Recent advances in the molecular biology of ameloblast gene products have revealed the enamel matrix to be highly heterogeneous. It contains proteins encoded by a number of different genes. Listed here are the principal proteins in the extracellular matrix of the developing enamel:  Amelogenins, important proteins in establishing and maintaining the spacing between enamel rods in early stages of enamel development.
 Ameloblastins, signaling proteins produced by ameloblasts from the early secretory to late maturation stages. Their function is not well understood; however, their developmental pattern suggests that ameloblastins play a much broader role in amelogenesis than the other proteins. Ameloblastins are believed to guide the enamel mineralization process by controlling elongation of the enamel crystals and to form junctional complexes between individual enamel crystals.
 Enamelins, proteins distributed throughout the enamel layer. These proteins undergo proteolytic cleavage as the enamel matures. Low-molecular-weight products of this cleavage are retained in the mature enamel, often localized on the surface of enamel crystals.
 Tuftelins, the earliest detected proteins located near the dentinoenamel junction. Their acidic and insoluble nature aids in the nucleation of enamel crystals. Tuftelins are present in enamel tufts and account for hypomineralization, i.e., enamel tufts have a higher percentage of organic material than the remainder of the mature enamel.
The maturation of the developing enamel results in its continued mineralization so that it becomes the hardest substance in the body. Amelogenins and ameloblastins are removed during enamel maturation. Thus, mature enamel contains only enamelins and tuftelins. The ameloblasts degenerate after the enamel is fully formed, at about the time of tooth eruption through the gum.
Cementum covers the root of the tooth.
The root is the part of the tooth that fits into its alveolus, or socket in the maxilla or mandible. Cementum is a thin layer of bonelike material that is secreted by cementocytes, cells that closely resemble osteocytes. Like bone, cementum is 65% mineral. The lacunae and canaliculi in the cementum contain the cementocytes and their processes, respectively. They resemble those structures in bone that contain osteocytes and osteocyte processes.
Unlike bone, cementum is avascular. Also, the canaliculi in cementum do not form an interconnecting network. A layer of cementoblasts (cells that resemble the osteoblasts of the surface of growing bone) is seen on the outer surface of the cementum, adjacent to the periodontal ligament.
Collagen fbers that project out of the matrix of the cementum and embed in the bony matrix of the socket wall form the bulk of the periodontal ligament. These fibers are another example of Sharpey’s fbers (Fig. 16.15). In addition, elastic fibers are also a component of the periodontal ligament. This mode of attachment of the tooth in its socket allows slight movement of the tooth to occur naturally. It also forms the basis of orthodontic procedures used to straighten teeth and reduce malocclusion of the biting and grinding surfaces of the maxillary and mandibular teeth. During corrective tooth movements, the alveolar bone of the socket is resorbed and resynthesized, but the cementum is not.
Dentin is a calcified material that forms most of the tooth substance.