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Studies on stem cells have been attracting intense scientific and public
attention, not only because of controversies surrounding the use of
embryonic stem cells but also because of very provocative data that
have been emerging on adult stem cells. Much of the public attention
and debate has been focused on the possibility that adult stem cells
may be used as a substitute for human embryonic stem cells or as a
justification for stopping work on them. This has somewhat diminished
attention on very heated scientific debates that take us to the very
heart of how the concept of stem cells is perceived. To this author, the
latter debates have not been unlike certain philosophical debates of the
last century.
Since the seminal studies of Till and McCulloch in the 1960s, the
popular paradigm on adult stem cells has been that lineage-restricted
stem cells are derived from pluripotent stem cells very early during
development. To many, and consistent with much data, the restriction
to particular lineages was considered absolute. In other words, there
was a sense of determinism in the stem quality of particular stem cells:
once they were allocated, they were programmed to specific roles in a
given tissue. Furthermore, some adult tissues were considered devoid
of detectable stem cell presence or activity. During the last decade, new
challenges to our previous notions about stem cells have arisen, one
example being the demonstration of stem cells in adult neuronal tissue
where they had been said not to exist. Our certainty about stem cell
biology has been challenged even further by recent reports that previously
designated tissue-restricted adult stem cells might not only be
multipotent but also pluripotent. In essence, the debate has become
similar to the that between Cartesian and Existentialist philosophers
many decades ago. Are stem cells fated to be particular stem cells
determined to particular lineage(s) or do they have they the capacity
to actualize diverse potentials in diverse environments? In other words,
do stem cells exercise “free will”? In a sense, we are debating in a
cellular context whether “essence precedes existence” or “existence
precedes essence” of stem cells. |
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