HHematopoietic stem cells (HSCs) perform the key task of producing all the body’s blood cells. They occupy tiny niches in the bone marrow, along with stromal (connective tissue) cells, fat cells, endothelial cells and other cell types. The laboratory of Paul Frenette, M.D., has found that some of these niche-cell neighbors are vital for keeping HSCs alive and for helping them regenerate and move. (HSCs travel back and forth between the bone marrow and the bloodstream.) This low-power immunofluorescence image shows bone marrow taken from the sternum (breastbone) of a mouse. While HSCs themselves are not visible in this image, we can see stromal cells (green) and the two main types of blood vessels found in bone marrow: arterioles (red) and sinusoidal cells (blue). Noboru Asada, M.D., Ph.D., formerly a postdoctoral fellow in Dr. Frenette’s lab, conducted the research. Dr. Frenette is a professor of medicine and of cell biology and director of the Ruth L. and David S. Gottesman Institute for Stem Cell and Regenerative Medicine Research.