26 SEPTEMBER 2014 • VOL 345 ISSUE 6204 1565 SCIENCE
Neural tube development
fect when H2O on the catalyst surface was
replaced by D2O, suggesting that cleavage of
the O-H or O-D bond is a critical step in the
reaction mechanism. These observations indicate that water plays a direct role in the CO
oxidation reaction (see the figure).
Saavedra et al. supported their experimental results with a comprehensive density functional theory (DFT) investigation.
Based on the evidence from the IR spectra,
their DFT model incorporates adsorbed water and hydroxyl groups associated with the
support. The results show that adsorbed
water at the gold-titania interface helps activate O2 at a very low energetic cost—so low,
that the process occurs spontaneously at low
temperatures. This is a very surprising revelation given the high energy barriers that
gold surfaces display toward O2 activation.
In the authors’ model, the most kinetically
important step is instead associated with
decomposition of a reactive intermediate via
proton transfer to water, in agreement with
their experimental observations.
In Saavedra et al.’s mechanism, hydroper-oxyl and hydroxyl species are involved in the
water-enhanced CO oxidation reaction. Hydroxyl species associated with both the support and the gold surface play roles in the
process. Furthermore, water helps to both
activate O2 and decompose reactive intermediates associated with CO2 generation. This
mechanism not only ties together observations made in several previous studies of the
water-enhanced CO oxidation reaction but
also lends credence to many seemingly conflicting claims.
Water plays a key role in a number of
other reactions carried out on gold catalysts
(12). By elucidating the role of water in CO
oxidation, the study by Saavedra et al. may
prove key to solving many of the puzzles of
gold catalysis. ■
REFERENCES AND NOTES
1. M.Haruta et al., J. Catal. 115,301(1989).
2. M. Daté, M. Okumura, S. Tsubota, M. Haruta, Angew. Chem.
Int. Ed. 43, 2129 (2004).
3. J. Saavedra et al ., Science 345, 1599 (2014).
4. C. K. Costello et al ., Appl. Catal. A Gen. 243, 15 (2003).
5. M.M.Schubert, A.Venugopal,M.J.Kahlich, V.Plzak,R.J.
Behm, J. Catal. 222, 32 (2004).
6. G.C.Bond,D. T. Thompson,GoldBull. 33,41(2000).
7. H. H. Kungetal .,
J.Catal.216, 425 (2003).
8. W. C. Ketchie, Y. Fang, M. Wong, M. Murayama, R. Davis,J.
Catal. 250, 94 (2007).
9. R. A. Ojifinni etal.,
J.Am.Chem.Soc. 130, 6801 (2008).
10. M. Ojeda, B.-Z. Zhan, E. Iglesia, J. Catal. 285, 92 (2012).
11. A. Bongiorno, U. Landman, Phys. Rev. Lett. 95, 106102
12. G.M.Mullen,J.Gong,T.Yan, M.Pan, C.B.Mullins, Top.
Catal. 56, 1499 (2013).
We gratefully acknowledge the support of the U.S. Department
of Energy (DE-FG02-04ER15587) and the Welch Foundation
(F-1436). G.M.M. acknowledges the NSF for a Graduate Research
During vertebrate embryogenesis, new tissues are forged through the devel- opment of specific cell types in partic- ular patterns. This patterning happens in the context of tissue growth, but how are these two phenomena coordinated? How does a tissue that changes
size over time maintain the proportions
of its different domains? Does patterning scale with tissue size? On page 1577 of
this issue, Kicheva et al. (1) answer these
long-standing questions. Their analysis of
mouse and chick embryos of different sizes
reveals that patterning proportions of different progenitor domains in the vertebrate
neural tube—the rudiment of the spinal cord—do not scale with growing size.
Instead, the authors propose a two-phase
process for how tissue regions grow in
Managing patterns and
proportions over time
By Olivier Pourquie
Tissue patterning is specified relative to growth
through differences between cell proliferation and the
register to one another, despite different
The neural tube is initially composed of
a single layer of neural progenitor cells that
gives rise to all cell types of the spinal cord.
These progenitors produce descendants
that stop dividing and migrate to the periphery of the neural tube where they differentiate into the various types of neurons.
Neurites from these neurons grow to establish the future functional circuitry, and glial
cells develop and occupy specific areas of
the spinal cord such as the white matter.
In response to exogenous signals pro-
vided by diffusing morphogens such as
Department of Genetics, Harvard Medical School and
Department of Pathology, Brigham and Women’s Hospital,
77 Avenue Louis Pasteur, Boston, MA 02115, USA. Institut
de Génétique et de Biologie Moléculaire et Cellulaire, CNRS
(UMR 7104), Inserm U964, Université de Strasbourg, Illkirch,
F-64700, France. E-mail: email@example.com
Pattern and size over time. During vertebrate neural tube development, morphogens released dorsally and
ventrally specify progenitor cell territories. Growth of the progenitor pools then becomes independent of morphogen
concentration and follows a differentiation program defined by transcription factors characteristic of each pool.
Growth kinetics is different between each territory and is controlled by a balance between the proliferation and the