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THE PERIODONTAL LIGAMENT 12/12/23 WHAT IS THE PDL? The Periodontal Ligament is a soft connective tissue structure which separates cementum from the alveolar bone It forms part of the PERIODONTIUM (PDL + Cementum + Alveolar Bone + Gingival Tissue) It originates from the neural crest cells which give rise to the dental follicle FUNCTIONS OF THE PDL Inflammation - Immune Cells Host defence - Immune Cells Repair and regeneration - Cementoblasts, Fibroblasts, Osteoblasts, Stem Cells Tooth eruption – Endothelial cells “Shock absorber” during mastication Orthodontic tooth movementORIGIN The PDL is derived from neural crest cells which give rise to the dental follicle This dental follicle gives rise to cementoblasts, osteoblasts and fibroblasts These cells then form the periodontiumCELLS OF THE PDLFIBROBLASTS Origin: Dental follicle Where? Embedded between collagen fibres Functions: Secrete collagen Produce and remodel the extracellular matrix Generate forces for tooth eruption Differentiate into cementoblastsCEMENTOBLASTS Origin: Dental follicle Where? Near cementum Functions: Secrete the cementum organic matrix for cementum repairOSTEOBLASTS Origin: Dental follicle Where? Near the alveolar bone Functions: Repair of alveolar boneEPITHELIAL CELLS Origin: Epithelial rests of Malassez – remnants of HERS Where? Near root, mainly at the apical and cervical areas Functions: Secrete enamel like proteins onto the root surface Pathology! Form lateral periodontal cysts in response to inflammationENDOTHELIAL CELLS Where? Inner lining of blood vessel walls Function: Tooth support Create hydrostatic pressure for tooth eruptionIMMUNE CELLS Function: Immunity Important role in inflammatory periodontal disease Types: Macrophages Mast cells Eosinophils T/B cellsSTEM CELLS Mesenchymal stem cells maintain homeostasis of the periodontiumVASCULAR SUPPLY The extensive blood supply of the PDL reflects its high turnover rate Mainly via the superior and inferior alveolar arteries which give rise to perforating arteries Posterior teeth > anterior teeth Mandibular teeth > maxillary teeth INNERVATION Mainly from branches from the apical region of the roots which perforate into the alveolar bone From the dental and interalveolar branches of the alveolar nerves 4 types of nerve endings... FREE NERVE ENDINGS Unmyelinated and branched Extend to the cementum layer Pain receptors and mechanoreceptorsRUFFINI’S CORPUSCLES Dendritic like In myelinated Schwann cells Mechanoreceptors COILED ENDINGS In the mid-region of the PDL Unknown functionSPINDLE-LIKE ENDINGS Not common Associated with the root apexFIBRES OF THE PDL PRINCIPAL FIBRES Connect to alveolar bone and cementum via Sharpey’s fibres (terminal endings in cementum and alveolar bone) [A = transseptal ligament] B = alveolar crest C = horizontal D = oblique E = apical F = interradicularHORIZONTAL FIBRESOBLIQUE FIBRESALVEOLAR CREST FIBRESAPICAL FIBRES What histological stain was used in these images?DEVELOPMENT OF PRINCIPAL FIBRES 1. Begins before root formation with the first fibres forming in the dental follicle region 2. Initial oblique fibres form between the developing root and alveolar bone 3. Tooth eruption begins 4. Initial oblique fibres are replaced by horizontal fibres 5. Tooth eruption continues and gingival ligament fibres begin to form 6. More oblique fibres form 7. Alveolar crest fibres replace horizontal fibresRemembering the principal fibres... Old Amazing Incredible HAT Oblique fibres connect alveolar bone to cementum Apical fibres connect alveolar bone to cementum in the root apex Interradicular fibres in multicuspid teeth connect alveolar bone to cementum Horizontal fibres connect alveolar bone to cementum Alveolar crest fibres connect alveolar crest to root dentine Transseptal ligament connect neighbouring teeth GINGIVAL LIGAMENT FIBRES Gingival ligament do not insert into alveolar bone! A = Circular surround neck of tooth B = Dentogingival connect acellular cementum to gingivae C = Dentoperiosteal connect acellular cementum to alveolar periosteum D = Alveologingival connect alveolar bone to gingival tissue SECONDARY FIBRES Secondary fibres do not attach to cementum or bone Randomly orientated In middle portion of PDL Between nerves and blood vesselsCOLLAGEN The PDL mainly consists of 50-75% type I collagen + 20% type III It also contains types V, VI, XII, XIV Functions: Fibrillar collagen provides the 3D frameworkELASTIC PROTEINS Functions: Tissue resilience Support blood/nerve supply Types: Oxytalan Elaunin ElastinATTACHMENT PROTEINS Function: Bind other proteins to cells Types: Fibronectin Laminin Tenascin Osteopontin Vitronectin Osteonectin ThrombospondinPROTEOGLYCANS Function: Tissue hydration and space filling Types: Decorin Biglycan VersicanPDL TURNOVER The high turnover rate of the PDL is mediated by increased cellular activity, matrix metalloproteinases, growth factors, cytokines etc. The collagen half-life in the PDL is 1 day vs 6 days in the alveolar boneMATRIX METALLOPROTEINASES Functions: Digest all extracellular matrix proteins Control of MMP expression: Gene expression Protein transcription Pro-enzyme activation/inhibition Tissue inhibitors of metalloproteinases (TIMPS)ANY QUESTIONS?Practice questions 1. Name 4 functions of the PDL (2 marks)