WATCHING PACEMAKERS FORM IN A PETRI DISH. The bandleaders of biological systems, pacemakers are cells or groups of cells capable of generating regularly repeating activity on their own or in response to outside signals. In humans, roughly 5000 cells in the sinoatrial node, located in the right roof of the atrium, generate the signals that regulate the rhythmic contractions of the heart. In efforts that may improve understanding of how natural pacemakers form and provide steady signals, researchers at Technion University in Israel (Yoav Soen, yoav@technion.ac.il) excised muscle cells and connective tissue (fibroblasts) from the ventricles of rats. Spreading these cells on a petri dish under the proper conditions caused the cells to proliferate, move around and eventually form a hardy network of fibers after 1-3 weeks. Using a CCD camera and real-time computer processing , the researchers detected rhythmic contractions in the cells. They also noticed rhythm disorders, such as alternations between irregular and regular rates of contraction. This suggested to them that one or more pacemakers had formed within the network. While in-vitro pacemaker activity has been observed before, the Technion optical technique is the first that monitors the cells noninvasively and continuously long enough to watch a cell network evolve and form a pacemaker system. Although the cell network is very different from a biological heart, it can provide insights into how its structure and density affects the development of a pacemaker. (Soen et al., Physical Review Letters, 26 April; See figure at Physics News Graphics)