Orbans Oral Histology and Embryology has become the classic text for successive generations of dental students
The oral cavity contains a variety of hard tissues and soft tissues. The hard tissues are the bones of the jaws and the tooth. The soft tissues include the lining mucosa of the mouth and the salivary glands.
The tooth consists of crown and root. That part of the tooth visible in the mouth is called clinical crown; the extent of which increases with age and disease. The root
portion of the tooth is not visible in the mouth in health. The tooth is suspended in the sockets of the alveolar bone by the periodontal ligament. The anatomical crown is covered by enamel and the root by the cementum. Periodontium is the term given to supporting tissues of the tooth. They include the cementum, periodontal ligament and the alveolar bone. The innermost portion of the crown and root is occupied by soft tissue, the pulp. The dentin occupies the region between the pulp and enamel in the crown, and between pulp and cementum in the root.
DEVELOPMENT OF TOOTH
The tooth is formed from the ectoderm and ectomesenchyme. The enamel is derived from the enamel organ which is differentiated from the primitive oral epithelium lining the stomodeum (primitive oral cavity). Epithelial mesenchymal interactions take place to determine the shape of the tooth and the differentiation of the formative cells of the tooth and the timing of their secretion. The ectomesenchymal cells which are closer to the inner margins of the enamel organ differentiate into dental papilla and the ectomesenchymal cells closer to the outer margins of the enamel organ become dental follicle. Dentin and pulp are derivatives of dental papilla while cementum, periodontal ligament and alveolar bone, are all derivatives of dental follicle. The cells that form these tissues have their names ending in blast.
Thus, ameloblast produces enamel, odontoblast dentin, cementoblast, cementum and osteoblast bone. These synthesizingcells have all the features of a protein secreting cell—well developed ribosomes and a rough endoplasmic reticulum (ER), Golgi apparatus, mitochondria and a vesicular nucleus, which is often polarized. The cells that resorb the tissues have their names ending in ‘clast’. Thus, osteoclast resorbs bone, cementoclast, cementum and odontoclast resorbs all the dental tissues. The ‘clast’ cells have a similar morphology in being multinucleated giant cells. Their ultra structural features include numerous lysosomes and ingested vacuoles.
Dentin is the first hard tissue of the tooth to form. Enamel starts its formation after the first layer of dentin has formed. The enamel formation is from its junction with dentin outwards, first in the cuspal/incisal and later in the cervical regions. Dentin formation is similar, but from the dentinoenamel junction, the formation is pulp ward. Cementum formation occurs after the root form, size, shape and number of roots is outlined by the epithelial root sheath and dentin is laid down in these regions. Formation of enamel, dentin and cementum takes place as a daily event in phases or in increments, and hence they show incremental lines. In dentin and cementum formation, a layer of uncalcified matrix forms first, followed by its mineralization. While in enamel formation enamel matrix is calcified, but its maturation or complete mineralization occurs as a secondary event. Mineralization occurs as a result of supersaturation of calcium and phosphorus in the tissue fluid. The formative cells concentrate the minerals from calcium phosphate (apatite) and secrete them into the organic matrix, in relation to specific substances like collagen, which act as attractants or nucleators for mineralization. The mechanism of mineralization is quite similar in all the hard tissues of tooth and in bone