Investigators led by a team at Massachusetts General Hospital (MGH) that previously discovered tiny channels in the skull have now found that cerebrospinal fluid (also known as “brain water”) can exit the brain into the skull’s bone marrow through these channels. The discovery, which is published in Nature Neuroscience, is important because immune cells produced in the spongy tissue of the skull’s bone marrow can screen the cerebrospinal fluid for signs of infection and other threats to the brain.