Figure 3. TEM of P₁₁-4 gels and fibrils. TEM micrographs show: (a) section through an unstained, undiluted P₁₁-4 gel following 7 days’ incubation at 35°C in "mineralizing" solution (22 mM NaHCO₃, 3.75 mM CaCl₂, 1.67 mM Na₂HPO₄, pH 7.4). Electron-dense deposits, some apparently aligned, may be seen (arrows). Scale bar corresponds to 250 nm. (b) P₁₁-4 fibrils from gels that were not incubated in "mineralizing solution", stained with uranyl acetate and prepared by dilution of the gels immediately before application to the TEM grid. Individual fibrils with clear helical twist can be seen. Scale bar corresponds to 100 nm. (c) Section through an undiluted P₁₁-4 gel stained with uranyl acetate after 7 days’ incubation at 35°C in "mineralizing" solution (22 mM NaHCO₃, 3.75 mM CaCl₂, 1.67 mM Na₂HPO₄, pH 7.4, supersaturated with respect to hydroxyapatite). Fibrils appear to be arranged in twisted bundles with a width of hundreds of nm to µm and lengths of several µm, reminiscent of collagen fibers. Electron-dense deposits associated with the bundles following in vitro mineralization may reflect sites where hydroxyapatite nucleation has taken place. Arrows show bundles of fibrils in cross-section (white arrow) and running longitudinally (black arrow). Scale bar corresponds to 1 µm.