TEETH DEVELOPMENT AND ABNORMALITIES: Discuss in detail the development of tooth. Elaborate the theories of tooth eruption

The teeth are formed in relation to the alveolar process. The epithelium overlying the convex border of this process becomes thickened and projects into the underlying mesoderm. This epithelial thickening is called dental lamina.

The dental lamina is, in fact, apparent even before the alveolar process itself is defined. As the alveolar process is semicircular in outline the dental lamina is similarly curved. 

The dental lamina now shows a series of local thickenings, each of which is destined to form one milk tooth. These thickenings are called enamel organs. There are ten such enamel organs (five on each side) in each alveolar process.

The stages in the formation of an enamel organ and the development of a tooth are as follows: 

1. Each enamel organ is formed by localized proliferation of the cells of the dental lamina. 

2. As the enamel organ grows downwards into the mesenchyme (of the alveolar process) its lower end assumes a cup-shaped appearance. The cup comes to be occupied by a mass of mesenchyme called the dental papilla. 

The enamel organ and the dental papilla together constitute the tooth germ. At this stage the developing tooth looks like a cap—it is, therefore, described as the cap stage of tooth development. 

3. The cells of the enamel organ that line the papilla become columnar. These are called ameloblasts. 

4. Mesodermal cells of the papilla that are adjacent to the ameloblasts arrange themselves as a continuous epithelium-like layer. The cells of this layer are called odontoblasts. 

The ameloblasts and odontoblasts are separated by a basement membrane. The remaining cells of the papilla from the pulp of the tooth. The developing tooth now looks like a bell (bell stage). 

5. Ameloblasts lay down enamel on the superficial (outer) surface of the basement membrane. The odontoblasts lay down dentine on its deeper surface. 

As layer after layer of enamel and dentine are laid down, the layer of ameloblasts and the layer of odontoblasts move away from each other. 

6. After the enamel is fully formed, the ameloblasts disappear leaving a thin membrane, the dental cuticle, over the enamel. 

The odontoblasts, however, continue to separate the dentine from the pulp throughout the life of the tooth. 

7. The alveolar parts of the maxilla and mandible are formed by ossification in the corresponding alveolar process. As ossification progresses, the roots of the teeth are surrounded by bone. The root of the tooth is established by continued growth into underlying mesenchyme. 

Odontoblasts in this region lay down dentin. As layers of dentin are deposited, the pulp space becomes progressively narrower and is gradually converted into a canal through which nerves and blood vessels pass into the tooth. In the region of the root there are no ameloblasts. 

The dentin is covered by mesenchymal cells that differentiate into cementoblasts. These cells lay down a layer of dense bone called the cementum. Still further to the outside, mesenchymal cells form the periodontal ligament which connects the root to the socket in the jaw bone.

The permanent teeth are formed as follows: 

a. The dental lamina gives off a series of buds, one of which lies on the medial side of each developing milk tooth. These buds from enamel organs exactly as described above. They give rise to the permanent incisors, canines and premolars. 

b. The permanent molars are formed from buds that arise from the dental lamina posterior to the region of the last milk tooth. The dental lamina is established in the 6th week of intrauterine life. At birth the germs of all the temporary teeth and of the permanent incisors, canines and first molars, show considerable development. 

The germs of the permanent premolars and of the second molars are rudimentary. The germ of the third molar is formed after birth. The developing tooth germs undergo calcification. All the temporary teeth and the permanent lower first molar begin to calcify before birth.

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