Bone Lining Cells and the Bone Fluid Compartment, an Ultrastructural Study
Critical data has been developed in recent years that supports the hypothesis that ion concentrations in bone fluid and general extracellular fluid are modulated by a functional membrane that separates the fluid of bone from the general ECF. The evidence for the postulated functional membrane in bone is based on the experiments of Geisler and Neuman1, and Ramp and Neuman2, Scarpace and Neuman3,4. The prime points of the data are that potassium is concentrated in the bone fluid and that mineralization of the matrix is increased when bone cells are disrupted or poisoned inferring that the cells serve to partition these fluids, excluding excess calcium from the matrix surfaces. The actual “membrane” and mechanisms of its action are not clearly established. Talmage has suggested that the cells lining the bone surface (osteoblasts) serve this partitioning function5. The Talmage model proposes that calcium enters the bone fluid compartment by passive diffusion from the general ECF to the bone fluid compartment by passing between the lining cells. In the Talmage model, calcium eflux from the bone occurs by entering the osteocytes and osteoblasts whereupon it is actively transported through the cells and through the plasma membrane to reach the general ECF. Entry into the cell would not be difficult as a downhill gradient would exist, i.e., intracellular calcium levels are approximately 10–5.
KeywordsBone Cell Endosteal Bone Functional Membrane Bone Membrane Bone Lining Cell
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