The Periodontium is the tissue which forms the supporting structure of a tooth. In it cementum lies between radicular dentin and periodontal ligament extending from cemento-enamel junction to the apical foramen. Formation of the cementum is called Cementogenesis which is carried out by cells called Cementoblasts. Differentiation of Cementoblasts. In the stages of tooth development, during the advanced bell stage, the Ameloblasts and Odontoblasts have started depositing enamel and dentin. As these depositions move cervically the inner and outer enamel epithelium at the cervical loop give an extension called the Hertwigs Epithelial Root Sheath. This bilayered sheath has Dental Papilla on one side and Dental sac on another. It sends signal to the cells of the dental papilla to differentiate into odontoblasts. These odontoblasts then start depositing the radicular dentin. This root dentin deposition sends signals for differentiation of Cementoblasts. Cementoblasts are known to differentiate from two sources. First Source, cells of the dental sac. After deposition of radicular dentin, the cells of Hertwigs Epithelial Root Sheath start degenerating. This degeneration exposes the ectomesenchymal cells of the dental sac to radicular dentin, which on receiving signals, differentiate into cementoblasts which subsequently deposit cementum. Second Source, cells of Hertwigs Epithelial Root Sheath. After deposition of radicular dentin the cells of Hertwigs Epithelial Root Sheath receives signals due to which they themselves undergo epithelial mesenchymal transformation and differentiate into cementoblasts, which subsequently deposit cementum. Cementoblasts. In the periodontium, on looking closely, we see cementoblasts present at the periphery of the cementum towards the periodontal ligament. Cementoblast differentiating from the cells of dental sac are similar to osteoblasts. They are plump, cuboidal or slightly flattened cells. Whereas cementoblasts differentiating from the cells of Hertwigs Epithelial Root Sheath are similar to fibroblasts. They are fusiform shaped cells. Cementoblasts are mononuclear cells with abundant rough endoplasmic reticulum, golgi apparatus and mitochondria. Cementogenesis. It has been shown that cementoblasts differentiate from the cells of dental sac are involved in the formation of cellular intrinsic fiber cementum. Whereas cementoblast differentiating from the cells of Hertwigs Epithelial Root Sheath are involved in the formation of acellular extrinsic fiber cementum. Cementum has two components, organic component and mineralized component. Formation of cementoid. As mineralization always follows organic matter deposition, there is an outermost thin layer of non-mineralized cementum just adjacent to the cementoblast, called cementoid. As the cells deposit cementum and move backwards the old layer of cementoid gets mineralized to form cementum and a new layer of cementoid gets deposited. Formation of organic matter. The organic matter of cementum is mainly formed of collagen fibers in which majority of them are type 1 collagen. These type 1 Collagen fibers are formed from two sources. First Source is the cementoblasts of cementum which secrete collagen fibers parallel to the surface of cementum called intrinsic fibers. They are 1-2 microns wide. The Second Source are the fibroblasts of periodontal ligament which form collagen fibers perpendicular to the cementum surface called extrinsic fibers. They are 5-7 microns wide. Fibers which help in tooth anchorage are called Sharpeys fibers. Mineralization. Mineralization is mainly done by hydroxyapetite crystals which are 55 nanometer wide and 8 nanometers thick. Incremental deposition. Cementum is deposited in a regular rhythmic manner with periods of activity and periods of rest, resulting in evenly spaced incremental lines or Incremental Lines of Salter. It is known that rate of formation of Cellular cementum is more than that of Acellular cementum because of which in Cellular cementum the incremental lines are more far apart and there is entrapment of cementoblasts within its own matrix, forming cementocytes. Formation of cementodentinal junction. As the odontoblasts differentiate, they start depositing dentin. This predentin layer is predominantly made of collagen fibers and lacks odontoblastic processes. After getting appropriate signals, the cementoblasts differentiate and in turn start depositing cementoid. Again predominantly made of collagen fibers which intermingle with fibers of predentin, such that after mineralization of both tissues a strong cemento-dentinal junction develops. As the first layer of radicular dentin lacks odontoblastic process, this layer is called the hyaline layer of Hopewell Smith, present just outside the Tomes Granular layer. Cementodentinal Junction in permanent teeth is straight and smooth, whereas in deciduous teeth is sometimes scalloped. Formation of cemento-enamel junction. During tooth development, ameloblast deposit enamel at cervical loop. Adjacent to it the Hertwigs Epithelial Root Sheath which gives way for dentin deposition. Then Hertwig's Epithelial Sheath is replaced by the differentiating cementoblasts, which in turn deposits cementum. This develops the cemento-enamel junction which is edge to edge type or sharp type. Sometimes as Hertwig's Epithelial Root Sheath gives way to cementoblasts, few sheath cells fail to move away from the radicular dentin, thus not allowing differentiation of cementoblasts and deposition of cementum. Resulting in failure of cementum deposition thus forming Gap type of cemento-enamal junction. But more commonly, the Hertwig's Epithelial Root Sheath correctly gives way to cementoblast differentiation with subsequent deposition of cementum. But if the reduced enamel epithelium is unable to carry out its protective role perfectly it may expose the enamel slightly, resulting in cementum deposition forming an overlap type of cemento-enamel junction.