學習目標. Summary 參考資料. Periodontal Ligament. Periodontium. Periodontal Ligament & Cementum. 口腔胚胎及組織學 Oral embryology & histology

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1 口腔胚胎及組織學 Oral embryology & histology Periodontal Ligament & 林靜毅 Jerry Lin DDS, DMSc Diplomate, American Board of Periodontology 台北醫學大學牙醫學士哈佛大學口腔生物醫學博士哈佛牙醫學院牙周病專科醫師哈佛牙醫學院牙周病學客座講座台大牙周病科兼任主治醫師台北醫學大學助理教授美國牙周病醫學會院士 學習目標 能了解頭臉部及口腔之發育 結構與功能, 並發展能有效吸收與傳播相關資訊之辭彙, 將胚胎及組織學上之基本概念, 應用於臨床上之診斷與治療 參考資料 1.Illustrated Dental Embryology, Histology, and Anatomy: Mary Bath-Balogh, Margaret J. Fehrenbach, 2nd Edition, Elsevier Saunders,2006 2E 2.Esstential ti of foral lhistology and dembryology--a clinical i l approach: James K Avery, Denial J Chiego,Jr 3rd Edition, Elsevier Mosby, Ten Cate's Oral Histology: Development, Structure, and Function: Nanci Anatonio, 6th ed. Mosby, 2003 Summary 提供學生探討頭臉部及口腔之發育, 認識牙齒及口腔組織之正常顯微結構, 並了解其功能與結構之關係, 進而對生理 病理與臨床牙科學上之問題能做比較與思考, 為各牙科臨床科目之重要基礎課程 Periodontal Ligament Periodontium The attachment apparatus of the tooth and consists of cementum, PDL, bone lining the alveolus, and part of the gingiva The PDL is composed of a complex vascular and highly cellular connective tissue that surrounds the tooth root and connects it to the inner wall of the alveolar bone The average width of the PDL is about 0.2 mm, ranging from 0.15 to 0.38 mm. The width decrease with age The thinnest portion is around the middle third of the root 1

2 Cells of the Periodontal Ligament Connective tissue cells Epithelial rest cells Immune system cells Cells associated with neurovascular elements Undifferentiated mesenchymal cells Connective Tissue Cells Fibroblasts Most common cells in PDL Appear as ovoid or elongated cells along the principal fibers Pseudopodia-like process Producing collagen which has high turnover rate Cementoblasts Line the ligament surface of the cementum Most often seen in resting phase does not remodel, cementoclasts (odontoclasts) are not normally found in the ligament Osteoblasts Osteoclasts Odontoclasts The epithelial rests of Malassez form a latticework in the PDL and appear as either isolated clusters of cells or interlacing stands Considered remnants of Hertwig s root sheath Contain keratinocyte growth factors Participate in the formation of periapical cysts and lateral root cysts Epithelial rest cells Neutrophils Lymphocytes Macrophages Mast cells Eosinophils Defense Cells Undifferentiated mesenchymal cells (Progenitor cells) Although it has been demonstrated that they are a source of new cells for the PDL, it is not known whether a single progenitor cell gives rise to daughter cells that differentiate into fibroblasts, osteoblasts, and cementoblasts, or whether there are separate progenitors for each cell line The most important elements of PDL are the principal fibers, collagenous and arranged in bundles The terminal portions of the principal fibers that are inserted into cementum and bone are termed Sharpey s fibers Periodontal Fibers 2

