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Second, histologically visible granules and cytoplasmic inclusions (germ
plasm) were observed in the vegetal cortex of frog eggs and embryos, and these
segregated predictably with the germline in the embryo ( Blackler, 1958;
Bounoure, 1934 ). Third, explants derived from different regions of the egg
formed different tissues when cultured in isolation ( Holtfreter, 1938a,b ). In
particular, animal pole explants formed ciliated epidermis; vegetal cells formed
endoderm; and equatorial cells formed a mixture of mesodermal, neural, and
endodermal tissues. Importantly, Nieuwkoop and colleagues showed that
recombination of animal and vegetal blastula explants caused mesoderm fate
to be induced in the ectodermal moiety ( Nieuwkoop, 1969; Sudarwati and
Nieuwkoop, 1971 ). Taken together, these studies showed that axis formation,
germ cell specification, and germ layer patterning likely depended to some
extent on localization of putative determinants to the vegetal pole of the egg.
Localized messenger RNAs became theoretically attractive candidates
for cytoplasmic determinants in frog eggs, and this idea was supported by
initial observations. Notably, mitochondria and RNAs (pentose nucleic
acids) were detected in the germ plasm ( Blackler, 1958 ). Additional
molecular studies using analytical hybridization of mRNAs suggested that
poly(A)
mRNAs were differentially distributed along the animal-vegetal
axis ( Carpenter and Klein, 1982 ). Vegetally enriched poly(A)
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mRNAs
could be translated into a unique set of proteins, suggesting that functional
protein activities were indeed being partitioned in the egg ( King and Barklis,
1985 ). The invention of molecular cloning techniques suggested that it
would be possible to isolate these specific localized mRNAs. Melton and
colleagues accomplished this using dissected animal and vegetal poles of
oocytes, identifying specific transcripts enriched in each hemisphere
( Rebagliati et al., 1985 ). The first vegetal cDNA isolated, vg1 ( gdf1 ), showed
striking localization to the vegetal cortex of full-grown oocytes and appeared
to follow a specific localization pathway during oogenesis ( Melton, 1987;
Weeks and Melton, 1987 ). By contrast, the animally enriched genes did
not associate with the animal cortex and animal pole-specific localization
mechanisms have yet to be identified. Consequently, most efforts have since
focused on vegetally enriched transcripts.
Many of the first localized RNAs to be discovered were maternal tran-
scripts in eggs or RNAs transported to different sites in specialized somatic
cells, such as migrating fibroblasts, oligodendrocytes, and neurons ( Kloc
et al., 2002b ). These observations led to the assumption that RNA localiza-
tion was a phenomenon limited to a few remarkable cases. Advances in
genome-level analyses, high-throughput processing methodology, and
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