Fig. 4.

pH-dependent mineral deposition on type I collagen. A: transmission electron micrograph of a partially mineralized bone in vitro showing collagen and periodic hydroxyapatite deposition (arrows). B: surface plasmon resonance showing phosphate and pH dependency of hydroxyapatite deposition in an artificial type I collagen layer. A more detailed description of the chip used is included in the section “a model for osteoblast epithelial-like structure and bone transport.” At time zero, responses are overlain for identical injection protocols. The baseline curves are identical for buffer (140 mM NaCl and 10 mM HEPES, pH = 7.4 or 6.81), 1 mM CaCl₂, or 5 mM phosphate (alone with buffer). At ~700 s the injection ended and the surface was washed with buffer. Later, the surface was washed with 10 mM EDTA to remove any stabilized calcium salts and return the response to baseline. At 1 mM CaCl₂ a dramatic phosphate dependence of large hydroxyapatite aggregate deposition occurs. Mineral deposition is reduced dramatically by dropping pH to 6.8. At the permissive pH 7.4, 5 and 2.5 mM phosphate injections generate stable hydroxyapatite deposition. These results indicate a strong pH dependence and multistep model for collagen mineralization. [From Blair et al. (6).]