Joining the circulatory systems of an old and a young animal is called heterochronic parabiosis. Here we see a cross-section of the small intestine of a young mouse that was parabiosed to an old mouse for two months. Postdoctoral research fellow Tahmineh Tabrizian, M.D., Ph.D., has found that factors in old-mouse blood “age” intestinal stem cells in young mice by impairing their ability to proliferate. (The stem-cell damage can’t be seen in this low-magnification image.) The next step in Dr. Tabrizian’s research: Identify the stem-cell aging factors present in old blood and see whether targeting those factors (using neutralizing antibodies, for example) helps rejuvenate stem cells in old mice and, ultimately, in old people. Dr. Tabrizian’s mentor is Derek M. Huffman, Ph.D., an assistant professor of molecular pharmacology and of medicine (endocrinology).