Human embryonic stem cells (HESCs) have long been recognized as having the potential to develop into any kind of tissue, thus offering the hope of producing artificial human organs needed to replace those damaged by injury or chronic disease. But now, Hebrew University researchers and colleagues abroad have discovered the exact mechanism by which this occurs and revealed this process, resolving a long-standing question as to whether stem cells develop through selective activation or selective repression of genes. The collaborative research group - which included Dr. Eran Meshorer of the genetics department at HU's Silberman Institute of Life Sciences and scientists from US National Institutes of Health, the Affymetrix company in California and the Hospital for Sick Children in Toronto - has revealed that HESCs express large proportions of their genome "promiscuously." This permissive expression includes lineage-specific and tissue-specific genes, non-coding regions of the genome that are normally "silent" and repetitive sequences in the genome which comprise the majority of the mammalian genome but are also normally not expressed. When HESCs differentiate into specific tissue types, they undergo global genetic silencing. But until this occurs, the cells maintain an open and active genome. This might be the secret of their success, since by maintaining this flexibility they maintain their capacity to become any cell type. Once silencing, or genetic repression, occurs, this ability is gone. Thus, one can say that stem cells stand at the ready until 'the last minute' - prepared to engage in selective activation into specific cells. To reveal how this occurs, the researchers created the first full-mouse genomic platform of DNA microarrays. Microarrays are glass-based chips that allow simultaneous detection of thousands of genes. The microarrays used in the study were not confined to specific genes but spanned the entire genome. Hundreds of such microarrays were required to cover the entire genome at different points during stem-cell differentiation. It was by observation of these sequences that the researchers were able to establish exactly how and at what point stem cells develop into specific tissue cells, and when the silencing occurs. WHEN THE DOG BITES, WHEN THE SCORPION STINGS Two people in Haifa were bitten by snakes recently, joining the 208 people who have been hurt by animals since the beginning of the year, Magen David Adom reports. The other animals included dogs, scorpions, a spider and snakes. The victims this time are 61-year-old resident of the village Ibellin, who encountered a snake while working in the fields of Kfar Masaryk. He lost consciousness while MDA medics were treating him and was evacuated in moderate condition to Rambam Medical Center. A 49-year-old resident of Kiryat Bialik was bitten by a snake while walking in his town. Onlookers called MDA, who found him in serious condition and rushed him to Rambam. MDA warns that the current heat wakes up snakes, who have accumulated much toxin over the winter and are looking for rodents. Regard every snake as a poisonous one and call 101. Do not allow the part of the body bitten to move, and never cut or suck the skin around the bite, MDA says. Don't run after the snake or give the victim food or water until he is treated. BREATHING FREE Children who live on tree-lined streets have lower rates of asthma, according to research published in the Journal of Epidemiology and Community Health.The researchers base their findings on asthma rates among four to five year olds and hospital admissions for the disease in children up to 15 living in 42 health service districts of New York City. US rates of childhood asthma soared 50% between 1980 and 2000, with particularly high rates in poor urban communities. In the city, asthma is the leading cause of admission to hospital among children. The medical data were then plotted against city data on the number of trees in each area, sources of pollution, racial and ethnic make-up, and population density. The city had an average of 613 street trees per square kilometer, and 9% of young children had asthma. Asthma rates in this age group fell by almost a quarter for every standard deviation increase in tree density, equivalent to 343 trees per square kilometer. This pattern held true even after taking account of sources of pollution, levels of affluence and population density - all factors likely to influence the results. But tree density had no impact on admissions to hospital for asthma among older children. The findings do not mean that the number of trees in any city is directly related to asthma rates, caution the authors, but trees may help curb asthma rates by encouraging children to play outdoors more or by improving air quality, they say. New York City is also planning to plant one million extra trees by 2017, which could provide the perfect opportunity to discover exactly what impact tree density has on asthma, they add. SMILE AFTER THE DENTIST Many people who received injected anesthesia in the dental chair are embarrassed when they speak strangely, bite their lip and drool until the shots wear off. Now the US Food and Drug Administration has approved a drug called OraVerse that quickly reverses the numbing effects. Developed by Novalar, a California company, the unique drug - which will go on sale in the US in October - can even be given to children from age 6. Children, the mentally and physically disabled and the elderly tend to bite their lip or tongue when they are numb from dental anesthesia, and sometimes this causes them to bleed. Another benefit of OraVerse, which went through years of clinical trials, is that dentists will be able to work on both sides of their patients' mouths during a single sitting. A survey of dentists and patients said the vast majority would be interested in such a product, whose cost is expected to run between $5 and $25.