A histological and electron-microscopic study of the architecture and ultrastructure of human periodontal tissues.

The structure of periodontal tissues is still far less understood than their clinical relevance would demand. Here the periodontal ligament and radicular cementum in healthy human teeth were studied by light microscopy, transmission and scanning electron microscopy. These observations showed that the extracellular matrix of periodontal ligament is composed of a loose plexus of wavy collagen fibrils immersed in a highly hydrated interfibrillar matrix. Only close to their cemental insertion do these fibrils gather in thick, parallel fascicles (Sharpey's fibres). As these cross the mineralization front, they become infiltrated by the mineral phase and continue directly with the cementum matrix. Sharpey's fibres, "extrinsic" and "intrinsic" fibres all appear to be the same fibres, which bend and branch repeatedly during their course within the thickness of the cementum. Because of its physical continuity with the cementum, a limited portion of the periodontal ligament approximately corresponding to the length of Sharpey's fibres remains unaffected by enzymatic digestion of the interfibrillar matrix while the rest of the ligament is completely dissolved. The findings here indicate that the periodontal ligament and dental cementum join by a continuity rather than a contiguity of structures; that the collagen-mineral relation in cementum has distinctive features in comparison to other hard tissues; that extrinsic and intrinsic fibres of cementum and the adjoining portion of periodontal ligament form a structural, mechanical and metabolic unit distinct from the central, more metabolically active portion of the periodontal ligament.