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  Tooth Development and Growth  
 
Teeth development and their growth are influenced by several predisposing factors. Among these factors, genetics constitute a large part of the individual’s teeth development and growth.
Growth and eruption of teeth in the mouth is caused by numerous, complex series of gene expressions that result in teeth formation that came from embryonic cells.

The ectomesenchyme is an embryonic connective tissue that is covered by one to two layers of thick epithelium. These are the cells that lines that newly formed primitive mouth. Development of teeth starts when the embryonic epithelial and the neural crest that arises from mesenchymal cells starts to interact with each other.

Later, a continuous band of thick epithelium forms around this primitive mouth, which would become the jaws after about 37 days.

From each of these epithelium bands, the dental and vestibular lamina is quickly formed. Sites of the future teeth inside the dental lamina, which has every series of epithelial outgrowths into the mesenchyme are formed. After this, tooth development proceeds in three stages. These are the bud, cap, and bell.

1. Bud stage – This stage is represented by the multiplication of the epithelium into the ectomesenchyme of the jaw. These supporting ectomesenchymal cells are packed closely beneath and around the bud.

2. Bud to Cap – This transition from bud to cap is represented by the onset of differences between tooth germs. This will later give rise to different teeth.

3. Cap stage – This stage undergoes a condensation of the ectomesenchyme. This condensation is the multiplication of the tooth bud into the ectomesenchyme that in turn increases the density of the adjacent epithelial outgrowth by the dental lamina. The lateral lamina results as the bud becomes larger, it drags along and tethers itself to the dental lamina. Formative elements of the tooth and its supporting tissues are now identified in this stage.

The epithelial outgrowth resembles a cap, hence the name. This cap sits on a ball of condensed ectomesenchyme cells, which are called the enamel organ. This enamel organ will become the tooth enamel, while the ectomesenchyme cells or dental papilla will become the dentin and the pulp. The condensed mesenchyme or dental papilla, which encapsulates the enamel organ or the dental follicle that later becomes the supporting tooth tissues.

4. Bell stage – Important changes happen in this stage. The first change is the histo-differentiation. This histo-differentiation begins at the late cap stage and continues to the bell stage. The same epithelial cells develop into functionally distinct components of the tooth. Cells at the center of the enamel organ become the stellate reticulum.
The second change is the enamel organ becoming bell-shaped. This occurs as the undersurface of the epithelial cap deepens and the tooth crown develops its final shape.

The outer dental epithelium comes from the cuboidal cells at the periphery of the enamel organ. While the inner dental epithelium forms the cells on the border of the dental papilla. The cervical loop is the meeting point of these two epithelia, which is found at the rim of the enamel organ.

At this point, the cells continue to divide until the crown grows to its full size. The tooth later gives rise to the epithelium component of the root.

The next change happens with the occurrence of three events. These are: (1) dental papilla is separated from the dental organ; (2) the dental lamina fragments into numerous discrete epithelial cell aggregates; and (3) the inner dental epithelium begins taking shape of the tooth crown.

The fragmentation of the dental papilla is essential as it signifies that tooth development continues inside the jaw tissues. This is important occurrence for the tooth to function; it has to penetrate the epithelium for it to reach the occlusion plane.

During the cap stage, nerve fibers start forming near the tooth and grow toward the dental follicle. The nerves develop around the tooth bud once they arrived there. They will then enter the dental papilla. These nerve fibers never proliferate into the enamel organ.

Aside form the nerve fibers, it is in the cap stage that blood vessels form in the dental follicle and enter the dental papilla. Entrance of the dental papilla is formed by groups of blood vessels. Number of blood vessels is determined and reached their maximum number at the crown stage. The dental papilla then forms in the pulp of the tooth.

Throughout a human beings life, the amount of pulp tissue in a tooth decreases, meaning the blood supply of a tooth decreases along with age. Since the enamel and the dentin is a mineralized tissue, they do not have blood vessels for it get their needed nutrients.

After these stages, tooth eruption occurs. Tooth eruption occurs in three stages.

  1. Deciduous teeth stage – This occurs when the first primary teeth appear.
  2. Primary dentition stage – This occurs at six months of age and lasts until the six years of age.
  3. Permanent dentition stage – This stage starts around the 11 years of age.

Tooth eruption is influenced by many factors such as dentition patterning. This determines specific tooth types and their positions in the jaw. Animals have a homodont type of teeth, or their teeth all have the same shape. Human teeth on the other hand, have different shapes or are heterodont. Heterodont has three types, namely: (1) incisiform; (2) caniniform; and (3) molariform.

There are two models that have been proposed to explain dentition patterning. These are:

  1. Field model – This model proposed that the responsible cells are present in the ectomesenchyme.
  2. Clone model – This model proposes that each tooth type is derived from a clone of ectomesenchymal cells that are programmed by the epithelium to produce teeth of a given pattern.

But a new theory stated that the ectomesenchyme eventually assumes a dominant role in dentition patterning. Molar development comes from the recombination of molar papilla with the incisor dental organ. Recombination of the incisor papilla with the molar dental organ on the other hand, results in incisor development.

 
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