Lecture 8 - Periodontium (Part 2) PDF

The Periodontium (Part 2) The Periodontal Ligament (PDL) By the end of this lecture, you should be able to: Identify the cellular and molecular structure of the periodontal ligament Describe how the structure of the PDL reflects its various functions Describe the consequences of imbalances in hard t...

The Periodontium (Part 2) The Periodontal Ligament (PDL) By the end of this lecture, you should be able to: Identify the cellular and molecular structure of the periodontal ligament Describe how the structure of the PDL reflects its various functions Describe the consequences of imbalances in hard tissue removal and deposition around the PDL “The Tooth” It also consists of the periodontal (“around the tooth”) tissues that support it and anchor it into the jaws: cementum, the periodontal ligament (PDL), and alveolar bone. Enamel Dentine Pulp PDL Alveolar bone Cementum “The Tooth” Adding the periodontal tissues to ”the tooth” is not arbitrary: their developmental origins are from the teeth, not from the jaw (more on that in the development of the periodontium lecture) Enamel Dentine Pulp PDL Alveolar bone Cementum The PDL- the basics The Periodontal Ligament (PDL) is the fibrous connective tissue that holds the tooth in its alveolus (socket). In healthy teeth, the PDL is a soft tissue between the cementum of the tooth root and the wall of the socket (the alveolar bone). PDL In radiographs, the PDL is radiolucent, appearing as a dark region between the tooth and socket. Ten Cate’s Oral Histology (8th Edition); Fig. 8-6 The PDL- the basics The average width of the PDL space changes with age: 0.21mm at 11-16yrs. 0.18mm at 32-52yrs. 0.15mm at 51-67yrs. PDL Ten Cate’s Oral Histology (8th Edition); Fig. 8-6 The PDL- the basics In soft tissue sections, the PDL is a network of thick collagen fiber bundles (types I, III, and XII) that anchor on two sides: the cementum and the alveolar bone. The PDL is incredibly important to consider when developing predictable regenerative or orthodontic therapies à it is a highly dynamic, constantly changing tissue! Cementum PDL Dentine Alveolar bone The PDL- functions The PDL has several important functions in supporting a tooth and its structure and cellular content reflect this. Cementum PDL (1) A cushion during occlusion (2) A pressure sensor (mechanoreception) (3) A key role in tooth drift (4) A source of new periodontal cells (5) A key role in tooth eruption? Dentine Alveolar bone The PDL- functions (1) A cushion during occlusion The thick collagen fiber bundles have considerable tensile strength. Their orientations around the tooth differ depending on the region (major fiber groups). Collagen fiber bundles are produced and maintained by fibroblasts, the most abundant cell type found in the PDL. Ten Cate’s Oral Histology (8th Edition); Fig. 9-28 The PDL- functions (1) A cushion during occlusion The thick collagen fiber bundles have considerable tensile strength. Their orientations around the tooth differ depending on the region (major fiber groups). Collagen fiber bundles are produced and maintained by fibroblasts, the most abundant cell type found in the PDL. Ten Cate’s Oral Histology (8th Edition); Fig. 9-28 The PDL- functions (1) A cushion during occlusion The thick collagen fiber bundles have considerable tensile strength. Their orientations around the tooth differ depending on the region (major fiber groups). Collagen fiber bundles are produced and maintained by fibroblasts, the most abundant cell type found in the PDL. Ten Cate’s Oral Histology (8th Edition); Fig. 9-28 The PDL- functions PDL (1) A cushion during occlusion Can visualize PDL fiber orientations with special histological stains and crosspolarized light. Alveolar bone Pulp Dentine The PDL- functions (2) A pressure sensor (mechanoreception) The PDL is well-innervated by mandibular and maxillary branches of the trigeminal nerve. These nerves are intimately connected with the ligament fiber bundles, sending sensory information back to the brain to let you know how hard you are biting and if you are occluding your teeth properly. Ten Cate’s Oral Histology (8th Edition); Fig. 9-33 The PDL- functions (3) A key role in tooth drift Teeth are able to gradually move through the jaw (tooth drift) in response to physical stimuli or during growth of the jawbones. This is mediated by the PDL: Pressure on the leading