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gastrulation and tailbud stages, T-GP cells were selected and transplanted back into
the endoderm. While most of them failed to migrate, 46 % did reach the genital
ridges. These results were very encouraging and led to the next key experiment that
asked if functional gametes could be produced from animal pole cells receiving
EGFP labeled germ plasm. To distinguish these cells, EGFP-labeled germ plasm
was injected into recipient transgenic frogs in which every cell expressed DsRed2.
Embryos with DsRed2 labeled animal pole cells containing EGFP-labeled germ
plasm were allowed to reach gastrula stage. These labeled cells were then trans-
planted into the blastocoel of a wild-type host frog where they could enter the endo-
derm and eventually migrate into the gonads. Both genetically marked DsRed2
sperm and oocytes were produced in gonads of wild-type hosts. These wild-type
host animals were then mated to generate embryos. A very small number of result-
ing embryos carried the DsRed2 genetic marker. Despite these small numbers, the
results do demonstrate that the presence of germ plasm alone was sufficient to redi-
rect cells fated to become somatic ectoderm or neural ectoderm, into the germ cell
lineage. The change in fate was accomplished despite what other cytoplasmic or
nuclear cues were present and despite cell signaling from other somatic cells. These
results also imply that germ plasm contains dominant repressive mechanisms to
shield pPGCs from somatic fates. Thus germ plasm has evolved in vertebrates as
well as in invertebrates and functions as autonomous determinants of the germ line.
Earlier approaches to settle the issue of germ plasm sufficiency had ectopically
placed either whole vegetal pole blastomeres, presumably containing germ plasm, or
migrating rhodamine (TRITC)-labeled PGCs within the blastocoel of unlabeled host
embryos. Using these methods, Wylie et al. ( 1985 ) found that TRITC labeled cells
were not detected within the host’s gonadal tissue, but that a very small number were
detected histologically in somatic tissues (Wylie et al. 1985 ). Furthermore, the
TRITC labeled cells appeared to have differentiated into somatic cells representative
of all three primary germ layers. There are several reasons why Wylie et al. ( 1985 )
may not have detected germ line TRITC labeled cells. These include the limited
persistence of TRITC, migration failure, loss of cell due to apoptosis, or insufficient
germ plasm. However, Tada and coworkers did not document the fate of the T-GP
cells that failed to enter the germ line, making it difficult to compare the results of the
two studies. Did any of the T-GP cells differentiate into somatic cells and survive?
These are important questions as they concern the ability of germ plasm to preserve
full potential within ectopic environments and outside any germ line niche.
It has been known for a long time that in order for Xenopus PGCs to migrate cor-
rectly into the genital ridges they must be correctly situated in posterior endoderm
(Fig. 8.7). If labeled pPGCs are implanted into anterior endoderm, the number of
PGCs reaching the genital ridge declines severely (Ikenishi and Tsuzaki 1988 ). The
Tada et al. ( 2012 ) study goes further and confirms that the endodermal environment
is only required for correct migration and not for survival or germ line maintenance.
In fact, if PGCs are isolated and cultured in a simple buffer on fibronectin, they
autonomously undergo the correct cellular movements required for directional
migration and at the normal developmental times (Morichika et al. 2010 ). What
initiates such a program and what types of cues are provided by the endoderm in
T. Aguero et al.