The embryological formation of teeth starts by the cooperative action of two types of tissue. The dental lamina tucks downwards from the surface epithelium of the mouth to meet a dental papilla growing towards the surface.
Brachydont teeth have a simple structure and do not grow very tall.
The crown of a brachydont tooth is made of enamel, the inner core is made of dentine, and the root is covered by cementum. All the teeth of humans and pigs, and the incisor teeth of cattle and sheep are of this type.
The premolar and molar teeth of pigs have a grinding surface covered by rounded bumps or tubercles; this is called a bunodont type of tooth (Figure 1-6).
Bunodont teeth may be used for grinding and crushing a variety of feeds. However, they would soon be worn away by the constant grinding of tough fibrous feeds such as those consumed by foraging ruminants.
Ruminants have a special type of tooth distinguished by a greater height (hypsodont) and complex curved ridges of enamel (selenodont).
Hypsodont teeth are long-lasting because they develop a greater depth to be worn down.
Selenodont ridges are composed of alternating layers of enamel, dentine and cementum, and they exert a powerful grinding action when top and bottom teeth are moved horizontally against each other.
The fate of the missing canine teeth in ruminants is quite interesting. The teeth of the upper jaw are inserted into two bones, the premaxilla and the maxilla. Many anatomists define the upper canine tooth as the tooth that is immediately posterior to the suture between the premaxilla and the maxilla. The lower canine is then defined as the tooth that articulates immediately anterior to the upper canine. Thus, the fourth incisor in the lower jaw may be claimed as a canine tooth (Andrews, 1981). In embryonic ruminants, the control system that causes the most anterior three pairs of teeth to become shaped as incisors appears to spread posteriorly and to take control over the developing canine tooth (Osborn, 1978). Although poultry do not have any teeth, the genetic information for tooth formation may still be present in an unexpressed form. When grafted onto jaw tissue from mice, the dental epithelium from chicks may develop ameloblasts that may synthesize enamel (Kollar and Fisher, 1980).