In vitro calcified tendon collagen: an atomic force and scanning electron microscopy investigation

Atomic force microscopy (AFM), scanning electron microscopy and X-ray energy dispersive spectroscopy have been performed on decalcified turkey tendons submitted to in vitro calcification in order to investigate the morphology and the surface relationships between the inorganic phase and the collagen fibres during deposition and compare with those found for physiologically calcified samples. 'Tapping mode' AFM was used to reduce the vertical force applied to the samples, which were examined without any preparation. A further characterization has been carried out by means of X-ray diffraction, infrared absorption and chemical analyses. The observations indicate that the inorganic phase deposited on collagen fibres during in vitro calcification is poorly crystalline B carbonated apatite. The composition, structure and dimensions of apatitic crystallites, as well as their orientation with respect to collagen fibrils, are very similar to those characteristic of physiologically calcified tissues. However, the crystallites seem to be nucleated on the fibril surface, without appreciably affecting the molecular packing of collagen.