Cementum is a specialized bony substance covering the root of a tooth. In layers, the enamel being the top, the cementum would be the third to last. From top to bottom: enamel is first, dentin is next, then cementum , then root, then pulp. Cementum is excreted by cells called cementoblasts within the root of the tooth and is thickest at the root apex. Its coloration is yellowish and it is softer than either dentin or enamel.
The principle role of cementen within the tooth is to serve as a medium by which the periodontal ligaments can attach to the tooth for stability. Hence, its bottom surface is tangent to the periodontal ligaments running through the jaw (via collagen fibers), and the upper portion of the surface is firmly cemented to the dentin of the tooth. It also meets the enamel lower on the tooth at the cemento-enamel junction. Here the cementum is known as acellular cementum due to its lack of cellular components, and covers approximately 1/3-1/2 of the root. The more permeable form of cementen, cellular cementen, covers 1/3-1/2 of the root apex, where it binds to the dentin. There is also a third type of cementen, afibrillar cementum, which sometimes extends onto the enamel of the tooth.
Cementen's chemical makeup of is similar to that of bone, but it lacks vascularization. Volumetrically, it is approximately 45% inorganic material (mainly hydroxyapatite), 33% organic material (mainly collagen) and 22% water. Although it is only found in the root of humans, other mammals have dentin in the crowns of their teeth.
