27 MAY 2011
formed; excess RA can reprogram a “distal”
amputation blastema (the site of progenitors
for regeneration) to produce more proximal
in addition to distal structures (
ing by WNT and FGF plays an essential role
in appendage regeneration in several differ-
ent systems (
), and WNTs may pro-
long regeneration in species that normally
lose this capacity at metamorphosis (
Whether this occurs through the maintenance
of progenitors by WNT and FGF, as seen in
the chick limb bud, remains to be seen. The
nding that embryonic limb progenitor cells
can be manipulated to maintain or extend
their plasticity opens a new door to analyzing
how these progenitors become committed to
particular fates, whether and to what extent
this process can be reversed, and the degree to
which such programming is shared between
limb development and regeneration.
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, 1433 (2007).
2. J. L. Galloway
, 400 (2009).
3. K. L. Cooper
, 1083 (2011).
4. A. Roselló-Díez
, 1086 (2011).
5. D. ten Berge
, 3247 (2008).
6. X. Zhao
, 1050 (2009).
7. J. Schlueter, T. Brand,
Cytogenet. Genome Res.
8. F. V. Mariani
, 401 (2008).
9. M. Maden,
, 672 (1982).
10. Y. Kawakami
, 3232 (2006).
11. G. Lin, J. M. Slack,
, 323 (2008).
A Logical Use for Atoms
Andreas Heinrich and Sebastian Loth
The coupled spins of individual atoms can
be used to demonstrate basic logic function.
t is increasingly more difﬁ
cult and costly
to continue to shrink silicon chip com-
ponents. Moore’s law, which describes
such progress, is running into serious trouble.
Technologists have been seeking out alterna-
tive architectures for computation. Some have
even pondered the possibility of shrinking the
components to the ultimate end of any road-
map—to the scale of atoms. On page 1062 of
this issue, Khajetoorians
. pre sent such
a radical leap forward (
) by demonstrating
spin-based logic operation in chains of mag-
netic atoms on a surface.
Spintronics promises to add functionality
to semiconductor devices by making use of
the spin—the magnetic degree of freedom—
of electrons that are ﬂ
owing through devices.
But can logic operations be performed with-
out pushing electrons (spin-polarized or not)
through wires? Indeed, several schemes have
been proposed to do just that. One example
is magnetic domain wall logic (
relies on the dipolar coupling between mag-
netic nanodots. Cascades of such dots can
meet at intersections where it was shown that
logic operations could be performed (
About 10 years ago (