Germ Cell Migration

After specification and just before or during the early stages of gastrulation, germ cell migration occurs. PGCs migrate out of the embryo proper and reside in extraembryonic tissues until gastrulation is complete. PGC migration in the mouse occurs between embryonic days 7.5 and 13.5, when the PGCs travel through the developing gut to become incorporated into the primitive gonad (see Figure 8.2). The migration of PGCs is a multistep process during which the cells migrate through different tissue types and environments (Anderson et al., 2000; Molyneaux et al., 2001; Molyneaux and Wylie, 2004; Santos and Lehmann, 2004). Both migratory and survival signals appear to be required during this period for successful PGC development. After embryonic day 7.5, there is a distinct population of cells that appear to have a germ-cell-specific gene-expression profile. Several factors are involved in germ cell survival as PGCs progress along their migratory route. For example, the gene Dead end, which is required for the initiation of migration in zebrafish, appears to act in the mouse as a survival factor. Mice with mutations in this gene display a decrease in PGC number by embryonic day 8.0 and have increased testicular germ cell tumors detected after birth (Youngren et al., 2005). Another gene, Tiar, encodes an RNA-binding protein that is necessary for PGC survival and that has been implicated as a regulator of apoptosis. The Tiar protein is required on embryonic day 11.5 during migration from the hindgut to the genital ridge, and mice lacking this protein fail to develop oogonia or spermatogonia (Beck et al., 1998). Furthermore, the growth factor Fgf2, its receptor Fgf2-IIIb, and other genes (e.g., the antiapoptotic gene Bax) are required for the survival of germ cells during the migration from the hindgut to the genital ridge (Sette et al., 2000; Stallock et al., 2003; Takeuchi et al., 2005).

Germ Plasm \

Oocyte

FIGURE 8.1 The pregastrulation specification of germline fate through the inheritance of germ plasm. Oocytes from species with predetermined germ cell specification contain a microscopically dense complex that is enriched in RNAs, RNA-binding proteins, mitochondria, and ribosomes. During the earliest embryonic cell divisions, germ plasm segregates to cells that will eventually give rise to the germ cell lineage. Cells that do not contain germ plasm give rise to somatic lineages. (See color insert.)

Germ Plasm \

Oocyte

FIGURE 8.1 The pregastrulation specification of germline fate through the inheritance of germ plasm. Oocytes from species with predetermined germ cell specification contain a microscopically dense complex that is enriched in RNAs, RNA-binding proteins, mitochondria, and ribosomes. During the earliest embryonic cell divisions, germ plasm segregates to cells that will eventually give rise to the germ cell lineage. Cells that do not contain germ plasm give rise to somatic lineages. (See color insert.)

Specification

Migration

Colonization

Pre-PGCs —-in ^t proximal isSA' epiblast e6.25

e7.5

e7.5

PGCs in extraembryonic mesoderm

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