Dentin Histology of the Teeth

Dentin is a specialized kind of bone formed by the odontoblasts but different in the sense that it does not contain complete cells but only cellular extensions, i.e., cytoplasmic extensions from the odontoblasts. These cross the full thickness of the dentin from the odontoblastic cell body that lies at the border between dentin and dental pulp to the junction between dentin and enamel. The tiny canals that house the odontoblastic extensions are recognizable as evenly spaced tubuli. This tubular nature is the histologic hallmark for dentin, not only in teeth but also in odontogenic lesions in which the nature of each mineralized material may not be recognizable at first sight. Sometimes, in the dentin incremental lines that run parallel to the dentino-enamel junction, are observed, the so-called lines of von Ebner.

Te organic matrix laid down by the odontoblasts is called predentin; it is recognisable by its 
pink staining in hematoxylin- and eosin-stained sections. In this matrix, calcifcation occurs in the 
form of spherical masses which as they increase in size gradually fuse together, thus transforming the predentin into the darker staining mineralised dentin. At the border between dentin and predentin, these calcospherites can be frequently seen, either separate or already in contact with the calcified dentin.

Several types of dentin are discerned. The layer of dentin immediately adjacent to the border with the enamel is called the mantle dentin. The more centrally located part of the dentin is the circumpulpal dentin. This distinction is relevant as some structural tooth alterations are confined to the circumpulpal dentin, leaving the mantle dentin uninvolved, as will be discussed later on. Mantle dentin is separated from the circumpulpal dentin by a zone of disturbed dentin formation called globular dentin, so-called because of the presence of interglobular spaces due to defcient mineralisation at this site. Both mantle 
and circumpulpal dentin are taken together as primary dentin. Te dentin that is formed during 
the functional life of the tooth is called secondary dentin. Its life-long gradual deposition causes a decrease of the pulp chambers over the course of time. Tertiary, or reparative dentin is formed as a pulpal response to some kind of trauma. It differs from both primary and secondary dentin in that it has a more irregular structure, not only containing odontoblastic extensions but also complete cell bodies of odontoblasts and thus resembling bone. That is why it is sometimes also called osteodentin.
Dentin that has lost its organic content due to disappearance of odontoblasts with concomitant 
disintegration of intratubular cytoplasmic processes is said to contain dead tracts. If these empty tubuli calcify, sclerotic or transparent dentin is formed. Both dead tracts and sclerotic dentin are only visible in undecalcifed ground sections.