Teeth are hard bony structures in the oral cavity that are variously modified to capture, tear, cut or grind food material before it is swallowed. Epidermal teeth are hard cornified epidermal structures of rare occurrence, as in the buccal funnel of cyclostomes and on the edges of tadpole jaws. Dermal teeth are found in mosts of the vertebrates.
TYPES OF TEETH
Polyphyodont dentition involves replacement of teeth from time to time several times in lifetime so that jaws are never left without teeth. Lower vertebrates having loose attachment of teeth lose teeth while feeding and capturing prey and hence teeth must grow again to replace the lost ones.
Diphyodont dentition is a characteristic of mammals in which milk teeth appear in the young ones but as they grow and jaw becomes larger, milk teeth are replaced by larger permanent ones to fit in the larger jaw bone.
Monophyodont teeth appear only once in lifetime and if they fall they are never again replaced by the new ones. Toothless animals have this kind of teeth and marsupials retain their milk teeth.
Based on the type of attachment of teeth on the jaw bone the following three types are found in vertebrates:
Acrodont teeth are attached on the top surface of the jaw bone as in fish and amphibians. This type of attachment is not very strong and teeth are lost easily and are replaced by new ones.
Pleurodontteeth are attached on the inner side and uppper side of the jawbone that brings larger surface area of tooth in contact with jawbone and hence attachment is stronger, as in lizards and urodeles. But this attachment is also not as strong as thecodont.
Thecodont dentition is found in mammals in which root of the tooth is firmly fixed in a socket of the jawbone, making the attachment strongest in vertebrates. This is a peg and socket attachment with the help of cementum that surrounds the root portion of the tooth.
Based on the kinds of teeth found there are two types of dentition:
Homodont dentition is found in the majority of vertebrates such as fish, amphibia and reptiles in which all teeth are functionally and anatomically of the same type, although their size may be variable depending on the location. Sometimes functionally some teeth may be specialized as fangs of snakes.
Heterodont dentition occurs in mammals in which there are 4 functionally different types of teeth, namely, flat incisors for cutting, long and pointed canines for tearing flesh and large and broad premolars and molars with flat grinding surface. Molars have no counterparts in the milk teeth.
There are also some other type of teeth as follows:
Secodont teeth have sharp cutting edges that function like scissors to cut flesh as in some primates and in carnivores.
Bunodont teeth are small with smaller cusps or tubercles on the surface for handling soft diet as in man, monkeys, rodents etc.
Brachydont teeth are smaller and low crowned suitable for feeding on soft diet.
Hypsodont teeth possess larger crown that can resist wear and tear of feeding on tough and fibrous diet as in ungulates.
Selenodont teeth are found in horses and other ungulates in which silica deposits around cusps and in the depressions of the grinding surface.This makes the grinding surface of teeth harder to prevent wearing.
Lophodontteeth are found in elephants which feed on the roughest diet that any mammal can feed on.The ridges on the grinding surface are in the shape of rounded lophs and the depressions are filled with silica.
DEVELOPMENT OF TOOTH
Teeth develop over the jaw bone where certain malpighian cells start actively multiplying forming a mass of cells called dental lamina or enamel organ. A dental papilla made of group of dermal cells appears below the dental lamina that supplies nourishment to the growing mass of cells. Cells of the outer layer of dermal papilla arrange themselves in a row and get differentiated into odontoblast cells. Epidermal cells of the dental lamina that cover the growing dentine are called ameloblasts.
The tooth gradually grows outwards and eventually gets exposed by penetrating through the skin covering the jaw bone. The dental papilla inside the pulp cavity remains active along with its blood supply and nerve intact. This development of tooth is identical to the development of dermal scales in fishes. Hence shark teeth are also called modified placoid scales.
COMPARATIVE ACCOUNT OF DENTITION
Cyclostomes have only epidermal teeth. Some fishes are toothless such as sturgeons, sea horse and pipe fish and others like lung fishes and Chimaera have teeth modified into crushing plates. Majority of fishes possess Polyphyodont, Acrodont and homodont dentition suitable for seizing prey.
