INCOMPLETE COVERAGE OF MAMMALIAN BONE-MATRIX BY LINING CELLS

A functional ''membrane'' is thought to exist that separates the gener al extracellular fluid from the bone extracellular fluid. Many investi gators have concluded that bone lining cells form this barrier. It is posited that alterations in this barrier may lead to the recruitment o f osteoclasts and contribute to the control of extracellular calcium h omeostasis. 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 theo ries of the physiologic roles of bone lining cells in osteoclast recru itment and calcium homeostasis.