“We have the bone marrow, which produces the blood cells, and when you need them, you need them urgently. But how do they get out?” said Svetlana Komarova, who studies bone biology at McGill University in Montreal.

Sure, textbooks showed entrances and exits, but they were enormous arteries and veins at the ends of bones — an impractical route if the body needs a quick burst of the cells forged within our marrow. But in a paper published Monday in Nature Metabolism, a team of German immunologists announced they’d found a whole network of tunnels that explain the escape. While their first observations were in mice, they found a similar map of secret capillaries in humans, too, which may ...