3 Collagen The term collagen derives from the greek work for glue, it yields gelatin on boiling The main constituent of skin, bone, cartilage, tendon and is also present in specialized structure such as basement membranes, vitreous, cornea, aorta and other tissue Triple-chain helix Collagen A protein composed of different amino acids, the most important of which are glycine, proline, hydroxylysine and hydroxyproline At least 19 recognized collagen species encoded by at tleast t25 separate genes, dispersed damong 12 chromosome The principal fibers are composed mainly of collagen type I, whereas reticular fibers are composed of collagen type III. Collagen type IV is found in the basal lamina Collagen Biosynthesis Occurs inside the fibroblasts to form tropocollagen molecules Tropocollagen molecules aggregate into microfibril that are packed together to form fibrils Collagen is synthesized by fibroblasts, chondroblasts, osteoblasts, odontoblasts, and other cells Collagen Biosynthesis Intracellular steps Transcription of individual genes Translation Hydroxylation of prolyl & Lysyl residues Helix formation & secretion Extracellular steps Processing Fibril formation Cross linking & fibril stabilization Principal Fibers Transseptal group Alveolar crest group Prevent the extrusion of the tooth Horizontal group Oblique group Bear the brunt of vertical masticatory stress & transform them into tension on the alveolar bone Apical group Interradicular group In the furcation area 3

4 Dentogingival group Alveologingival group Circular group Dentoperiosteal group Transseptal fiber system Gingival Ligament Oxytalan & Eluanin Although the PDL does not contain mature elastin, two immature forms are found: oxytalan and eluanin Oxytalan fibers run parallel to the root surface in a vertical direction and bend to attach to the cementum Regulate vascular flow Oxytalan fibers have been shown to develop de novo in the regenerated PDL Other fibers Small collagen fibers associated with the larger principal collagen finers have been described Run in all directions Indifferent fiber plexus Extracellular Matrix Ground substance Ground Substance Filling spaces between fibers & cells Main components: Glycosaminoglycans Hyaluronic acid proteoglycon Glycoprotein Fibronectin Laminin Glycolipids Calcified masses: cementicles Calcified epi rests Calcified Sharpey s fiber Calcified, thrombosed vessels Functions of PDL Physical Formative Remodeling Nutritional sensory 4

5 Physical Functions Provision of a soft tissue casing to protect the vessels and nerves from injury by mechanical forces Transmission of occlusal forces to the bone Attachment of the teeth to the bone Maintenance of the gingival tissues in their proper relationship to the teeth Resistance to the impact of occlusal forces (shock absorption) Shock absorption Tensional theory When a force is applied to the crown, the principal fibers first unfold and straighten and then transmit forces to the alveolar bone, cause an elastic deformation of the bony socket. Finally when the alveolar bone has reached its limit, the load is transmitted to the basal bone Shock absorption Viscoelastic system theory The displacement of the tooth is largely controlled by fluid movements, with fibers having only a second role When forces are transmitted to the tooth, the extracellular fluid passes from the PDL into the marrow spaces of bone through foramina in the cribriform plate After depletion of tissue fluids, the fiber bundles absorb the slack and tighten, which lead to a blood vessel stenosis. Arterial back pressure causes ballooning of the vessel and passage of the blood ultrafitrates into the tissue, thereby replenishing the tissue fluid Transmission of Occlusal Forces to the Bone The arrangement of the principal fibers is similar to a suspension bridge or a hammock. When an axial force is applied to a tooth, a tendency toward displacement of the root into the alveolar occurs The oblique fibers alter their wavy, untensed pattern; assume their full length; and sustain the major part of the axial force The apical portion of the root moves in a direction opposite to the coronal portion In areas of tension, the principal fiber bundles are taut rather than wavy. In areas of pressure, the fibers are compressed. Formative & Remodeling Function Cells of the PDL participate in the formation and resorption of cementum and bone, which occur in physiologic tooth movement; in the accommodation of the periodontium to occlusal forces; and in the repair of injuries The PDL is constantly undergoing remodeling. Fibroblasts form the collagen fibers, and the residual mesenchymal cells develop into osteoblasts, cememtoblasts, and fibroblasts affect the rate of formation of collagen, cementum and bone The rate of collagen synthesis is twice as fast as that in the gingiva and four times as fast as that in the skin Nutritional & Sensory Function The PDL supplies nutrients to the cementum, bone, and gingiva by way of the blood vessels and also provides lymphatic drainage In relation to other ligaments and tendons, PDL is highly hl vascularized This high blood vessel content may provide hydrodynamic damping to applied forces, as well as high perfusion rates to the PDL 5

