The tooth anatomy includes the crown, which is the portion of the tooth exposed to the oral cavity, and one or more roots, which are enveloped in bone and the periodontium.
The crown of each tooth has 5 surfaces, as follows:
Buccal (facing the cheek or lip)
Lingual (facing the tongue)
Mesial (between the teeth)
Distal (between the teeth)
Chewing (occlusal for molars and premolars, incisal for incisors and canines)
In the transverse section, the tooth has 3 distinct layers, as follows:
A surface enamel layer covering only the crown
An inner layer of dentin in both the crown and the root
The core area, known as the pulp, which contains nerves, arteries, and veins 
Radiographically, the layers are easily identifiable because they have different radiopacities. Enamel is the most mineralized of the calcified tissues of the body, and it is the most radiopaque of the 3 tooth layers. Dentin is less radiopaque than enamel and has a radiopacity similar to that of bone. The pulp tissue is not mineralized and appears radiolucent. 
The anatomy of the tooth is displayed in the image below.
Primary (ie, deciduous) teeth number 20,  and secondary (ie, adult) teeth number 32. A phase of mixed dentition exists, depending on the age of the patient (typically, 6-14 y). This phase is associated with simultaneous exfoliation or the eruption of primary and secondary teeth (see Tables 1-2).
Table 1. Calcification and Eruption Sequence of Primary Dentition (Open Table in a new window)
|Primary Teeth Calcification Begins (Weeks In Utero)||Enamel Completed (Months after Birth)||Eruption (Months after Birth)|
|First molar||14. 5-16.5||6||13-19|
Table 2. Calcification and Eruption Sequence of Secondary Dentition (Open Table in a new window)
|Permanent Teeth Calcification Begins (Months)||Eruption (Years)|