Cementum

1. Cementum Prof. Olfat M. Gaballah

2. Contents • Introduction • Cementogenesis • Physical and chemical properties • Types of cementum • Incremental line of cementum • Cells of cementum • Cemento-enamel junction • Age change of cementum • Clinical consideration

4. Introduction • Cementum is avascular mineralized ectomesenchymal tissue covering anatomical root, • It acts as a medium for attachment of collagen fibers that bind the tooth to the surrounding structures. • It resembles bone in a few of its characteristics but is avascular, non-innervated, and unable to remodel. • Cementum thickness varies from one tooth to another and in different regions of the same tooth. • Cementum has the highest fluoride content of all the mineralized tissues.

5. Physical properties • Color: it is light yellow. It can be distinguished from enamel by its lack of luster and its darker hue. • Hardness: It is less than that of dentin. • Thickness:it is thinnest at CEJ ( 20- 50 um ) and gradually increases in thickness toward the apical foramen ( 100 -150). • Permeability: it is permeable from the PDL and dentin sides ( Cellular cementum is more permeable than cellular cementum )

6. Chemical properties Inorganic Materials Organic Materials • On a dry weight base it consists of 45% to 50% inorganic materials. • The inorganic materials consist mainly of calcium and phosphate in the form of hydroxyapatite crystals. • Numerous trace elements are found in cementum with varying amounts like copper, iron, magnesium, and potassium. • It also has the greatest fluoride level of all mineralized tissues. The most prevalent ionic substitution is fluoride, which is found in higher concentrations in acellular cementum than in cellular cementum. • It contains about 50% to 55% organic materials and water by weight The organic portion of cementum consists of • Collagen fibrils (90%) and non collagenous protein (10%) • Type I collagen is the predominant collagen of cementum. • Noncollagenous proteins identified in cementum include: polysaccharides (proteoglycans), glycoproteins, and, phosphoproteins. • Also, Alkaline phosphatase, bone sialoprotein, dentine matrix protein, dentine sialoprotein, fibronectin, osteocalcin, osteonectin, osteopontin, proteoglycanes, proteolipids, and several growth factors. • Enamel proteins also have been suggested to be present in cementum

7. Steps of cementogenesis • Itconsistsoftwosteps • 1-OrganicMatrix Formation • 2-Mineralization

8. Cementogenesis Hard tissue formation reaches the future CEJ [cervical loop] HERS proliferates downwards Inner cells induce dental papillae cells to differentiate into odontoblasts Formation of radicular predentin Rupture of HERS [cell rest of mallassez] Thedental follicle comes in contact with dentin matrix Differentiation of cementoblasts & cementum formation.

9. Following complete crown formation, hertwig's epithelial root sheath (which is driven by the coronoapical expansion of the inner and outer enamel epithelium) transmits an inductive message to the ectomesenchymal peripheral pulp cells, potentially by secreting some enamel proteins. • These cells develop into odontoblasts and form a layer of predentine. • The ectomesenchymal cells from the inner region of the dental follicle come into touch with the predentin shortly after hertwig's epithelial root sheath is disrupted. • After receiving a reciprocal inductive signal from dentine or the surrounding Hertwig's epithelial root sheath cells, infiltrating dental follicle cells develop into cementoblasts, which create cementum. • Apoptosis occurs in the cells of the Hertwig's epithelial root sheath, and some cells from the fragmented root sheath form discrete masses surrounded by a basal lamina, known as epithelial cell rests of Malassez, which persist in the mature periodontal ligament (ECRM are not simply residual cells, but they play an important role in PDL maintenance and regeneration).

10. Organic matrix formation • the newly differentiated cementoblasts first elaborate the organic matrix or "cementoid". That consists of collagen fibres and ground substances. • The cementoblast extends its process (fibroblast-like behaviour) into unmineralized dentin and deposits collagen intrinsic fibres that mix with the collagen fibres of dentin matrix. • The mineralization of mantel dentine begins internally and does not reach the dentine surface until the fibres intertwine, at which point the mineralization spreads and the dentine cementum junction is formed. • Adjacent fibroblasts of the dental follicle produce collagen fibres that become embedded in the cementum matrix, allowing the tooth to be attached to the surrounding bone.

