General Dentistry Concepts: Tooth
A tooth is a hard, calcified structure which is found in the jaws or mouth. Teeth are used to break down food for consumption. Some animals, especially carnivores, use their teeth for hunting and as defense. The roots of the teeth are surrounded by the gums. Teeth are not made from bone, but of numerous tissues which vary in their density and hardness. The cellular tissues which eventually become teeth, originate from the embryonic germ layer, which is called the ectoderm.
While there is considerable variation in their form and position, the structure of the teeth is similar across the vertebrates. The teeth of mammals contain deep roots. This pattern is also found in some fish in addition to crocodilians. In most teleost fish, the teeth are actually attached to the outer surface of the bone. The teeth found in lizards are attached to the inner surface of their jaw. In cartilaginous fish, which includes sharks, the teeth are attached using tough ligaments which connect to the hoops of cartilage which form the jaw.
Some animals only develop a single set of teeth. Other animals will develop multiples sets. As an example, sharks grow a new set of teeth about every two weeks to replace their worn teeth. Rodent’s incisors grow and continually wear down through gnawing, which helps them maintain a constant length. Many rodents, including guinea pigs and rabbits, have continuously growing molars in addition to their incisors.
Teeth are not always attached to the jaw like they are in mammals. In many reptiles and fish, the teeth are attached to the palate floor of the mouth. They form additional rows inside those on the jaws proper. While they are not true teeth in the normal sense, the dermal denticles of sharks are almost identical in structure. Teeth appear to have first evolved in sharks and they are not found in more primitive jawless fish. Lampreys have tooth-like structures on the tongue, which are composed of keratin, not dentine or enamel. They also do not bear a relationship to true teeth.
Living amphibians typically have smaller teeth, if they have any teeth at all, as they often consume soft foods. In reptiles, the teeth are generally simple and conical in shape. There can be some variation between species, as an example the venom-injecting fangs found on snakes. The pattern of incisors, canines, premolars and molars are only found in mammals. The number of teeth vary drastically between species. Zoologists use a standardized dental formula to describe the specific pattern in any given group.
Teeth are among the most distinctive and longest lasting features found on mammals. Paleontologists often use the teeth to identify a fossil species and determine their relationships to one another. The shape of the animal's teeth are closely related to its diet. As an example, plant matter is difficult to digest. This results in herbivores having numerous molars for effective chewing and grinding. Carnivores, on the other hand, have canine teeth which are used to kill their prey and to tear through tough meat.
Mammals, are generally considered diphyodont. This means they develop two sets of teeth in their lifetime. In humans, the first set of primary or baby teeth begin to appear around about six months of age. During adolescents, the baby teeth fall out and are replaced with the adult teeth. Tooth eruption is known as teething and can be painful. Kangaroos, elephants, and manatees are unusual among mammals because they are polyphyodonts, meaning their teeth are continually replaced during their lifetime.