For centuries, the skeleton has been recognized as a means of locomotion, protection of
vital organs, and provision of a readily available store of calcium and phosphorus. Even
more ancient is the knowledge that bone marrow is hidden deep within bone. Every day,
200 billion red blood cells are made in the marrow, delivered to the circulation to serve their
120-day-long duty, and then die (1). The precise equilibrium between destruction and
production of these blood cells has been appreciated for over 60 years but only recently has …