somatic lineages that ultimately per-
ish, the germ cells have the potential to form a new individual and progress to the next
generation. In the process, they carry the diploid genome through embryogenesis,
divide it into two haploid sets during meiosis, and arrange it for the union with another
haploid genome at fertilization to start the process over and over again (Lesch and Page
2012 ). The founder cells of the germline are called primordial germ cells (PGCs).
PGC formation generally begins when the future father or mother is still an
embryo, but the mechanism by which PGCs are established is not conserved across
species (Schoenwolf 1997 ). In many animals specification of germ cells occurs by
preformation, i.e., through the inheritance of preformed determinants known as the
germ plasm (Weissmann 1885 ). Certain RNAs and proteins synthesized during
oogenesis are transported to, and stored at, a specific location of the oocyte.
Following fertilization, cells that inherit this material will become PGCs.
Experiments on anuran amphibians (frogs and toads) led to the recognition of the
role of germ plasm in vertebrate germ cell specification. During oogenesis in
Xenopus laevis, the germ plasm forms when electron-dense granules become asso-
ciated with an aggregate of mitochondria, known as the mitochondrial cloud. The
structure forms on one side of the cell nucleus and is then transported to the vegetal
cortex of the oocyte. It contains specific proteins and RNA, and, after fertilization,
it accumulates in the vegetal-most blastomeres within the presumptive endoderm.
The descendants of these blastomeres will become PGCs (Whitington and Dixon
1975 ). It has been demonstrated that the dazl gene, with its RNA localized to the
germ plasm, plays a critical role in PGC specification (Houston and King 2000 ).
Preformation seems to be the common mechanism for germ cell specification in
teleosts as well. Zebrafish germ cells arise at the periphery of the developing blasto-
derm in the animal hemisphere. The embryos contain germ plasm, and although they
express dazl, its RNA does not initially localize in the germ plasm but within the veg-
etal yolk (Maegawa et al. 1999 ). It accumulates in the germ plasm at a later time, after
an animally directed movement (Hashimoto et al. 2004 ; Theusch et al. 2006 ). This
indicates that the mechanisms of segregation of dazl in relation to the bulk of germ
plasm are not conserved between Xenopus and zebrafish. Instead, zebrafishes synthe-
size vasa mRNA during oogenesis (Yoon et al. 1997 ). It is a component of the germ
plasm, which, after fertilization, localizes to the cleavage furrows and ends up in four
cells by the 32-cell stage. These four cells will give rise to the PGCs. Preformation
might be the mechanism for avian germline segregation as well. A vasa protein has
been identified in material associated with the mitochondrial cloud in chicken eggs.
The chicken vasa protein localizes in cleavage furrows of the early embryo until it
ends up in six to eight cells in the ~300-cell embryo (Tsunekawa et al. 2000 ).
Alternatively, PGCs form later in development and their determination is not directly
dependent on maternal molecules. In these animals, PGCs are selected from multipo-
tent embryonic cells via signals generated by neighboring cells through
a process termed induction (Extavour and Akam 2003 ). In reptiles and mammals, no
1 Egg Activation at Fertilization