Periodontal Ligament & Cementum 學習目標參考資料 Periodontium

口腔生理學（含顎咬合）學習目標 Oral physiology (occlusion included) ‧1. let the student to understand the Periodontal Ligament & base knolwedge of oral physisology. ‧2.The student can firsther studying Cementum the advance courses of dental science. 臺北醫學大學牙醫學系張維仁老師 E-mail [email protected]

參考資料

‧1.Applied Oral Physiology, 2nd ed. Christopher L.B.Lavelle Butterworths & Co.Lts, 1998 Periodontal Ligament & Cementum ‧2.Physiology for dental students 1st ed. D.B.Ferguson. Butterworths & Co.Lts, 1998.

Periodontal Ligament

• The PDL is composed of a complex vascular and highly cellular connective tissue that surrounds the tooth root and Periodontium connects it to the inner wall of the alveolar bone • The average width of the PDL is about 0.2 mm, ranging from The attachment apparatus of the tooth 0.15 to 0.38 mm. The width and consists of cementum, PDL, bone decrease with age lining the alveolus, and part of the • The thinnest portion is around gingiva the middle third of the root Connective Tissue Cells • Fibroblasts – Most common cells in PDL – Appear as ovoid or elongated cells along the principal fibers – Pseudopodia-like process Cells of the Periodontal – Producing collagen which has high turnover rate Ligament • Cementoblasts – Line the ligament surface of the cementum – Most often seen in resting phase Connective tissue cells – Cementum does not remodel, cementoclasts (odontoclasts) are Epithelial rest cells not normally found in the ligament Immune system cells • Osteoblasts Cells associated with • Osteoclasts neurovascular elements • Odontoclasts Undifferentiated mesenchymal cells

Epithelial rest cells Defense Cells

• The epithelial rests of Malassez form a • Neutrophils latticework in the PDL • Lymphocyte and appear as either isolated clusters of cells s or interlacing stands • Considered remnants of • Macrophage Hertwig’s root sheath s • Contain keratinocyte growth factors • Mast cells • Participate in the formation of periapical • Eosinophils cysts and lateral root cysts

Undifferentiated mesenchymal cells (Progenitor cells) Periodontal Fibers • Although it has been demonstrated that • The most important they are a source of new cells for the PDL, elements of PDL are the principal fibers, it is not known whether a single progenitor collagenous and cell gives rise to daughter cells that arranged in bundles differentiate into fibroblasts, osteoblasts, • The terminal portions and cementoblasts, or whether there are of the principal fibers that are inserted into separate progenitors for each cell line cementum and bone are termed Sharpey’s fibers Collagen Collagen

• The term collagen derives from the greek • A protein composed of different amino acids, the most important of which are glycine, proline, work for “glue”, it yields gelatin on boiling hydroxylysine and hydroxyproline • The main constituent of skin, bone, • At least 19 recognized collagen species encoded by at least 25 separate genes, cartilage, tendon and is also present in dispersed among 12 chromosome specialized structure such as basement • The principal fibers are composed mainly of membranes, vitreous, cornea, aorta and collagen type I, whereas reticular fibers are composed of collagen type III. Collagen type IV other tissue is found in the basal lamina • Triple-chain helix

Collagen Biosynthesis Collagen Biosynthesis • Intracellular steps • Occurs inside the – Transcription of individual fibroblasts to form genes tropocollagen molecules – Translation • Tropocollagen molecules – Hydroxylation of prolyl & aggregate into microfibril Lysyl residues that are packed together to – Helix formation & secretion form fibrils • Extracellular steps • Collagen is synthesized by – Processing fibroblasts, chondroblasts, – Fibril formation osteoblasts, odontoblasts, – Cross linking & fibril and other cells 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 Gingival Ligament Oxytalan & Eluanin

• Dentogingival group • Although the PDL does not contain mature • Alveologingival group elastin, two immature forms are found: oxytalan • Circular group and eluanin • Dentoperiosteal • Oxytalan fibers run parallel to the root surface in group a vertical direction and bend to attach to the cementum • Transseptal fiber system • 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 Extracellular Matrix • Run in all directions • Indifferent fiber plexus Ground substance

Ground Substance Physical Functions • Filling spaces between fibers & cells • Provision of a soft tissue “casing” to protect the • Main components: vessels and nerves from injury by mechanical – Glycosaminoglycans forces • Hyaluronic acid • proteoglycon • Transmission of occlusal forces to the bone – Glycoprotein • Fibronectin • Attachment of the teeth to the bone • Laminin • Maintenance of the gingival tissues in their – Glycolipids proper relationship to the teeth • Calcified masses: cementicles • Resistance to the impact of occlusal forces – Calcified epi rests (shock absorption) – Calcified Sharpey’s fiber – Calcified, thrombosed vessels Shock absorption Shock absorption • Viscoelastic system theory • Tensional theory – The displacement of the tooth is largely controlled by – When a force is applied to the crown, the fluid movements, with fibers having only a second role principal fibers first unfold and straighten and – When forces are transmitted to the tooth, the extracellular fluid passes from the PDL into the marrow then transmit forces to the alveolar bone, spaces of bone through foramina in the cribriform plate cause an elastic deformation of the bony – After depletion of tissue fluids, the fiber bundles absorb socket. the slack and tighten, which lead to a blood vessel – Finally when the alveolar bone has reached stenosis. Arterial back pressure causes ballooning of the vessel and passage of the blood ultrafitrates into its limit, the load is transmitted to the basal the tissue, thereby replenishing the tissue fluid bone