side of the tooth initiates a bone-resorption response. Tension on the trailing side initiates a bone deposition response. Saffar et al. (1997) Periodontology 2000 The PDL- functions (3) A key role in tooth drift A healthy periodontal ligament is required for orthodontic tooth movement. Without it, teeth will not move! Roberts (2006) Seminars in Orthodontics The PDL- functions (4) A reservoir of periodontal cells There is a lot of tissue turnover within the PDL: Collagen fiber bundles are broken down and re-assembled. Alveolar bone is deposited or resorbed. Cementum layers are added and removed. This requires a reservoir of cells to replenish these tissues. The PDL- functions (4) A reservoir of periodontal cells The PDL contains: Fibroblasts Osteoblasts Cementoblasts Osteoclasts Epithelial cells Macrophages Neurovascular cells Stem cells Collagen production and turnover From: Biorender Fibroblast The PDL- functions (4) A reservoir of periodontal cells Osteoblasts The PDL contains: Fibroblasts Osteoblasts Cementoblasts Osteoclasts Epithelial cells Macrophages Neurovascular cells Stem cells Alveolar bone production From: Biorender The PDL- functions (4) A reservoir of periodontal cells Cementoblasts The PDL contains: Fibroblasts Osteoblasts Cementoblasts Osteoclasts Epithelial cells Macrophages Neurovascular cells Stem cells Cementum production From: Biorender The PDL- functions (4) A reservoir of periodontal cells The PDL contains: Fibroblasts Osteoblasts Cementoblasts Osteoclasts Epithelial cells Macrophages Neurovascular cells Stem cells Osteoclasts Hard tissue resorption From: Biorender The PDL- functions (4) A reservoir of periodontal cells Dentine Epithelial Rests of Malassez The PDL contains: Fibroblasts Osteoblasts Cementoblasts Osteoclasts From: Biorender Epithelial cells Macrophages Neurovascular cells Stem cells Function not known. Acellular cementum production maybe? Ten Cate’s Oral Histology (8th Edition); Fig. 9-27 The PDL- functions (4) A reservoir of periodontal cells The PDL contains: Fibroblasts Osteoblasts Cementoblasts Osteoclasts Epithelial cells Macrophages Neurovascular cells Stem cells From: Biorender Immune responses The PDL- functions (4) A reservoir of periodontal cells Blood vessels Nerves The PDL contains: Fibroblasts Osteoblasts Cementoblasts Osteoclasts Epithelial cells Macrophages Neurovascular cells Ten Cate’s Oral Histology (8th Edition); Fig. 9-34 Nerves and blood vessels Stem cells From: Biorender The PDL- functions (4) A reservoir of periodontal cells The PDL contains: Fibroblasts Osteoblasts Cementoblasts Osteoclasts Epithelial cells Macrophages Nerovascular cells Turkkahraman et al. (2020) Am. J. Orthodont. Dent. Ortho. From: Biorender Stem and progenitor cells Produce new periodontal cells for repair and regeneration The PDL- functions (5) A key role in tooth eruption? The mechanism(s) of tooth eruption are poorly understood. Some research has demonstrated that PDL development is needed to maintain tooth eruption. But other studies have shown that this isn’t the case. Does the PDL have a role in tooth eruption? Research is ongoing! Ten Cate’s Oral Histology (8th Edition); Fig. 9-24 Imbalance in the PDL The PDL normally maintains a balance between hard tissue removal (resorption) and hard tissue deposition. Cementum PDL Alveolar bone Imbalance in the PDL Cementum Trauma, infection/disease, and other factors can contribute to an imbalance in these processes. Dentine Excessive resorption can lead to root and alveolar bone resorption of varying severity. If severe enough, this can lead to tooth loss. PDL Resorption Alveolar bone Imbalance in the PDL Andersson et al. (1984) Int. J. Oral Surg. Trauma, infection/disease, and other factors can contribute to an imbalance in these processes. Excessive alveolar bone or cementum deposition can lead to partial or complete dental ankylosis (fusion of teeth to the jawbone). This has consequences for tooth extraction and orthodontic tooth movement. Dental ankylosis Imbalance in the PDL Cementum Andersson et al. (1984) Int. J. Oral Surg. Dentine Dental ankylosis PDL Resorption Alveolar bone By the end of this lecture, you should be able to: Identify the cellular and molecular structure of the periodontal ligament Describe how the structure of the PDL reflects its various functions Describe the consequences of imbalances in hard tissue removal and deposition around the PDL