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to predetermine their fate as PGCs. Indeed, material with the properties of germinal
granules, a major component of germ plasm, was found in the equatorial region of
axolotl oocytes, suggesting such a possibility (Williams and Smith 1971 ).
However, an alternative hypothesis was that PGCs arise from common totipotent
cells with the capacity to form germ cells or somatic cells. This was first suggested
by transplantation experiments in the newt Triturus where the presumptive lateral
mesoderm of gastrulae was replaced by primitive ectoderm of donors, without a
noticeable reduction in the number of PGCs (Kotani 1957 ). Later, Kocher-Becker
and Tiedemann ( 1971 ) demonstrated the induction of PGCs, along with mesoder-
mal and endodermal structures, when explanted primitive ectoderm (animal cap) of
embryos from Triturus was exposed to a “vegetalizing factor” isolated from extracts
of chicken embryos. While the nature of this PGC inducing factor has never been
determined, these studies laid the groundwork for subsequent studies to determine
the mechanisms that underlie PGC induction.
Fig. 8.10 Development of the germ line in axolotl embryos. The precursors of axolotl primordial
germ cells develop from cells in the ventral lip of the blastopore (blue). They pass over the lateral
lips of the blastopore during the final stages of gastrulation and reside in the presumptive interme-
diate mesoderm (red). Commitment to the germ line occurs at about stage 25. PGCs are then dis-
placed medially by the expansion of the lateral plate. They develop in close association with the
mesonephric ducts (dark gray), which come to occupy a more lateral position. By stage 45, the
PGCs have reached the genital ridge, and they are positioned ventro-lateral to the dorsal aorta
(white), and medial to the mesonephric ducts in the aorta–gonad–mesonephros (AGM) region of
the embryo
T. Aguero et al.