Transmission of Occlusal Forces to the Formative & Remodeling Bone Function • The arrangement of the principal fibers is similar to a suspension bridge • Cells of the PDL participate in the formation and or a hammock. resorption of cementum and bone, which occur in • When an axial force is applied to a physiologic tooth movement; in the accommodation tooth, a tendency toward of the periodontium to occlusal forces; and in the displacement of the root into the repair of injuries alveolar occurs • The PDL is constantly undergoing remodeling. • The oblique fibers alter their wavy, Fibroblasts form the collagen fibers, and the untensed pattern; assume their full residual mesenchymal cells develop into length; and sustain the major part of osteoblasts, cememtoblasts, and fibroblasts affect the axial force the rate of formation of collagen, cementum and • The apical portion of the root moves in bone a direction opposite to the coronal portion • The rate of collagen synthesis is twice as fast as • In areas of tension, the principal fiber that in the gingiva and four times as fast as that in bundles are taut rather than wavy. In the skin areas of pressure, the fibers are compressed

Nutritional & Sensory Function Nutritional & Sensory Function

• The PDL supplies nutrients to the cementum, bone, • The PDL is abundantly supplied with sensory and gingiva by way of the blood vessels and also nerve fibers capable of transmitting tactile, pressure, and pain sensations by the trigeminal provides lymphatic drainage pathways • In relation to other ligaments and tendons, PDL is • The bundles divide into single myelinated fibers, highly vascularized which ultimately lose their myelin sheaths and end in one of four types of neural termination: • This high blood vessel content may provide – Free ending hydrodynamic damping to applied forces, as well – Ruffini-like mechanoreceptors, primarily in the as high perfusion rates to the PDL apical area – Coiled meissner’s corpuscles, mainly in the midroot region – Spindlelike pressure and vibration endings, mainly in the apex Free ending

Ruffini-like mechanoreceptors

Nerve terminals in a human Coiled meissner’s corpuscles

Spindlelike endings

Functions of PDL Cementum

Physical The calcified, avascular Formative mesenchymal tissue that forms Remodeling Nutritional the outer covering of the anatomic sensory root

Cementum Cementum

• Acellular (primary) • Both consist of a • The two main sources of collagen fibers in cementum calcified interfibrillar cementum are • Cellular (secondary) matrix and collagen – Sharpey’s fiber (extrinsic): embedded portion of the cementum fibrils 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 Cementum Acellular Cementum

• The major proportion of the organic matrix • Acellular cememtum is the first cementum formed, covers approximately the cervical third or half of the of a cementum is composed of type I (90%) root and type III (about 5%) collagens. • Formed before the tooth reaches the occlusal plane. • Sharpey’s fibers are composed of mainly Its thickness ranges from 30 to 230 μm type I collagen where the type III collagen • Sharpey’s fibers make up most of the structure of appears to coat the type I collagen of acellular cementum. Most fibers are inserted at Sharpey’s fibers approximately right angle into the root surface. Their size, number and distribution increase with function

Acellular Cementum Cellular Cementum

• Sharpey’s fibers are • Formed after the tooth completely calcified, reaches the occlusal with the mineral plane crystals oriented • Cellular cementum is parallel to the fibrils less calcified than the • Acellular cementum acellular type also contains calcified intrinsic collagen fibrils

Cellular Cementum Cementum

• More irregular and • Both acellular cementum and cellular cementum are contains cells arranged in lamellae separated by incremental lines (cementocytes) in parallel to the long axis of the root individual spaces • These lines represent “rest periods” in cementum (lacunae) that formation and are more mineralized than the adjacent communicate with cementum each other through a • The inorganic content of cementum (hydroxyapatite) is system of 45% to 50%, which is less than that of bone (65%), enamel (97%), or dentin (70%) anastomosing canaliculi • Acellular cementum and cellular cementum are very permeable. The permeability of cementum diminishes with age Schroeder’s Classification Schroeder’s Classification • Acellular afibrillar cementum (AAC) – Contains neither cells nor extrinsic or intrinsic collagen • Cellular intrinsic fiber cementum (CIFC) fibers, except for mineralized ground substance. Coronal cementum. (1-15 μm) – Contains cells but no extrinsic collagen fibers. Formed by cementoblasts. It fills resorption • Acellular extrinsic fiber cementum (AEFC) lacunae – Composed almost entirely of densely packed bundles of Sharpey’s fibers. Cervical third of roots. (30-230 μm) • Intermediate cementum • Cellular mixed stratified cementum (CMSC) – Poorly defined zone near the cementodentinal – Composed of extrinsic & intrinsic fibers and may contain junction of certain teeth cells. Co-product of fibroblasts and cementoblasts. – Contain cellular remnants of Hertwig’s sheath Apical third of the roots, apices and furcation areas. embeded in calcified ground substance (100-1000 μm)

Cementoenamel Junction Cementoenamel Junction

• Three types of • Studies with the SEM relationships: indicate that the – In about 60-65%, cementoenamel cementum overlaps junction exhibits all of the enamel the above forms and – In about 30%, edge- to-edge butt joint shows considerable exists variation when trace – In 5-10%, the circumferentially cementum and enamel fail to meet

Cementoenamel Junction Summary

介紹口腔中正常的組織，解剖、生理和口腔組織的演化及生長過程，使同學們對人體口腔有基本概念。