11. Perforating fibres or "Sharpey's fibres" are the embedded section of periodontal ligament fibres in the cementum . Then the cementoblasts have now entered a quiescent stage near the cementum front, ready to act when needed, whether for continued growth or repair. • The cementum is applied in successive layers or increments (rhythmic procedure) until it reaches its ultimate thickness. • Cementoblasts become entrapped on the cementum matrix and reside in lacunae in the apical part of the root as cementum deposition progresses. • The integration of some cementoblasts within the matrix appears to be due to the cementoblasts' rapid matrix deposition. • On the cementum surface lining, a thin layer of cemented tissue is normally visible.

12. Mineralization • Mineralization begins when some cementoid tissue has been set down. Calcium and phosphate ions found in tissue fluids are deposited in the matrix. Crystals of hydroxyl apatite are organized along the fibrils. Apatite crystals can be seen aggregated into groups of nucleation centres, similar to bone calcifications.

13. Cementoid • Growth of cementum is a rhythmic process, as a new layer of cementoid is formed, the old ones become calcified. • A thin layer of cementoid tissue is usually observed on the cemental surface. • The cementoid tissue is formed of the uncalcified cementum matrix

14. Types of cementum 1-Acellular Cementum ( extrinsic fibers cementum ) ( primary cementum ) 2-Cellular Cementum ( intrinsic fibers cementum) (secondary cementum ) 3-Intermediate Cementum. 4-Acellular Afibrillar Cementum

15. Acellular Cementum • It is formed early in tooth development and is crucial during the initial stages of tooth eruption and root development • This type of cementum does not contain cementocytes • Forms majority of the primary cementum • Seen more in the cervical and middle thirds of the root. • Contains a lot of extrinsic fibers (Sharpey’s fibers) • The diameter of extrinsic fibers is roughly calculated at 3—6 μm. They show branching and anastomosing • The main function of this type of cementum is tooth anchorage • This type of cementum is formed slowly, hence the incremental lines are more close to each other.

16. AC

17. Cellular cementum • This type of cementum contains cementocytes within the lacunae. • Usually formed after eruption hence forms majority of secondary cementum. • Seen more towards the apical part of the root. • This type of cementum is formed rapidly so the incremental lines are far from each other. • Lacunae with canaliculi are seen, these canaliculi are directed towards the periodontal ligament ( source of nutrient material to the cementocytes ) . • Mainly involved in repair and adaptation of cementum ( functioning cementum )

18. Cementocyte cell Ground section Decalcified section

19. intermediate cementum • It is a highly calcified non-collagenous material that is nearly identical in its structure to the aprismatic enamel. • It is situated between the granular layer of tomes and cementum. It is restricted in the root apex as patches because the Cells of Hertwig’s epithelial root sheath become trapped in a rapidly deposited dentin or cementum • It has been suggested that the intermediate cementum is formed by the inner enamel epithelium of the root sheath. • The function of intermediate cementum is probably to seal the surface of the sensitive root dentin and cement the dentin with cementum. • Some investigators reported that no such type of cementum is found.

20. Acellular afibrillar cementum AAC • cementum overlaps the enamel for a short distance just coronal to the cemento – enamel junction . This occurs due to early separation of reduced enamel epithelium near the cervix , allowing the dental follicle to come in contact with the enamel surface. then the Connective tissue-derived cells deposit AAC. On enamel surface

21. Incremental Lines Of Cementum • Both cellular and acellular cementum are formed in successive layers. • The intervals between these successive depositions are represented by the incremental lines of Salter • Histochemical studies indicate that the incremental lines of salter are highly mineralized areas with less collagen and more ground substance. • Deposition of cementum probably continues throughlife.

