Time for tooth development

Entire primary dentition initiated between 6 and 8 weeks of embryonic development.
Successional permanent teeth initiated between 20th week in utero and 10th month after birth Permanent molars between 20th week in utero (first molar) and 5th year of life (third molar)

Dentinogenesis

Dentin formation, known as dentinogenesis, is the first identifiable feature in the crown stage of tooth development. The formation of dentin must always occur before the formation of enamel. The different stages of dentin formation result in different types of dentin: mantle dentin, primary dentin, secondary dentin, and tertiary dentin.

Odontoblasts, the dentin-forming cells, differentiate from cells of the dental papilla. They begin secreting an organic matrix around the area directly adjacent to the inner enamel epithelium, closest to the area of the future cusp of a tooth. The organic matrix contains collagen fibers with large diameters (0.1-0.2 μm in diameter). The odontoblasts begin to move toward the center of the tooth, forming an extension called the odontoblast process. Thus, dentin formation proceeds toward the inside of the tooth. The odontoblast process causes the secretion of hydroxyapatite crystals and mineralization of the matrix. This area of mineralization is known as mantle dentin and is a layer usually about 150 μm thick.

Whereas mantle dentin forms from the preexisting ground substance of the dental papilla, primary dentin forms through a different process. Odontoblasts increase in size, eliminating the availability of any extracellular resources to contribute to an organic matrix for mineralization. Additionally, the larger odontoblasts cause collagen to be secreted in smaller amounts, which results in more tightly arranged, heterogenous nucleation that is used for mineralization. Other materials (such as lipids, phosphoproteins, and phospholipids) are also secreted.

Secondary dentin is formed after root formation is finished and occurs at a much slower rate. It is not formed at a uniform rate along the tooth, but instead forms faster along sections closer to the crown of a tooth. This development continues throughout life and accounts for the smaller areas of pulp found in older individuals. Tertiary dentin, also known as reparative dentin, forms in reaction to stimuli, such as attrition or dental caries.

The dentin in the root of a tooth forms only after the presence of Hertwig's epithelial root sheath (HERS), near the cervical loop of the enamel organ. Root dentin is considered different than dentin found in the crown of the tooth (known as coronal dentin) because of the different orientation of collagen fibers, the decrease of phosphoryn levels, and the less amount of mineralization.

Disturbances to interarch alignment are

a. Excessive overbite where the incisal edge of the maxillary incisors extend to the cervical third of the mandibular incisors

b. Excessive overjet where the maxillary teeth overjet the mandibular teeth by more than 3mm

c. End-to-end relationship: edge-to edge bite where the anterior teeth meet at there incisal edge with  no overjet or overbite; cusp-to bite where the posterior teeth meet  cusp to cusp with no interdigitation

d. Crossbite where the normal faciolingual relationship of the maxillary to the mandibular teeth is altered for the anterior.teeth. the mandibular  tooth or teeth are facial  rather than lingual to the maxillary teeth for the posterior teeth, normal inercuspaton is not seen

Permanent teeth

1. The permanent teeth begin formation between birth and 3 years of age (except for the third molars)

2. The crowns of permanent teeth are completed between 4 and 8 years of age, at approximately one- half the age of eruption

The sequence for permanent development

Maxillary                     

First molar → Central incisor → Lateral incisor → First premotar → Second pmmolar  → Canine → Second molar → Third molar

Mandibular

First molar → Central incisor → Lateral incisor → Canine → First premolar → Second premolar → Second molar → Third molar

Permanent teeth emerge into the oral cavity as

                                      Maxillary                       Mandibular

Central incisor               7-8 years                        6-7 years

Lateral incisor                8-9 years                        7-8 years

Canine                           11-12 years                    9-10 years

First premolar                10-Il years                      10-12 years

Second premolar            10-12 years                  11-12 years

First molar                       6-7 years                      6-7 years

Second molar                 12-13 years                    11-13 years

Third molar                      17-21 years                    17-21 years

The roots of the permanent teeth are completed between 10 and 16 years of age, 2 to 3 years after eruption

Maxillary Third Permanent Molar

They are the teeth most often congenitally missing

Facial: The crown is usually shorter in both axial and mesiodistal dimensions. Two buccal roots are present, but in most cases they are fused. The mesial buccal cusp is larger than the distal buccal cusp.

Lingual: In most thirds, there is just one large lingual cusp. In some cases there is a poorly developed distolingual cusp and a lingual groove. The lingual root is often fused to the to buccal cusps.

Proximal: The outline of the crown is rounded; it is often described as bulbous in dental literature. Technically, the mesial surface is the only 'proximal' surface. The distal surface does not contact another tooth.

Occlusal: The crown of this tooth is the smallest of the maxillary molars. The outline of the occlusal surface can be described as heart-shaped. The mesial lingual cusp is the largest, the mesial buccal is second in size, and the distal buccal cusp is the smallest.

Root Surface:-The root may have from one to as many as eight divisions. These divisions are usually fused and very often curved distally.

LOCATION OF THE TEETH

Normally, a human receives two sets of teeth during a lifetime.

The first (deciduous or primary) set consists of 20 teeth ("baby" teeth).

The second (permanent) set usually consists of 32 teeth. In each quadrant, there are eight permanent teeth: two incisors, one cuspid, two bicuspids, and three molars 

The tooth positioned immediately to the side of the midline is the central incisor, so called because it occupies a central location in the arch.

To the side of the central incisor is the lateral incisor. Next is the cuspid, then the two bicuspids (the first bicuspid, followed by the second bicuspid). The last teeth are three molars. After the second bicuspid comes the first molar, followed by the second molar, followed by the third molar or more commonly called the "wisdom tooth."

Another method of describing the location of teeth is to refer to them as anterior or posterior teeth .

Anterior teeth are those located in the front of the mouth, the incisors, and the cuspids. Normally, these are the teeth that are visible when a person smiles.

The posterior teeth are those located in the back of the mouth-the bicuspids and molars.