Evolution And History Of The Periodontal Ligament - A Review

Abstract

The periodontium is defined as those tissues supporting and investing the tooth, comprises of root cementum, periodontal ligament, bone lining the tooth socket and that part of the gingiva facing the tooth. The widespread occurrence of periodontal diseases and the realization that lost tissues can be repaired and perhaps regenerated has generated considerable interest in the factors and cells regulating their formation and maintenance. It is important to understand that each of the periodontal components has its very specialized structure and these structural characteristics directly define function. Indeed, proper functioning of the periodontium is only achieved through structural integrity and interaction between its components. EVOLUTION Although the various methods by which teeth are fixed in their position upon the bones which carry them pass by gradational forms into one another, so that a simple and at the same time absolutely correct classification is, impossible, yet for the purpose of description for principal methods may be enumerated, namely, attachment by means of fibrous membrane, by a hinge, by ankylosis and by implantation in bony sockets. There is a fundamental difference between the attachment of reptilian and mammalian teeth. In the ancestral reptiles the teeth are ankylosed to the bone. In mammals they are suspended in their sockets by ligaments. The evolutionary step from reptile to mammal included a series of co- ordinated changes in the jaws. The central point of these changes is the radicalReconstructionof the mandible. In reptiles the mandible consists of a series of bones united by sutures. Only the upper most bones, the dentary, carry the ankylosed tooth. The change from the manyboned reptiles to the single-¨boned mammalian mandible brought with it a radical change in the mode of growth. In the reptile, the mandibular and maxillary teethmovewith the bones to which they are fused. In the mammal the teeth have to �moveas units independent of the bones, and this movement is made possible by the remodelling of the periodontium. The evolutionary change from the reptiles to the mammals replaces the ankylosis of tooth and bone to a ligamentous suspension of the tooth. This change permits movement of mammalian teeth and the continued repositioning necessitated by jaw growth or tooth wear. CELLS OF THE PERIODONTAL LIGAMENT FIBROBLASTS