Oral Histology Lecture Notes PDF

Oral Histology Lec and Lab 10 ‫ محمد عبود‬.‫ م‬.‫م‬ Cementum Dental sac consists of ectomesenchymal cells and fibers. From the sac, the supporting tissues of the tooth (period...

Oral Histology Lec and Lab 10 ‫ محمد عبود‬.‫ م‬.‫م‬ Cementum Dental sac consists of ectomesenchymal cells and fibers. From the sac, the supporting tissues of the tooth (periodontium) are developed, which are: Cementum PDL Alveolar bone Gingiva (ectodermal in origin) The cementum is a specialized mineralized tissue covering the root surfaces and, occasionally, small portions of the crown of the teeth. It has many features in common with bone tissue. However, the cementum is different from bone in that it contains no blood or lymph vessels, has no innervation, does not undergo physiologic resorption or remodeling, but is characterized by continuous deposition throughout life. The prime function of the cementum is to give attachment to collagen fibers of the periodontal ligament (PDL). Physical Characteristics: 1-Color: Light yellow, lighter in color than dentin, can be distinguished from enamel by its lack of luster (dull surface). 2-Softer than dentin. Soft and thin cementum in the cervical area means that is can be removed readily with abrasion when gingival recession exposes the root causing sensitivity. 3- Thickness:, the thickness of cementum at the cervical area is (20-50 µm), and it gradually increase in thickness till reach its maximum about (150-200 m) at the apex. 4- Permeability: more permeable than dentin. Permeability decreases with age. 5- Hardness: fully mineralized cementum has less hardness than dentin, because it contains less mineral than bone or dentin. Chemical Composition: Cementum is approximately 45-50% (inorganic) hydroxyapatite by weight and the remaining portion 50-55% (organic) is collagen and noncollagenous matrix proteins. The inorganic contents are composed of calcium and phosphate. Also, the cementum has higher levels of fluoride content than enamel and dentin. The organic component are composed of collagen type I, and noncollagenous proteins: (1) CAP (cementum derived attachment protein) which promotes attachment of mesenchymal cells to extracellular matrix. (2) Cementum is rich in Glucoseaminoglucans like chondroitin sulphate and located around cementum lacunae. 1 Structure of cementum: Cellular elements Fibrous elements Calcified interfibrillar matrix Cellular elements: 1- Cementoblasts: Cementoblasts line the root surface and when active, they contain numerous mitochondria, well developed Golgi complex, and open-faced nucleus. Its function is the formation of the cementoid including the intrinsic collagen fibers and the interfibriller matrix. Cellular intrinsic fiber cementum 2- Cementocytes: Cementocytes are seen located in lacunae in cementum matrix and typically have numerous processes lying in canaliculi. Because cementum is avascular tissue, thus the processes of the cementocytes are oriented toward the tooth surface for nutrition of cementum. Cementocytes Cementocyte lacunae in ground sectio 2 Fibrous element:  Extrinsic fiber system (E) The sharpey’s fibers constitutes the external fiber system of the cementum & are produced by fibroblast in the PDL, oriented more or less perpendicularly to the C.D.J.  Intrinsic fiber system (I) Produced by cementoblast & is composed of fibers oriented more or less parallel to long axis of the root. Interfibrillar matrix : Consists of proteoglycans and glycoproteins formed by cementoblast, undergo mineralization through precipitation of hydroxyl apatite crystals. Classification of cementum: cementum can be classified based on: 1. Presence or absence of cells. i) Acellular cementum (primary). ii) Cellular cementum (secondary). 1. Type of fibers. a) Extrinsic fiber cementum. b) Intrinsic fiber cementum. c) Mixed fiber cementum. 1. Presence or absence of cell and on the type of fibers. i) Acellular extrinsic fiber cementum (primary cementum) (AEFC). ii) Cellular intrinsic fiber cementum (secondary cementum) (CIFC). iii) Mixed cementum: a) Cellular mixed fiber cementum (CMFC) b) Cellular mixed stratified cementum (CMSC). iv) Afibrillar acellular cementum (AAC). Types Origin of Fibers Location Function Acellular (primary) Extrinsic From cervical margin to Anchorage the apical third Cellular (secondary) Intrinsic Middle to apical third Adaptation and repair and furcations Mixed (alternating layers Intrinsic and Apical portion and Adaptation of acellular and cellular) extrinsic furcations Acellular afibrillar — enamel in the cervical No known function along area the cementoenamel junction 3  i. Acellular extrinsic fiber cementum (primary cementum) (AEFC)  Formed first.  Contains mainly bundles of Sharpeys’ fiber (extrinsic, perpendicular to cementum).  Develops slowly by cementoblasts which remains on its surface.  Does not contain cementocyte.  Location: From cervical margin to the apical third.  Well mineralized cementum.  Main function is anchorage (attachment). ii. Cellular intrinsic fiber cementum (secondary cementum) (CIFC)  Formed after half of root cementum is formed.  It contains mainly intrinsic fibers (parallel to the root surface), so they don’t assist for tooth attachment, has no Sharpeys’ fiber.  Develops more rapidly.  It contains cementocytes.  Location: Middle to apical third and furcation areas of root.  Less mineralized than acellular cementum.  Its function is adaptation and repair. iii. Mixed cementum:  It contains both extrinsic and intrinsic fibers.  It contains cementocyte.  Located: in the apical third and the furcation area.  Its function: in the adaptation. It has 2 subtypes: a) Cellular mixed fiber cementum (CMFC)  The intrinsic fibers are fewer in number and smaller in size than extrinsic fibers. b) Cellular mixed stratified cementum (CMSC)  Has alternating layers of acellular extrinsic and cellular intrinsic cementum. iv. Acellular afibrillar cementum (AAC)  It contains no cementocyte.  This type of cementum contains no collagen fibers.  Location: in the cervical region only when cementum overlying enamel.  Function: not known.  Formed following the loss of reduced enamel epithelium. 