Cementum is a mineralized connective tissue that is present on the root of the tooth. It extends from cemento-enamel junction, where it is thinnest, covers the entire surface of the root up to the apex, where it is thickest. Though sometimes, it may extend a little onto the inner dentinal wall of the root canal and also extend coronal to the cemento-enamal junction, onto the enamal surface of the crown. All although cementum is a part of the tooth but due to its intermediary position between the radicular dentin and periodontal ligament, cementum functionally belongs to the periodontium. Development. During the stages of tooth development, cementum develops in the Advanced Bell stage. In the tooth germ, the Hertwig's Epithelial Root Sheath plays a key role in its development. The cells that deposit cementum are called Cementoblasts. Periodontium. If we take a look at the periodontium microscopically, we see osteoblasts lining the alveolar bone and fibroblasts in the periodontal ligament. On focusing on the histology of cementum, we see two types of cells. Cementoblasts, which are present on the surface of the cementum towards the periodontal ligament and cementocytes, which are present within the cementum. They are actually intrapped cementoblasts with numerous cytoplasmic extensions predominantly directed towards the periodontal ligament. If we look at the organic matter, we see 2 types of collagen fibers. Extrinsic fibers- these fibers run perpendicular to the surface of cementum. They are extensions of the Principal fibers of the periodontal ligament and are therefore made by the fibroblasts. They are 5-7 microns wide. Intrinsic fibers- these fibers run parallel to the surface of cementum. They are formed by the cementoblasts and are only 1-2 micron wide. Cementum has been classified in a number of ways. The first classification is based on the presence of cementocytes. Here cementum can be of 2 types. The cementum which contains cementocytes is called Cellular Cementum. Cellular Cementum has a rapid rate of deposition which leads to the entrapment of cementoblasts, presence of a cementoid layer and incremental lines which are positioned wide apart. Whereas, the cementum with no cementocytes is called Acellular Cementum. Acellular Cementum has slow rate of deposition which results in no entrapment of cementoblasts, absence of cementoid layer and incremental lines which are closer together. The 2nd classification is based on the nature and origin of the organic matter. Here, cementum can be of 4 types. The cementum which contains only the extrinsic fibers placed perpendicular to the surface, is called Extrinsic Fiber Cementum. The cementum which contains only the intrinsic fibers placed parallel to the surface, is called Intrinsic Fiber Cementum. The cementum which contains both intrinsic fibers and extrinsic fibers placed perpendicular to each other, is called Mixed Fiber Cementum. The cementum which contains no collagen fibers, is called Afibrillar Cementum. The most commonly used classification is Schroeder Classification, which is based on the presence of cementocytes and nature and origin of the organic matter. Here, cementum is of 5 types. Acellular Extrinsic Fiber Cementum. In single rooted teeth Acellular Extrinsic Fiber Cementum covers the whole root or around 60-90% of the root, towards the cervical line. Whereas, in multi-rooted teeth, it covers the cervical 2/3rd of the root. Its thickness ranges from 50-200 microns. The cementocytes are absent, among fibers only extrinsic fibers are present, showing the cementum's major role in the anchorage of the tooth. Cellular Intrinsic Fiber Cementum. In single rooted teeth it may be absent entirely or present in the apical portion of the root. In multi-rooted teeth it is present in the apical third and the furcation area. Histologically, it contains cementocytes and has only the intrinsic fibers, showing that they do not play a role in the anchorage of the tooth, but a major role in the adaptation and repair. Generally, Cellular Intrinsic Fiber Cementum is present as part of the Cellular Mixed Stratified Cementum. In which, it is present intermittently between Acellular Extrinsic Fiber Cementum. Here it is interesting to note that the transition of cementum from an acellular type to a cellular variety is still not fully understood. Though, few researchers had suggested, that functional stimuli may be the reason. That is, the force generated by the tooth contact and mastication, should be held responsible for the onset of Cellular Intrinsic Fiber Cementum in the apical third. But many researchers did not accept this theory. They said that even teeth with no antagonist have cellular cementum. Moreover, teeth which are impacted, show cellular cementum. Cellular Mixed Stratified Cementum. In single rooted teeth, this type of cementum is not generally seen, if present is seen near the root apex. In multi-rooted teeth it is seen in the apical third and furcation area of the root. It is around 100-1000 microns thick. This cementum shows both Cellular Intrinsic Fiber Cementum, with intrinsic fibers and cementocytes and a Cellular Extrinsic Fiber Cementum, with extrinsic fibers but no cementocytes. Both these types are present in alternate stratified layers. Here it is important to understand, that the extrinsic fibers entrapped in between the layers lose their contact with the Principal Fibers of the periodontal ligament. Therefore, they do not assist in the anchorage. The anchorage is provided by the extrinsic fibers present only in the peripheral most layer. A cementum type called Cellular Mixed Fiber Cementum is also known, in which the cementum shows presence of cementocytes, intrinsic fibers and extrinsic fibers, in a non-stratified mixed manner. But it was not classified by Schroeder in his classification. Acellular Afibrillar Cementum. This type of cementum is occasionally present in any type of tooth, as isolated patches on the enamel, just coronal to the cemento-enamel junction. It consists of a mineralized matrix with no cementocytes and no collagen fibers. It has no function in tooth anchorage or repair. Intermediate Cementum. Now Intermediate Cementum has confused researchers for long and its origin is still controversial. In all teeth, at the junction of of dentin and cementum, a narrow homogeneous structure is seen, just 5-8 microns thick. It is present on the entire root. Now some researchers like, Arthur Hopewell-Smith, considered this structure as part of dentin. Whereas, some researchers like, Bencze, considered it as a part of cementum. So currently, the understanding for this narrow structure is that, in the region of Acellular Extrinsic Fiber Cementum, it is referred to as the Hyaline Layer of Hopewell-Smith. Whereas, in the region of Cellular Mixed Stratified Cementum, it is referred to as the Intermediate Cementum.