4058.3.2 Cell Autonomous Specification of Germ Line
Although germ plasm was shown to be both required and sufficient for determina-
tion of the Drosophila germ line, the question remained whether this was also true
in vertebrate systems. Early on, Smith ( 1965 ) had found that nuclei from PGCs, but
not other cells, remain totipotent upon nuclear transplantation into enucleated eggs
of the frog Rana pipiens (Smith 1965 ). His results suggested that germ plasm com-
ponents conferred a totipotent “status” to the genetic material within the nucleus.
But whether germ plasm contained all the determinants to convert a naïve cell into
the germ cell lineage remained untested. Tada and coworkers took advantage of the
unique abundance of mitochondria in germ plasm to generate a transgenic frog in
which EGFP was fused to a mitochondrial transport signal to selectively label germ
plasm (Tada et al. 2012 ; Taguchi et al. 2012 ). At the eight-cell stage, the EGFP
labeled germ plasm was removed by micropipette and injected into an ectopic loca-
tion, an animal pole cell. Interestingly, the injected germ plasm behaved, as it would
have in vegetal pole cells, by becoming localized to the peripheral cytoplasm early
on and migrating to a perinuclear location by gastrulation (Fig. 8.6). These ectopic
EGFP-germ plasm cells, referred to as T-GP, did not migrate properly into the geni-
tal ridges, but continued to express germ plasm markers such as nanos, xdazl, and
xpat at least until tailbud stage 28. For proper migration into the genital ridges,
perhaps T-GP cells had to begin within the endoderm. To test that idea, at
Fig. 8.6 Continuity of germ plasm in Xenopus (female cycle is depicted). Schematic showing
germ plasm (red circles) with mitochondria (green) within the Bb. The Bb moves to vegetal pole
later in oogenesis. During early cleavage stages, germ plasm is asymmetrically inherited by a few
vegetal pole blastomeres and remains peripheral (at this point called pPGC). At gastrula, germ
plasm becomes perinuclear (PGCs). During tailbud stages, PGCs begin to migrate laterally and
dorsally and exit the endoderm at tadpole stage 40, migrating along the dorsal mesentery to the
presumptive gonads. PGCs enter developing gonads at tadpole stages; here PGCs divide mitoti-
cally to form 16-cell interconnected nests of oogonia. These cells enter meiosis becoming oocytes.
At all stages either nuage or germ plasm is present
8 Mechanisms of Vertebrate Germ Cell Determination