Colloid science: Double bubbles are no trouble

Science 308, 537–541 (2005)

Emulsions — droplets of one liquid suspended in another — are widely used in technology, food processing and cosmetics. Double emulsions, in which each droplet encapsulates a smaller droplet of another liquid, provide further possibilities. For example, they allow the release of drugs or flavourings from the inner droplet to be governed by the size and composition of the outer droplet. Gelling or polymerizing the outer droplet results in the formation of a robust capsule for controlled release.

It has been hard to control the absolute and relative sizes of both droplets in such double emulsions. But A. S. Utada et al. have now come up with an answer: a versatile microfluidic system that can deliver not only precisely determined droplet sizes, but also specified numbers of inner droplets in each outer one.

The inner, outer and surrounding fluids are all squirted simultaneously and coaxially into the neck of a tapered capillary, with the innermost fluid being delivered into the mix through a separate micropipette. A theoretical model allows the sizes of the droplets to be predicted from the instrumental geometry and liquid flow rates, providing anything from thin-walled fluid capsules to multi-compartment polymer vesicles.

Philip Ball

Neurobiology: Astrocytes star in inhibition

J. Neurosci. 25, 3638–3650 (2005)

Astrocytes are well known for their structural role in the nervous system: these star-shaped cells hold neurons in place. They also contribute to neuronal function, including the activity of synapses — junctions between neurons that allow them to communicate with each other. Over the past decade, the role of astrocytes in the development of excitatory synapses has been revealed. It now seems that they modulate inhibitory synapses as well.

Sarina B. Elmariah et al. investigated the influence of astrocytes on synapses that use γ-aminobutyric acid (GABA) — the principal inhibitory neurotransmitter in the human brain. Drugs that target GABA receptors include anaesthetics and anti-epileptics, hence the additional significance in understanding how these synapses develop.

The authors cultured astrocytes with neurons from the rat hippocampus. They found that, in the absence of astrocytes, inhibitory synaptic terminals were rare by the fourth day in vitro. Conversely, their presence led to a sevenfold increase in the number of these terminals at this same stage. Astrocytes also produced an increase in the number and synaptic localization of GABA-receptor clusters. Further experiments led the authors to suggest that astrocytes may achieve these effects in part by indirectly stimulating signalling between neurons that is mediated by neurotrophin proteins.

Roxanne Khamsi

Materials science: Carbon through the phases

Phys. Rev. Lett. 94, 145701 (2005)

What happens to carbon when the heat is turned up and the pressure on? Carbon is subjected to extreme conditions in many geophysical and industrial processes: using a semi-empirical model known as the ‘long-range carbon bond-order potential’, Luca M. Ghiringhelli et al. have looked into carbon's behaviour in such situations.

Two very different ‘phases’ of pure carbon are familiar: graphite (the ‘lead’ in pencils) is usefully soft and opaque, whereas the less-everyday diamond is famously hard and clear. Squeezing graphite under high pressure eventually turns it into diamond, and diamond decays to graphite at room temperature and pressure — although extremely slowly.

Ghiringhelli and colleagues use their model to compute the temperatures and pressures at which transitions between graphite, diamond and a third — liquid — phase of carbon occur in conditions up to 12,000 kelvin and 400 gigapascals (4 million times Earth's atmospheric pressure). Experimental data under extreme conditions are scarce, but where they do exist their agreement with the model is good. The results of the simulation also allow the hotly disputed existence of two distinct liquid phases of carbon to be discounted, the authors claim.

Andreas Trabesinger

Molecular biology: Quick start

Mol. Cell 18, 171–183 (2005)

At least 2,500 genes are switched on in just six minutes when cells of budding yeast leave the resting state to begin the cell division cycle, according to Marijana Radonjic, Jean-Christophe Andrau and colleagues.

Eukaryotic cells, which include yeast and mammalian cells, spend most of their lives in the resting state, with their ability to proliferate maintained but inhibited. How this quiescent condition is regulated has implications for understanding cancer, development and ageing.

From Radonjic and colleagues' survey of yeast gene activity, it seems that RNA polymerase II, the enzyme responsible for much gene transcription, is poised and raring to go while the cells are resting. This is contrary to models of gene activation in which transcription of most genes is limited by the rate at which RNA polymerase II is recruited to the start site of the gene.

Radonjic et al. found that, in quiescent yeast cells, the enzyme was already predominantly bound next to the start sites of hundreds of genes that were immediately switched on as the cells began division. The authors suggest that such positioning allows a cell to respond rapidly, and begin growing, when it encounters nutrients in an environment where organisms compete strongly for resources.

Helen Dell

Virology: Polar flu

Biol. Lett. doi:10.1098/rsbl.2004.0253 (2005)

Newly spawned particles of influenza virus are released from only one face of virus-infected cells. Debra Elton et al. propose that this polar release has its roots in the asymmetric distribution of a viral protein in the cell nucleus.


The flu virus infects the epithelial cells that line the respiratory tract. It then coerces the cells into making new viral particles, which are released only from the exposed cell surface (the apical surface). Virus production begins with the replication of the viral genome in the nucleus, giving numerous RNA molecules. These are then packaged with the viral nucleoprotein (NP) and exported from the nucleus.

Using infected cells in culture, together with staining techniques, Elton et al. found that when export was inhibited, NP accumulated at the nuclear periphery in an apical distribution. The same occurred temporarily when export was not inhibited, or when NP alone was transfected into uninfected cells. The latter finding suggests that NP must be interacting with something in the cell that is similarly distributed. One potential candidate, the authors speculate, is chromatin — the protein-bound complex into which nuclear DNA is packaged.

Amanda Tromans