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.

MANDIBULAR CUSPIDS

Mandibular canines are those lower teeth that articulate with the mesial aspect of the upper canine.

Facial: The mandibular canine is noticeably narrower mesidistally than the upper, but the root may be as long as that of the upper canine. In an individual person,the lower canine is often shorter than that of the upper canine. The mandibular canine is wider mesiodistally than either lower incisor. A distinctive feature is the nearly straight outline of the mesial aspect of the crown and root. When the tooth is unworn, the mesial cusp ridge appears as a sort of 'shoulder' on the tooth. The mesial cusp ridge is much shorter than the distal cusp ridge.

Lingual: The marginal ridges and cingulum are less prominent than those of the maxillary canine. The lingual surface is smooth and regular. The lingual ridge, if present, is usually rather subtle in its expression.

Proximal: The mesial and distal aspects present a triangular outline. The cingulum as noted is less well developed. When the crown and root are viewed from the proximal, this tooth uniquely presents a crescent-like profile similar to a cashew nut.

Incisal: The mesiodistal dimension is clearly less than the labiolingual dimension. The mesial and distal 'halves' of the tooth are more identical than the upper canine from this perspective. In the mandibular canine, the unworn incisal edge is on the line through the long axis of this tooth.

MANDIBULAR FIRST MOLAR

It is the first permanent tooth to erupt.

Facial Surface:- The lower first permanent molar has the widest mesiodistal diameter of all of the molar teeth. Three cusps cusps separated by developmental grooves make on the occlusal outline The mesiobuccal cusp is usually the widest of the cusps. The mesiobuccal cusp is generally considered the largest of the five cusps. The distal root is usually less curved than the mesial root.

Lingual: Three cusps make up the occlusal profile in this view: the mesiolingual, the distolingual, and the distal cusp which is somewhat lower in profile. The mesiobuccal cusp is usually the widest and highest of the three. A short lingual developmental groove separates the two lingual cusps

Proximal: The distinctive height of curvature seen in the cervical third of the buccal surface is called the cervical ridge. The mesial surface may be flat or concave in its cervical third . It is highly convex in its middle and occlusal thirds. The occlusal profile is marked by the mesiobuccal cusp, mesiolingual cusp, and the mesial marginal ridge that connects them. The mesial root is the broadest buccolingually of any of the lower molar roots. The distal surface of the crown is narrower buccolingually than the mesial surface. Three cusps are seen from the distal aspect: the distobuccal cusp, the distal cusp, and the distolingual cusp.

Occlusal There are five cusps. Of them, the mesiobuccal cusp is the largest, the distal cusp is the smallest. The two buccal grooves and the single lingual groove form the "Y" patern distinctive for this tooth

Roots :-The tooth has two roots, a mesial and a distal.

Contact Points; The mesial contact is centered buccolingually just below the marginal ridge. The distal contact is centered over the distal root, but is buccal to the center point of the distal marginal ridge.

Roots: Lower molars have mesial and distal roots. In the first, molar, the mesial root is the largest. It has a distal curvature. The distal root has little curvature and projects distally.

MAXILLARY CENTRAL INCISORS

Viewed mesially or distally, a maxillary central incisor looks like a wedge, with the point of the wedge at the incisal (cutting) edge of the tooth.

Facial Surface- The mesial margin is nearly straight and meets the incisal edge at almost a 90° angle, but the distal margin meets the incisal edge in a curve. The incisal edge is straight, but the cervical margin is curved like a half moon. Two developmental grooves are on the facial surface.

Lingual Surface:- The lingual aspect presents a distinctive lingual fossa that is bordered by mesial and distal marginal ridges, the incisal edge, and the prominent cingulum at the gingival. Sometimes a deep pit, the lingual pit, is found in conjunction with a cingulum.

Incisal: The crown is roughly triangular in outline; the incisal edge is nearly a straight line, though slightly crescent shaped

Contact Points: The mesial contact point is just about at the incisal, owing to the very sharp mesial incisal angle. The distal contact point is located at the junction of the incisal third and the middle third.

Root Surface:-As with all anterior teeth, the root of the maxillary central incisor is single. This root is from one and one-fourth to one and one-half times the length of the crown. Usually, the apex of the root is inclined slightly distally.