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8.4.3 De Novo Formation of Germ Cells
A handful of studies suggest that in some ascidians, germ cells can be regenerated
at postembryonic stages of development. One example is Ciona intestinalis, where
all cells expressing Vasa-protein were removed during embryogenesis. These cells
are then formed de novo in juveniles post-metamorphosis (Takamura et al. 2002a).
However, vasa-mRNA was detected in cells within a more anterior region of the tail
that was not removed in the experiment, and it is possible that those cells might be
the source of the germ cells at later stages.
In the colonial ascidian Polyandrocarpa, buds only develop organs after being
separated from the parent. No vasa-mRNA was detected in buds that have separated
from the parent, and vasa-positive cells appear only later in developing animals.
These data suggest that in this particular species, germ cells might be formed de
novo in each asexual cycle of reproduction (Sunanaga et al. 2007 ). More detailed
studies are needed to test whether small, vasa-positive circulatory germ line stem
cells are entering the bud before separation, or whether germ cells are indeed formed
by epigenesis, and if so, by what mechanism.
When adult bodies and buds are removed from colonies of Botryllus primigenus,
Botrylloides violaceus or B. schlosseri, new bodies can regenerate from the remaining
vasculature in a process termed “vascular budding.” One study in Botryllus primigenus
suggested that vasa-positive cells are generated de novo during vascular budding, as
vasa-mRNA expression was undetectable immediately after ablation of bodies and
buds, but became detectable in 30 day specimens (Sunanaga et al. 2006 ). However, a
later study in the same species showed that circulating vasa-positive cells are in fact
entering the developing vascular buds (Kawamura and Sunanaga 2011 ). As described
above, circulating vasa-positive cells are present in all botryllid species, and therefore,
these cells are likely the source of the germ line in vascular buds.
Taken together, these studies suggest that de novo formation of germ cells may
occur in some but not all ascidian species, and more detailed studies are needed to
investigate this phenomenon.
8.5 The Enigma of PGC Specification in Urodeles
8.5.1 The Origin of PGCs in Urodele Amphibians
The origin of the germ line in urodele amphibians (salamanders) was a source of
considerable controversy during the early twentieth century. By that time the
endodermal origins of anuran (frog) primordial germ cells (PGCs) had been well
established; however, conflicting reports placed the origins of urodele PGCs in either
endoderm or mesoderm. To address this, Humphrey ( 1925 ) analyzed early development
in a number of urodele species, and he identified PGCs in the dorsal-most aspect of the
posterior lateral mesoderm adjacent to the mesonephric ducts (Humphrey 1925 ). He
later showed that the PGCs and mesonephric ducts were eliminated following unilateral
T. Aguero et al.