Anatomy of surprise: Scientists discover hidden blood networks that cross through bone

For years, physiologists looking closely at bones noticed something puzzling. It was a microscopic prison break, blood cells slipping unseen from the enclosed depths of the bone marrow into the general circulation.

“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, 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 shed light on how certain drugs work.