22. Cementodentinal Junction • The junction between cementum and dentin is smooth in permanent dentition and sometimes scalloped in deciduous dentition. • Sometimes dentin is separated from cementum by a zone known as intermediate cementum. This zone has features of both the cementum and dentin. • This appears as a structure-less layer and is also called as “ hyaline layer of Hopewell smith”

23. Cemento -Enamel Junction

24. Cementoenamel Junction • The relation between cementum and enamel is variable. It is generally seen in three types – • 1. Point junction (butt junction)– cementum and enamel meet each other at a sharp point. • 2. Overlap junction – cementum overlaps the enamel for a short distance. This occurs due to early separation of reduced enamel epithelium near the cervix early. allowing dental follicle to come in contact with the enamel surface. • 3. Gap junction – cementum and enamel do not meet. This happens due to delay separation of Hertwig’s epithelial root sheath at the cervical area so, the root cementum failed to deposit.

25. Cells Of Cementum • Cementoblast • Cementoclast • Cementocytes

26. Cementoblast • It is the formative cell of cementum • Cementoblasts are large cuboidal cells containing basophilic cytoplasm and vesicular nuclei. • Cementoblasts show all features characteristic of cells capable of protein synthesis and secretion. • They exhibit a well-developed rough endoplasmic reticulum, and Golgi apparatus, numerous mitochondria, a large nucleus that contains prominent nucleoli, and abundant cytoplasm. • Cementoblasts lay down the organic matrix with collagenous fibers where the hydroxyapatite becomes crystallized

27. Cementocyte • Cementocyte is similar to osteocyte. It is seen in a space designated as a lacuna. The cell is spider shape & possess radiating processes mostly directed toward the periodontal ( source of nutrition ) • These processes may branch & frequently anastomose with those of neighboring cells. • The cytoplasm of cementocytes in deeper layers of cementum contains few organelles, the endoplasmic reticulum appears dilated, & mitochondria are spares. • These characteristics indicated that cementocyte is marginally active cells.

28. Cementoclast • It resembles osteoclast. • It is mononucleated or multinucleated giant cells, often located in Howship’s lacunae • It originated from circulating monocytes. • Resorption of cementum occurs under certain conditions Like tooth shedding , trauma, pathological diseases, Calcium deficiency.

29. Functions of Cementum • Anchorage: the primary function of cementum is act as a medium for attachment of collagen fibers that bind the tooth to alveolar bone. • Adaptation:deposition of cementum helps to compensate for the occlusal/ incisal wear of the tooth substance. • Repair: minor root fractures and resorption is repaired by deposition of new cementum. • The types of repair of cementum : anatomic repair – repair reestablishes the outline of the root. Functional repair – if the resorption or defect is large then only a thin layer of cementum is deposited and alveolar bone proliferates so that a proper functional relationship is established. • The anatomical root outline is not reconstructed and a bay-like recess remains.

30. Age Change of Cementum Permeability ( by age the permeability of cementum decreases gradually ). The permeability from the periodontal side is lost except in the most recently formed layer of cementum. While the permeability from the dentin side remains only in the apical region • Hypercementosis Hypercementosis is an abnormal thickening of tcementum. It may affect one tooth, entire dentition, or only a part of the tooth. If the overgrowth improves the functional quality of cementum, it is termed cementum hypertrophy. While, if the overgrowth occurs associated with a pathological condition, it is termed cementum hyperplasia.

32. Clinical consideration • Cementum is painless to scale and will repair itself by further deposition. • Cementum serves to seal the ends of the dentinal tubules to decrease root sensitivity and to prevent the spread of periodontal-originated infection to the pulp. • Cementum has more resistance to resorption than bone • This difference in resistance may be attributed to that, the bone is richly vascularized, whereas cementum is avascular.For this reason, the proper orthodontic movement is made possible without cementum resorption but bone resorption leads to tooth migration. • Cementum resorption may occur after trauma or excessive forces or improper orthodontic treatment. • Cementum resorption ( root resorption ) occurs during tooth shedding ( physiological process )

33. References • Nanci a: wheeler's dental anatomy, physiology, and Occlusion.11th edition - November 9, 2019 • Kumar GS: Orban’s oral histology and embryology. 15Th Edition 2019 • Avery: essentials of oral histology and embryology. A Clinical approach, 3rd edition, elsevier's publisher, Philadelphia. • https://2u.pw/oERMmFM • https://2u.pw/tDbjzPM