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germ plasm per se (Padmanabhan and Richter 2006 ; Mei et al. 2013 ; Mora and King
unpublished). As discussed below, these proteins appear to have a function in oogen-
esis separate from their role during embryogenesis in PGCs. Thus, their RNAs may
be stored within germ plasm for later use, although also translated during oogenesis.
Clues as to when germinal granule components become active may be found in the
changing appearance of the granules going from round and compact in early oogen-
esis to irregular and less electron dense late in oogenesis. After fertilization, germinal
granules form irregularly shaped branching structures and we know that Dead-end is
translated then and soon after, nanos. Between late neurula (st 18) and early tailbud
(st 25) they change into irregular string-like bodies and finally into round granular
bodies again at the feeding tadpole (st 46) (reviewed in Kloc et al. 2001 ).
After fertilization events, the islands of germ plasm begin a continual process of
aggregation driven by actin contractions during divisions and later, ingression along
cleavage planes via microtubules. The end result is that by the 32-cell stage, germ plasm
is concentrated into ~32 large pools within the vegetal most blastomeres (Savage and
Danilchik 1993 ). During cleavage, germ plasm is found close to the plasma membrane
and segregates asymmetrically into one of the two daughter blastomeres, thus preserv-
ing the amount of germ plasm inherited by the one, presumptive primordial germ cell
(pPGC). The other daughter cell enters the somatic endoderm lineage. This changes
during gastrulation when the germ plasm moves in a microtubule dependent manner, to
a perinuclear position. Subsequent cellular divisions then generate cells both of which
contain germ plasm and hence, if they survive, are fated to give rise to gametes. These
cells are now called primordial germ cells (PGCs). Gastrulation, then, marks an impor-
tant time period when the germ line segregates from the future somatic lineages. PGCs
will divide approximately three times, once each at gastrulation, tail bud, and late tail-
bud stages to yield between 20 and 50 PGCs (Dziadek and Dixon 1977 ; Kamimura
et al. 1980 ). The wide range in numbers of PGCs is likely the result of the random dis-
tribution of germ plasm into early vegetal blastomeres and the chance that some cells
will not receive enough germ plasm to preserve germ line fate (Fig. 8.5).
PGCs remain in the endoderm from their inception until late tailbud stages (40–41),
3 days post-fertilization. During this time, PGCs have a characteristic migration pat-
tern, discussed later, that brings PGCs to the dorsal most midline of the endoderm, and
subsequently, along both sides of the dorsal mesentery. A few days later, PGCs arrive
at the genital ridge or presumptive gonads, squeeze in between the gonadal epithelial
cells and there begin to divide mitotically. What regulates the number of divisions these
cells undergo before they enter meiosis, is not known (al- Mukhtar and Web 1971 ; Kloc
et al. 2004a, b). PGCs are sexually indifferent and will develop into either female or
male gametes depending on the levels of estradiol present in the somatic gonad
(Villalpando and Merchant-Larios 1990 ). Frogs reach sexual maturity some 6 months
later. With the fusion of the gametes at fertilization and subsequent embryogenesis, the
process of segregating the germ line begins again. The “life-cycle” of the germ line
from gametes to PGCs and back again highlights the continuity of the germ cell lineage
through generations while somatic cell lineages are terminated with each individual’s
death (Fig. 8.6). The functional significance of these morphological changes must await
a better understanding of translational control in the germ line.
T. Aguero et al.