Crown stage

Hard tissues, including enamel and dentin, develop during the next stage of tooth development. This stage is called the crown, or maturation, stage by some researchers. Important cellular changes occur at this time. In prior stages, all of the inner enamel epithelium cells were dividing to increase the overall size of the tooth bud, but rapid dividing, called mitosis, stops during the crown stage at the location where the cusps of the teeth form. The first mineralized hard tissues form at this location. At the same time, the inner enamel epithelial cells change in shape from cuboidal to columnar. The nuclei of these cells move closer to the stratum intermedium and away from the dental papilla.

The adjacent layer of cells in the dental papilla suddenly increases in size and differentiates into odontoblasts, which are the cells that form dentin. Researchers believe that the odontoblasts would not form if it were not for the changes occurring in the inner enamel epithelium. As the changes to the inner enamel epithelium and the formation of odontoblasts continue from the tips of the cusps, the odontoblasts secrete a substance, an organic matrix, into their immediate surrounding. The organic matrix contains the material needed for dentin formation. As odontoblasts deposit organic matrix, they migrate toward the center of the dental papilla. Thus, unlike enamel, dentin starts forming in the surface closest to the outside of the tooth and proceeds inward. Cytoplasmic extensions are left behind as the odontoblasts move inward. The unique, tubular microscopic appearance of dentin is a result of the formation of dentin around these extensions.

After dentin formation begins, the cells of the inner enamel epithelium secrete an organic matrix against the dentin. This matrix immediately mineralizes and becomes the tooth's enamel. Outside the dentin are ameloblasts, which are cells that continue the process of enamel formation; therefore, enamel formation moves outwards, adding new material to the outer surface of the developing tooth.

MAXILLARY LATERAL INCISORS

it is shorter, narrower, and thinner.

Facial: The maxillary lateral incisor resembles the central incisor, but is narrower mesio-distally. The mesial outline resembles the adjacent central incisor; the distal outline--and particularly the distal incisal angle is more rounded than the mesial incisal angle (which resembles that of the adjacent central incisor. The distal incisal angle resembling the mesial of the adjacent canine.

Lingual: On the lingual surface, the marginal ridges are usually prominent and terminate into a prominent cingulum. There is often a deep pit where the marginal ridges converge gingivally. A developmental groove often extends across the distal of the cingulum onto the root continuing for part or all of its length.

Proximal: In proximal view, the maxillary lateral incisor resembles the central except that the root appears longer--about 1 1/2 times longer than the crown. A line through the long axis of the tooth bisects the crown.

Incisal: In incisal view, this tooth can resemble either the central or the canine to varying degrees. The tooth is narrower mesiodistally than the upper central incisor; however, it is nearly as thick labiolingually.

Contact Points: The mesial contact is at the junction of the incisal third and the middle third. The distal contact is is located at the center of the middle third of the distal surface.

Root Surface:-The root is conical (cone-shaped) but somewhat flattened mesiodistally.

Gingiva

The connection between the gingiva and the tooth is called the dentogingival junction. This junction has three epithelial types: gingival, sulcular, and junctional epithelium. These three types form from a mass of epithelial cells known as the epithelial cuff between the tooth and the mouth.

Much about gingival formation is not fully understood, but it is known that hemidesmosomes form between the gingival epithelium and the tooth and are responsible for the primary epithelial attachment. Hemidesmosomes provide anchorage between cells through small filament-like structures provided by the remnants of ameloblasts. Once this occurs, junctional epithelium forms from reduced enamel epithelium, one of the products of the enamel organ, and divides rapidly. This results in the perpetually increasing size of the junctional epithelial layer and the isolation of the remenants of ameloblasts from any source of nutrition. As the ameloblasts degenerate, a gingival sulcus is created.

TOOTH MORPHOLOGY

Descriptive anatomy

• Median sagittal plane: the imaginary plane in the center that divides right from left.
• Median line: an imaginary line on that plane that bisects the dental arch at the center.
• Mesial: toward the center (median) line of the dental arch.
• Distal: away from the center (median) line of the dental arch.
• Occlusal plane: A plane formed by the cusps of the teeth. It is often curved, as in a cylinder. We will speak often of the occlusal surface of a tooth.
• Proximal: the surface of a tooth that is toward another tooth in the arch.
• Mesial surface: toward the midline.
• Distal surface: away from the midline.
• Facial: toward the cheeks or lips.
• Labial: facial surface of anterior teeth (toward the lips).
• Buccal: facial surfaceof anterior teeth (toward the cheeks).
• Lingual: toward the tongue.
• Occlusal: the biting surface; that surface that articulates with an antagonist tooth in an opposing arch.
• Incisal: cutting edge of anterior teeth.
• Apical: toward the apex, the tip of the root.

ERUPTION OF THE PERMANENT TOOTH

- At the time at which the deciduous tooth erupts the tooth bud for the permanent tooth has already been building up enamel and dentin.

- When the permanent tooth starts to erupt, pressure on the root of the deciduous tooth causes resorption by the osteoclasts.

- Wolff's law states that when two hard tissues exert pressure on one another the softer of the tissues will be resorbed.

- The dentin and cementum of the root of the deciduous tooth is softer than the enamel of the permanent tooth that is why the root of the deciduous tooth is resorbed.

- Most permanent teeth have erupted and have been in use for 2 years before the root is completely formed.