Incomplete coverage of mammalian bone matrix by lining cells

A functional “membrane” is thought to exist that separates the general extracellular fluid from the bone extracellular fluid. Many investigators have concluded that bone lining cells form this barrier. It is posited that alterations in this barrier may lead to the recruitment of osteoclasts and contribute to the control of extracellular calcium homeostasis. The present ultrastructural studies, however, demonstrated that significant portions of the resting bone are in contact with the general extracellular fluid. In certain regions of the cochlea, up to 88% of the bone surface is in contact with extracellular fluid. These anatomic observations require a critical reevaluation of current theories of the physiologic roles of bone lining cells in osteoclast recruitment and calcium homeostasis.