6 Nutritional & Sensory Function The PDL is abundantly supplied with sensory nerve fibers capable of transmitting tactile, pressure, and pain sensations by the trigeminal pathways The bundles divide into single myelinated fibers, which h ultimately t l lose their myelin sheaths and end in one of four types of neural termination: Free ending Ruffini-like mechanoreceptors, primarily in the apical area Coiled meissner s corpuscles, mainly in the midroot region Spindlelike pressure and vibration endings, mainly in the apex Nerve terminals in a human Free ending Ruffini-like mechanoreceptors Coiled meissner ss corpuscles Spindlelike endings The calcified, avascular mesenchymal tissue that forms the outer covering of the anatomic root Acellular (primary) cementum Cellular (secondary) cementum Both consist of a calcified interfibrillar matrix and collagen fibrils The two main sources of collagen fibers in cementum are Sharpey s fiber (extrinsic): embedded portion of the principal fibers of the PDL and formed by fibroblasts Fibers that belong to the cementum matrix (intrinsic) and are produced by the cememtoblasts Cementoblasts also form the noncollagenous components of the interfibrillar ground substance, such as proteoglycans, glycoproteins, and phosphoproteins 6

7 The major proportion of the organic matrix of a cementum is composed of type I (90%) and type III (about 5%) collagens. Sharpey s fibers are composed of mainly type I collagen where the type III collagen appears to coat the type I collagen of Sharpey s fibers Acellular Acellular cememtum is the first cementum formed, covers approximately the cervical third or half of the root Formed before the tooth reaches the occlusal plane. Its thickness ranges from 30 to 230 μm Sharpey s fibers make up most of the structure of acellular cementum. Most fibers are inserted at approximately right angle into the root surface. Their size, number and distribution increase with function Acellular Sharpey s fibers are completely calcified, with the mineral crystals oriented parallel to the fibrils Acellular cementum also contains calcified intrinsic collagen fibrils Cellular Formed after the tooth reaches the occlusal plane Cellular cementum is less calcified than the acellular l type More irregular and contains cells (cementocytes) in individual spaces (lacunae) that communicate with each other through a system of anastomosing canaliculi Cellular Both acellular cementum and cellular cementum are arranged in lamellae separated by incremental lines parallel to the long axis of the root These lines represent rest periods in cementum formation and are more mineralized than the adjacent cementum The inorganic content of cementum (hydroxyapatite) is 45% to 50%, which is less than that of bone (65%), enamel (97%), or dentin (70%) Acellular cementum and cellular cementum are very permeable. The permeability of cementum diminishes with age 7

8 Schroeder s Classification Acellular afibrillar cementum (AAC) Contains neither cells nor extrinsic or intrinsic collagen fibers, except for mineralized ground substance. Coronal cementum. (1-15 μm) Acellular extrinsic fiber cementum (AEFC) Composed almost entirely of densely packed bundles of Sharpey s fibers. Cervical third of roots. ( μm) Cellular mixed stratified cementum (CMSC) Composed of extrinsic & intrinsic fibers and may contain cells. Co-product of fibroblasts and cementoblasts. Apical third of the roots, apices and furcation areas. ( μm) Schroeder s Classification Cellular intrinsic fiber cementum (CIFC) Contains cells but no extrinsic collagen fibers. Formed by cementoblasts. It fills resorption lacunae Intermediate t cementum Poorly defined zone near the cementodentinal junction of certain teeth Contain cellular remnants of Hertwig s sheath embeded in calcified ground substance Cementoenamel Junction Three types of relationships: In about 60-65%, cementum overlaps the enamel In about 30%, edge-toedge butt joint exists In 5-10%, the cementum and enamel fail to meet Cementoenamel Junction Studies with the SEM indicate that the cementoenamel junction exhibits all of the above forms and shows considerable variation when trace circumferentially Cementoenamel Junction 8