Skeleton keys

How the body puts flesh on its bones depends on the bones themselves

ANATOMY used to be a straightforward business. The body was divided up neatly into organs and systems that each had well-defined tasks. Indeed, for the past 150 years “Gray's Anatomy”—the 1858 textbook that defines the genre—has dissected the body along these thematic lines: nervous, circulatory, digestive and so on.

But the lines are becoming increasingly blurred. Gerard Karsenty of Columbia University and his colleagues report another such smudging in Cell this week. They have found that people's bones do much more than just provide scaffolding for their floppy innards. They have caught the skeleton behaving as part of the endocrine system, the scheme by which the body uses hormones to signal its needs.

It has long been known that the human skeleton constantly constructs and destroys cells, according to the stresses that its bones experience. Some cells produced in bone, called osteoblasts, build bone where it is needed. Another set of cells, osteoclasts, destroy it where it is deemed no longer necessary.

The researchers decided to examine the role of osteocalcin, a protein produced by osteoblasts. To do so, they used some mice that had been bred to lack the gene that instructs the body to make osteocalcin. The rodents were rather rotund, because osteocalcin helps regulate the cells that produce insulin in the pancreas and release it into the bloodstream. Insulin, in turn, controls the levels of sugar in the bloodstream by directing how much of it is taken up by the liver. Mice that produced no osteocalcin lacked this hormonal weight-control mechanism.