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Nuclear activity in oocytes
Differentiated cells can be experimentally reprogrammed back to pluripotency by
nuclear transfer, cell fusion or induced pluripotent stem cell technology. Nuclear
transfer and cell fusion can lead to efficient reprogramming of gene expression.
The egg and oocyte reprogramming process includes the exchange of somatic
proteins for oocyte proteins, the post-translational modification of histones and
the demethylation of DNA. These events occur in an ordered manner and on a
defined timescale, indicating that reprogramming by nuclear transfer and by cell
fusion rely on deterministic processes.
The remarkable stability of cell differentiation under normal conditions can be
reversed experimentally by nuclear transfer, cell fusion and induced pluripotent
stem (iPS) cell technology1-5. This provides an opportunity to generate
pluripotent embryonic cells from adult cells of the same individual and hence
opens the possibilit y of cell replacement without the need for
iPS cell technology makes use of the overexpression of transcription factors (FIG.
1a) and has been extensively reviewed, so it is not discussed in detail6-9. To
generate entirely unrelated cell types by iPS cell technology, treated cells must be
grown for multiple cell divisions over a long period of time (up to 3 weeks). By
contrast, nuclear transfer and cell fusion do not involve the overexpression of
new genes, and instead make use of natura components present in eggs and
some early embryos to initiate new transcription.
There are two kinds of nuclear transfer experiments: egg-NT involves the transfer
of a single somatic nucleus to an unfertilize d enucleated egg (in both mammals
and amphibians)1 (FIG. 1b); and ooc-NT involves the transplantation of multiple
somatic cell nuclei into the germinal vesicle (the nucleus) of a growing meiotic
prophase amphibian oocyte (an immature egg)10 (FIG. 1c). Note that the terms
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egg and oocyte refer to different developmental stages in amphibians and
mammals: amphibian eggs are in metaphase II of meiosis, which is equivalent to
mouse metaphase II stage oocytes, whereas their immediate precursors, oocytes,
are blocked in meiotic prophase I, which is equivalent to mouse germinal vesicle
stage oocytes.
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