Collagen structural organization in uncalcified and calcified human anterior longitudinal ligament

Collagen structure and collagen-apatite structural relationship has been investigated in human anterior ligament, where the mineral deposition occurs on collagen fibrils morphologically different from those of bone and tendons. Ultrastructural observations made on replicas of cryoprotected and freeze fractured uncalcified samples display a "helicoidal" morphology of the collagen fibrils. X-ray diffraction analysis carried out using conventional and synchrotron radiation sources revealed that the D-axial spacing is 65.0 nm and the electron density distribution inside the repeating period is very similar to those of tendon collagen in the same conditions of hydration. The short D-period can be interpreted as due to a greater angle of molecular crimping and/or molecular tilt compared to that of tendon. Air drying does not cause any appreciable variation in the D-axial period and induces an increase of the gap/overlap ratio that can be ascribed to telopeptide disorder. In spite of the different morphology of the collagen fibrils, the structural relationship between collagen and the mineral phase in calcified ligament is very close to that observed in bone and tendons. The apatitic phase is laid down in blocks along the collagen fibrils with the same axial periodicity, D = 65.0 nm, as that of uncalcified collagen fibrils. The mean height of the mineral blocks, which are 0.45D long, is even higher than in bone and masks any further fluctuation of the electron density due to the organic matrix.