Development and Structure of the Periodontal Ligament
Development of PDL
At the late bell stage, the Hertwig’s root sheath grows apically, mapping out the shape of the root. When the root dentine has formed, Hertwig’s root sheath degenerates, allowing cells from the dental follicle to come into contact with the root dentine; these cells differentiate into cementoblasts. Once cementum formation has begun, collagen fibres within the dental follicle orientate themselves into bundles and the ends become embedded into the developing surface of the cementum and alveolar bone; these are known as Sharpey’s fibres.
Structure of the PDL
The periodontal ligament is made up of two groups of fibres: the gingival fibre groups and the principal fibre groups (Fig.).
1.. Gingival fibre groups
The gingival fibre groups of the periodontal ligament include:
■ Dentino-gingival fibres (free gingival fibres) are attached to the cementum and fan out into the gingival tissue.
■ Alveolo-gingival fibres arise from the alveolar crest and run coronally into the attached and free gingiva.
■ Circumferential fibres (circular) encircle the neck of the tooth.
■ Alveolar crest fibres run from the cervical cementum to the alveolar crest.
2. Principal fibre groups
The principal fibre groups of the periodontal ligament are:
■ Oblique fibres which run obliquely from alveolar bone to tooth.
■ Apical fibres which radiate from the apex of the tooth to the adjacent alveolar bone.
■ Horizontal fibres which run horizontally from the cementum to the adjacent alveolar bone.
■ Inter-radicular fibres which are found between the roots of multi-rooted teeth and run from the root to the adjacent alveolar bone.
Cells of PDL
The predominant cells found within the periodontal ligament are fibroblasts. Cementoblasts cover the surface of the cementum and osteoblasts and osteoclasts cover the surface of alveolar bone. Remnants of the disintegrated Hertwig’s root sheath remain into adult life and can be found between the collagen fibres of the periodontal ligament. They are known as the epithelial cell rests of Malassez.