4118.3.4 Summary
In summary, three critical activities appear to be required in PGCs to protect them
from somatic differentiation: (1) translation of sequestered maternal germ line
mRNAs, (2) translational repression of the endoderm determinant VegT, and perhaps
other RNAs, and (3) transient genome-wide suppression of mRNA transcription at
and before the MBT to ensure that somatic differentiation programs remain inactive
when zygotic transcription is initiated in the rest of the embryo. Major outstanding
questions remain: How are germ plasm maternal RNAs translationally activated and
in what temporal order? What maternal transcription factors initiate the germ line
gene program? What initiates different programs of migration in PGCs? What is the
relationship between PGCs and adult ovarian germ stem cells? In this new age of
deep-sequencing and proteomics, many RNAs and proteins will be discovered as
germ plasm components. Putting these components into a viable gene network
essential to specify the germ line remains the challenge. The hope is that understand-
ing the mechanisms that suppress and reactivate the totipotent potential of germ cells
will allow us to reprogram and program other stem cells for cell-based therapies.
8.4 Germ Cell Specification in Ascidians
Ascidians are marine invertebrate organisms that belong to phylum Chordata
together with cephalochordates and vertebrates. They share many features of the
ancestral Chordata, such as a notochord, dorsal hollow nerve tube and a post-anal
tail. After fertilization, ascidian embryos undergo a typical form of mosaic develop-
ment. Cell fate determination is regulated by maternal RNAs and proteins, which
are deposited into specific regions of eggs and early embryos. Embryonic develop-
ment gives rise to a tadpole larva with simple tissue organization. After settlement,
the larva undergoes metamorphosis, the tail is absorbed, and after several days, the
juvenile starts feeding (Fig. 8.8).
In both solitary and colonial ascidians, the germ line is segregated early in devel-
opment, and the specification of primordial germ cells is dependent on maternally
localized determinants. This process has been studied in greater detail in solitary
ascidians, such as Ciona intestinalis, as their embryonic development is more suit-
able to observation and experimentation, but data from colonial species suggest that
the mechanisms of germ cell specification are highly similar between both groups.
However, in some ascidian species, the segregation of germ line and soma may not
be absolute, as some observations suggest that germ cells can be regenerated later in
development and even in adult animals.
In contrast to solitary ascidians, sexual reproduction and asexual reproduction
occur simultaneously in colonial ascidians, a situation that requires regeneration of
gonads in each asexual generation. To accomplish this, colonial species
retain long- lived germ line progenitors that are specified during embryogenesis and
persist throughout the life of the animal.
8 Mechanisms of Vertebrate Germ Cell Determination