4 Cementoenamel Junction (CEJ) Also is called cervical line. There are three types of junction between cementum and enamel: Cementum overlaps the enamel junction: In approximately 60% of teeth cementum overlapping the cervical end of enamel. This type occurs due to the degeneration of the reduced enamel epithelium (R.E.E.) allowing the dental sac cells to come in defective contact with enamel and differentiate into ameloblast, which produce afibrillar cementum, it does not contain collagen fibers. Edge to edge junction: In approximately 30% of human teeth the cementum and enamel meet at sharp line forming a butt joint. Gap junction: In approximately 10% of teeth, have a gap between the cementum and enamel, this condition occurs due to delay in the separation epithelial root sheath at the cervical portion, in this case C.E.J. this can result in dental hypersensitivity as the gingiva recedes exposing the underlying root dentin. 5  Cementodentinal junction: The dentin surface upon which cementum is deposited is relatively smooth in permanent teeth. The cementodentinal junction in deciduous teeth, however, is sometimes scalloped. Intermediate layer is found between cementum and dentin that function as permeability barrier. Deciduous teeth Permanent teeth Resorption and repair of cementum Cementum resorption is carried out by multinucleated cementoclast, while cementum repair is done by the cementoblasts. The presence of unmineralized surface layer of collagen fibers protects the cementum against resorption. Reversal line: The line that separates the change from cementum resorption to repair. Clinical consideration of cementum: 1) Hypercementosis: It is an abnormal thickening of cementum, may be diffuse or circumscribed. It may affect all teeth of the dentition, be confined to a single tooth, or even affect only parts of one tooth. Cementum hypertrophy: If the overgrowth occurs in functional teeth as prong like extension and improves the functional qualities of the cementum, found in teeth exposed to great stress. The prong like extension of cementum provide a large surface area for attaching an increased number of sharpey’s fibers, thus a firmer anchorage of the tooth to the surrounding bone is assured. Cementum hyperplasia: If the overgrowth occurs in nonfunctional teeth that show a decreased number of sharpey’s fibers. Found: in areas which enamel drops have developed on dentin, in irregular shape. Also found around degenerated epithelial rests in knop like projections called excementosis. When found associated with chronic periapical inflammation, the cementum will be extensively deposited around the root like a cuff. When found in areas of injuries to cementum, its shape will be as spike like extensions and calcification of sharpey’s fibers 6  Hypercementosis 2) Cementum is more resistance to resorption than is in alveolar bone and for this reason orthodontic tooth movement is made possible by bone resorption. 3) Cementum resorption can occur due to trauma and excessive occlusal force. 4) Cementicles: they are ovoid or round calcified structure that exhibits by calcification of the degenerated periodontal tissue or the epithelial rests of Malssez. Cementicles may be: Free in the periodontal ligament, Attached the cementum, or Embedded in the cementum during its growth by age. 5) Variation in thickness of cementum was observed in diabetics. It was found to be thicker in type 2 diabetes than in normal subjects. 6) Ankylosis Abnormal cemental deposition can sometimes lead to fusion of bone and cementum. These teeth will not show post eruptive movements. 7) Cemental caries can be seen on exposed surfaces of cementum associated with gingival recession of older individuals. 8) Determination of age of an individual is an accepted procedure in forensic dentistry through counting of the incremental lines, in cementum. 7 Incremental lines of Salter: Both cellular and acellular cementum show incremental lines which indicate periodic formation, these lines are highly mineralized areas with less collagen and more ground substance; these lines are parallel to the long axis of the tooth. Cementogenesis: It takes place in two phases: Matrix formation Mineralization There are 3 cell types responsible for the cementogenesis: Cementoblasts Cementocytes Fibroblasts Cementum formation in the developing tooth is preceded by the deposition of dentin along the inner aspect of Hertwig‘s epithelial root sheath (HERS). Once dentin formation breaks occur in the epithelial root sheath allowing the newly formed dentin to come in direct contact with the connective tissue of the dental sac, the undifferentiated mesenchymal cells derived from the dental sac differentiate into cementoblasts. Once the cementoblast are differentiated, they are rapidly increased in size and show an increase in number of cytoplasmic organelles, so the cells will be ready for the formation and secretion of cementum matrix. 8 Matrix formation: Cementoblast will form the organic matrix of cementum that is called cementoid, it is consisted of collagen fibrils embedded in amorphous protein and polysaccharides. Cementoid is similar to predentin, and osteoid tissue in bone. The cementoid is mineralized to form cementum. So the next step is: Mineralization:  After part of cementum matrix has been laid down, its mineralization begins, the minerals originate from the tissue fluids, where calcium and phosphate ions precipitate into matrix, within the surface and between collagen fibers of cementum.  The long axis of the mineral crystals are arranged parallel to the long axis of the collagen fibers which is parallel to the tooth axis.  a thin layer of cementoid tissue is only seen on the surface of the cementum, while a new layer is apposed. The cementoid tissue is lined by cementoblasts.  After reaching the full thickness, the cementoblasts enter a quiescent stage.  During matrix formation fibroblasts of the PDL form collagen fibers (sharpey`s fibers), which become embedded in the matrix to provide attachment of the root to the surrounding bone. Those fibers are also called perforating fibers. AEFC: Acellular extrinsic fiber cementum: HERS: Hertwig’s root sheath 9