March 9, 2010 vol. 107 no. 10 4487–4790

Proceedings of the National Academy of Sciences of the United States of America www.pnas.org PNASIn This Issue

PNAS March 9, 2010 vol. 107 no. 10 4487–4488www.pnas.org/cgi/doi/10.1073/iti1010107

Cryopreservation of biological materials often depends on the rapid removal of water from the material to be preserved. Re-searchers have long preserved cells and tissues in a vitrifi ed state by suspending them in cryoprotectant liquids before im-mersing them in liquid nitrogen. Vitrifi cation keeps cells meta-bolically dormant and helps avoid the formation of ice crystals, which could damage cells. To determine the factors that drive vitrifi cation, Young Song et al. (pp. 4596–4600) studied the physical phenomena that occur when a droplet of the cryopro-tectant 1,2-propanediol plummets into liquid nitrogen. When ejected into liquid nitrogen, the droplet rises to the surface, buoyed by a vapor cloud generated by the evaporation of the surrounding liquid nitrogen. After skittering around on the surface, the droplet sinks when its temperature reaches that of liquid nitrogen, in a similar fashion to water sprinkled on a hot skillet. Using mathematical modeling, the authors found that the size, temperature, and composition of the droplet determine whether the droplet gets vitrifi ed or crystallized. The fi ndings could enable the development of high-throughput, automated methods of cryopreservation of biologi-cal samples such as blood and oocytes, according to the authors. — P.N.

Atrazine can induce chemical castration in male frogs

Modeling phase transition in levitating droplets

In 2004, the European Union banned atrazine, the most com-monly used pesticide in the world, because of research linking it to endocrine disruptions in amphibians and other animals. However, the US Environmental Protection Agency (EPA) concluded in 2006 that there was insuffi cient data to determine whether atrazine affects amphib-ian development. Tyrone Hayes et al. (pp. 4612–4617) examined the

long-term effects of atrazine exposure on reproductive function and development in an all-male population of African clawed frogs and report that atrazine induced feminization and chemical castration in some of the animals. The authors com-pared 40 control frogs with 40 frogs reared from hatching until sexual maturity, both living in an environment with a 2.5 ppb atrazine concentration, which is in the range that animals experience in areas where the pesticide is found and is below EPA drinking water standards. All the adult control frogs showed male-typical external features, whereas four of the other group of frogs had some female-typical features. Two of these frogs had ovaries, and the other two successfully mated with male frogs and produced viable eggs. The atrazine-exposed frogs suffered from depressed testosterone levels, decreased breeding gland size, demasculinized la-ryngeal development, suppressed mating behavior, reduced spermatogenesis, and decreased fertility. These data provide evidence that atrazine may play a role in declining amphibian populations, according to the authors. — B.A.

Converting cellulose into ethanol

Researchers have developed a method of processing cel-lulose from plants and other non-edible natural products into individual sugar molecules that then can be fermented by bacteria to produce ethanol. Plant cellulose contains long chains of useful sugars that store plenty of energy, but it is largely inaccessible due to the diffi culty of breaking down cellulose. Joseph Binder and Ronald Raines (pp. 4516–4521) converted cellulose to simple sugars with a combination of an ionic liquid, acid, and chroma-tography and suggest that their technique could be developed into a large-scale industrial pro-cess to produce biofuel or other products. After the completion of the short (approximately 2–4

A droplet levitating on top of liquid nitrogen.

Two genetic male African clawed frogs copulating.

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4488 www.pnas.org/cgi/doi/10.1073/iti1010107

In� ammation-mediated vascular remodeling transforms tissue grafts

Balancing diversity and accuracy in recommendation performance

hours) chemical reaction, Binder and Raines report that bacteria and yeast could easily ferment the extracted sugars into ethanol. Compared to other industrial processes, the technique does not require as much dangerous, concentrated acid. The authors say the reaction can be used on many sources of lignocellulose, including non-food feedstocks such as switchgrass, wood, crop residues, and paper wastes. The technique is currently constrained by the expense of the ionic liquid, and must remain effi cient if it is to be scaled up to a full-size biorefi nery, according to the authors. — P.D.

Synthetic vascular grafts are widely used to reconstruct congenital heart defects, but revisions to the grafts are frequently necessary because of their inability to grow with the patient. Ongoing clinical studies have found that tissue-engineered vascular grafts constructed from bone marrow cell (BMC)-seeded polymer tubes can develop into functional living blood ves-sels, enabling continued vessel growth and providing a more effective strat-egy than non-living grafts. To test the theory that stem cells in the seeded BMCs differentiate into the engineered vessel’s endothelial and smooth muscle cells, Jason Roh et al. (pp. 4669–4674) implanted human BMC-seeded grafts into immunodefi cient mice and tracked the grafts’ evolution into functional tissue. The authors found that the seeded BMCs did not incorporate into the maturing neovessels; instead the scaffolds appeared to transform via infl ammation-mediated vascular remodeling. The research-ers identifi ed the remodeling mechanism as a parallel to natural vascular formation, and suggest that their fi ndings challenge core tissue engineer-ing concepts such as the role of seeded BMCs as progenitor cells and the detrimental nature of the infl ammatory response to biomaterials. The study may help tissue engineers develop vascular structures in vivo by harnessing the body’s natural repair and regeneration mechanisms, according to the authors. — J.M.

Tissue-engineered vascular grafts transform into living blood

vessels in mice.

Recommender systems use data on past user preferences to predict possible future likes and interests. Whereas some recommender systems extrapolate a user’s choices to suggest similar items, other systems examine the diversity among the consumer’s choices to identify items that users would be unlikely to fi nd themselves. Tao Zhou et al. (pp. 4511–4515) combined these two prediction methods by blending the output from a pair of algorithms—one for accuracy- and one for diversity-based recommendations—with a param-eter that adjusts the relative contribution of each. The algorithms employ processes analogous to random diffusion and heat diffusion, and expand a user’s history of selections across a broad range of items. According to the authors, based on four different metrics, the hybrid system performed better than popularity-based ranking or collaborative fi ltering via user similarity, two well-known recommender systems. A system that couples the various known strategies for accurately predicting a consumer’s interests and opinions could allow the recommendation process to become more personalized in the future, the authors say. — T.J.

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