TYPES OF TEETH

The human permanent dentition is divided into four classes of teeth based on appearance and function or position.

Incisors, Canines, Premolars & Molars

The very first histological evidence of tooth development appear during the second month of intrauterine life. Calcification of deciduous incisors begins at 3-4 months in utero.

MAXILLARY CUSPIDS (CANINE)

The maxillary cuspid is usually the longest tooth in either jaw. canines are considered the corner stones of the dental arch They are the only teeth in the dentition with a single cusp.

Facial Surface:- The facial surface of the crown differs considerably from that of the maxillary central or lateral incisors. In that the incisal edges of the central and lateral incisor are nearly straight, the cuspid has a definite point, or cusp.  There are two cutting edges, the mesioincisal and the distoincisal. The distoincisal cutting edge is the longer of the two. The developmental grooves prominent on the facial surface  extending two-thirds of the distance from the tip of the cusp to the cervical line.  The distal cusp ridge is longer than the mesial cusp ridge

Lingual Surface:  Distinct mesial and distal marginal ridges, a well-devloped cingulum, and the cusp ridges form the boundries of the lingual surface. The prominent lingual ridge extends from the cusp tip to the cingulum, dividing the lingual surface into mesial and distal fossae.

Proximal: The mesial and distal aspects present a triangular outline. They resemble the incisors, but are more robust--especially in the cingulum region

Incisal: The asymmetry of this tooth is readily apparent from this aspect. It usually thicker labiolingually than it is mesiodistally. The tip of the cusp is displaced labially and mesial to the central long axis of this tooth.

Root Surface:-The root is single and is the longest root in the arch. It is usually twice the length of the crown.

CEMENTUM vs. BONE

Cementum simulates bone
1) Organic fibrous framework, ground substance, crystal type, development
2) Lacunae
3) Canaliculi
4) Cellular components
5) Incremental lines (also known as "resting" lines; they are produced by continuous but phasic, deposition of cementum)

Differences between cementum and bone
1) Cementum is not vascularized
2) Cementum has minor ability to remodel
3) Cementum is more resistant to resorption compared to bone
4) Cementum lacks neural component
5) Cementum contains a unique proteoglycan interfibrillar substance
6) 70% of bone is made by inorganic salts (cementum only 46%)

Relation of Cementum to Enamel at the Cementoenamel Junction (CEJ)

"OMG rule"

In 60% of the teeth cementum Overlaps enamel
In 30% of the teeth cementum just Meets enamel
In 10% of the teeth there is a small Gap between cementum and enamel

Cementum & Cementogenesis

Cementum formation is called cementogenesis and occurs late in the development of teeth. Cementoblasts are the cells responsible for cementogenesis. Two types of cementum form: cellular and acellular.

Acellular cementum forms first. The cementoblasts differentiate from follicular cells, which can only reach the surface of the tooth's root once Hertwig's Epithelial Root Sheath (HERS) has begun to deteriorate. The cementoblasts secrete fine collagen fibrils along the root surface at right angles before migrating away from the tooth. As the cementoblasts move, more collagen is deposited to lengthen and thicken the bundles of fibers. Noncollagenous proteins, such as bone sialoprotein and osteocalcin, are also secreted. Acellular cementum contains a secreted matrix of proteins and fibers. As mineralization takes place, the cementoblasts move away from the cementum, and the fibers left along the surface eventually join the forming periodontal ligmaments.

Cellular cementum develops after most of the tooth formation is complete and after the tooth occludes (in contact) with a tooth in the opposite arch. This type of cementum forms around the fiber bundles of the periodontal ligaments. The cementoblasts forming cellular cementum become trapped in the cementum they produce.

The origin of the formative cementoblasts is believed to be different for cellular cementum and acellular cementum. One of the major current hypotheses is that cells producing cellular cementum migrate from the adjacent area of bone, while cells producing acellular cementum arise from the dental follicle. Nonetheless, it is known that cellular cementum is usually not found in teeth with one root. In premolars and molars, cellular cementum is found only in the part of the root closest to the apex and in interradicular areas between multiple roots.

Nerve and vascular formation

Frequently, nerves and blood vessels run parallel to each other in the body, and the formation of both usually takes place simultaneously and in a similar fashion. However, this is not the case for nerves and blood vessels around the tooth, because of different rates of development.

Nerve formation

Nerve fibers start to near the tooth during the cap stage of tooth development and grow toward the dental follicle. Once there, the nerves develop around the tooth bud and enter the dental papilla when dentin formation has begun. Nerves never proliferate into the enamel organ

Vascular formation

Blood vessels grow in the dental follicle and enter the dental papilla in the cap stage. Groups of blood vessels form at the entrance of the dental papilla. The number of blood vessels reaches a maximum at the beginning of the crown stage, and the dental papilla eventually forms in the pulp of a tooth. Throughout life, the amount of pulpal tissue in a tooth decreases, which means that the blood supply to the tooth decreases with age. The enamel organ is devoid of blood vessels because of its epithelial origin, and the mineralized tissues of enamel and dentin do not need nutrients from the blood.