Cementum and Cementogenesis PDF

Cementum (C.) is a thin layer of calcified, avascular c.t. that cover the dentin of the root. Cementum is the least known of all the mineralized tissue in the body. For example, very little is known about the origin and differentiations of C.forming cells (cementoblasts) and it has been questioned...

Cementum (C.) is a thin layer of calcified, avascular c.t. that cover the dentin of the root. Cementum is the least known of all the mineralized tissue in the body. For example, very little is known about the origin and differentiations of C.forming cells (cementoblasts) and it has been questioned whether these cells are a subpopulation of osteoblasts or have a unique type. Its known that both of these cells have receptors for parathormone(PTH) and parathormone receptor protein(PTHrP). Furthermore, they respond similarly to many of the factors that regulate their activity. C. varies in thickness at different levels of the root. Its thickest at the root apex and in the interradicular areas of multirooted teeth and thinner cervically. C. is contiguous with p.d.l. on its outer surface and is firmly adherent on its deep surface with dentin. Its prime function is to give attachement to collagen fibers of p.d.l. It therefore is a highly responsive mineralized tissue, maintaining the integrity of the root, healping to maintain the tooth in its functional position, and being involved in tooth repair and regeneration.  Cementogenesis 1- Matrix formation 2- Maturation Collagen Ground Hydroxy apatite fiber type I substance crystals  Cementogenesis For cementogenesis to begin, Hertwig epithelial root sheath (HERS) must fragment. HERS is a collar of epithelial cells derived from the apical prolongation of the enamel organ. Once the root sheath disintegrates, the newly formed surface of root dentin comes into contact with the undifferentiated cells of the dental sac. This then stimulates the activation of cementoblasts to begin cementogenesis. The external shape of each root is fully determined by the position of the surrounding Hertwig epithelial root sheath. It is believed that either 1) HERS becomes interrupted; 2) infiltrating dental sac cells receive a reciprocal inductive signal from the dentin; or 3) HERS cells transform into cementoblasts. After some cementum matrix has been laid down, its mineralization begins. Mineralization of cementoid is a highly ordered event and not the random precipitation of ions into an organic matrix. Gla proteins osteocalcin and osteonectin act as nucleators for mineralization, due to their strong affinity for calcium, BSP (bone sialoprotein) and alkaline phosphatase promote mineralization while osteopontin regulate growth of apatite crystals. The major proteoglycan located exclusively in the nonmineralized cementum was keratan sulfates–lumican and fibromodulin. This finding suggests that they have a regulatory role in cementum mineralization. Insulin like growth factor (IGF) present in developing and matured cementum monitors mineralization and controls cell differentiation. Cementum is less readily resorbed than bone, a feature that is important for permitting orthodontic tooth movements. The reason for this feature is unknown but it may be related to: 1- Differences in biological properties between bone and c.. 2- The properties of cementoid tissue. 3- The increase density of Sharpey fibers (particularly in acellular c.). 4-The proximity of epithelial cell rest of Malessez to the root surface. Unlike bone , c. does not have a lamellar appearance and has no marrow spaces. Hertwig’s epithelial root sheath at end of forming root. At side of root, sheath is broken up, and cementum formation begins  1- Matrix formation Cementum is formed during root Cementoblasts formation D HER Future C E J Epith. Diaph.  Chemical Composition 45-50 % Inorganic 50-55% Organic substances substances Collagen Hydroxyapatite crystals protein Polysaccharides Trace elements Cementum contains the greatest amount of fluoride in all mineralized tissues  Types Of Cementum 1- Acellular cementum 2- Cellular cementum 3- Intermediate cementum 4- Afibirllar cementum  Cementum Structure Cementocytes Malassez Cementoid layer Acellular cementum Cellular cementum Acellular Cementum Thickness is 20-50 µ. It is clear and structureless Covers the coronal half of the root. Incremental lines of Salter are parallel to the surface. Sharpey’s fibers space can be seen in it. Alternating layers of a cellular and cellular cementum could be seen.  Cellular Cementum Lacunae of cementocytes Incremental lines of Salter Cementocytes PDL side Dentin side Cementocytes  Cellular Cementum Viable Dentin side superficial cementocytes Degenerated deep layer’s cementocytes PDL side Fig. 11.20 Fibre orientation in acellular and cellular cementum. The root surface is seen in polarised light, the different colours reflecting different orientations of the collagen fibres. A = Acellular cementum; B = cellular cementum (Ground, longitudinal section; polarised light; × 50).  Intermediate Cementum Premature degeneration of epith. Root sheath of Hertwig ( after odontoblasts differentiation and before dentin formation) Contains entrapped epithelial cells It occur at apical 2/3 of premolars and molars roots and rare in incisors and deciduous teeth Intermediate Cementum  Cemento Enamel Junction 10% cementum and enamel doesn’t meet because of 60% 30% cementum cementum meets the enamel delayed separation of epith root sheath of Hertwig (area overlaps E in a sharp line of dentin not covered by C). (afibrillar cementum)  Cemento Dentinal Junction C D Smooth in permanent teeth Scalloped in deciduous teeth Incremental Lines of Salter In acellular C In cellular C They are hypermineralized area with less collagen fibers and more ground substance  Functions Of Cementum 1- Acts as a medium for attachment of collagen fibers of PDL (Sharpey’s fibers). 2- The continuous Cementoid T formation of cementum keeps the attachment apparatus intact. Cementoblast 3- Cementum deposition apically compensate for the attrition. 4- It is a major reparative tissue ( as in case of fracture or resorption of root) Cementoblast is a protein forming and secreting cell. Maturation occur layer Collagen fibers + by layer for the ground substance. collagen D fibers Cementum Large open face nucleus Cementoblasts Cementoblast RER Cementoid layer Golgi apparatus Mitochondria Secretory granules Alkaline phosphatase  HYPERCEMENTOSIS  Is an abnormal thickening of cementum.  May be diffuse or circumscribed.  May affect all teeth of the dentition, be confined to a single tooth, or even affect only parts of one tooth.  If the overgrowth improves the functional qualities of the cementum, it is termed cementum hypertrophy.  If the overgrowth occurs in non-functional teeth or if it is not correlated with increased function, its termed hyperplasia. Hypercementosis. Localised D D May affect one tooth or all teeth Hypercementosis Attached cementicle o  CLINICAL CONSIDERATIONS  Cementum is more resistant to resorption than is bone, & it is for this reason that orthodontic tooth movement is made possible. Cementum resorption can occur after trauma or excessive occlusal forces.  In most cases of repair, there is a tendency to re- establish the former outline of the root surface by cementum. This is called anatomic repair.  However, if only a thin layer of cementum is deposited on the surface of a deep resorption, the root outline is not reconstructed, & a bay like recess remains.  In such areas the periodontal space is restored to its normal width by formation of a bony projection, so that a proper functional relationship will result. the outline of the alveolar bone in these cases follows that of the root surface. This is called functional repair.