In amphibians teeth are located on jaw bones, palatine and vomer bones and are Polyphyodont. Tadpoles lack true teeth and their jaws have horny epidermal ridges which are used to scrape algae on which they feed.
Among reptiles, turtles lack teeth and have horny beak. In others, teeth are generally confined to jaw bones but in snakes and lizards may occur on palatine and pterygoid bones. Fangs of snakes are modified upper maxillary teeth and in Heloderma they are lower maxillary or mandibular.
Modern birds lack teeth but Archaeopteryx had thecodont dentition and so were the toothed birds Ichthyornis and Hesperornis.
DENTITION IN MAMMALS
Mammals as a rule possess heterodont, diphyodont and thecodont dentition. However, some mammals lack teeth as given below in detail.
Among monotremes, the spiny anteater or echidna (Tachyglossus and Zaglossus) lacks teeth.
Edentates, as the name suggests are toothless such as the giant anteater of South America.
Among the aquatic Cetacea baleen whales have no teeth, such as the blue whale, Balaenoptera musculus and, the whalebone whale.
Among humans, and astonishingly, males in “Bhudas” tribe of Hyderabad Sindh in Pakistan
are genetically so predisposed that they never grow teeth all their lives.
The Dental Formula
Mammals have heterodont dentition having four types of teeth meant for different function in handling food in the oral cavity. Incisors in front are flat teeth designed for cutting food into pieces and the canines next to them are generally long and pointed spike-like used for tearing flesh by carnivore animals. Premolars and molars are located on the posterior side of the jaw, have flat surface with tubercles called cusps and are used for grinding food of plant origin. They are therefore well developed in herbivore animals. Number and arrangement of teeth in mammals is specific in different groups of animals so much so that mammalian orders can be identified by their teeth and dental formula, which is written for one half of the upper and lower jaw as follows:
3 – 1 – 4 – 3 x 2 = 44. This dental formula belongs to horse and pig.
3 – 1 – 4 – 3
PRIMATES. Primates are basically arboreal animals whose ancestors were insectivores and some of them still continue with their original diet. Generally they possess larger pointed canines and flat molars.
UNGULATES. Ungulates belong to two orders, the even-toed Artiodactyla and the odd-toed Perissodactyla that includes horses and rhinoceroses. They are all herbivores and fleet footed grazers and browsers, with teeth adapted for grinding tough vegetation. Musk deer also has large upper canines hanging on either side of the jaws. Hippopotamus has large canines too with sharp edges meant for defense against the lurking crocodiles in their amphibious habitat.
Pigs and horses have full set of 44 teeth as depicted in the dental formula given above. Premolars and molars are similar in shape and size, have flat grinding surface, with silica deposits between cusps. Such grinders are called selenodont and are designed to grind tough grasses. Wild boars have large canines for digging roots of plants and also for defence.
INSECTIVORES. In insectivores such as shrews, hedgehogs and moles, all teeth are pointed and grinding teeth possess peg-like cusps for crushing hard exoskeleton of insects on which they feed. In the mole genera Scalopus and Condylura milk teeth are retained throughout life.
CHIROPTERA. In bats milk teeth are shed before birth and they are born with permanent teeth. Insectivorous bats have conical cusps on the grinding teeth for crushing insects.
RODENTIA. Rodents include rats, mice, squirrels and guinea pigs which possess chisel-shaped front incisors for gnawing nuts and hard objects. These teeth grow throughout life due to the wide opening of the pulp cavity but they are worn out equally fast. Premolars and molars have flat grinding surface.
LAGOMORPHA. In rabbits also the upper incisors are chisel shaped adapted for gnawing, canines are absent and cheek teeth are modified for grinding. As in other herbivores, a big diastema is present between incisors and premolars.
PROBOSCIDIA. The order includes elephants which have upper incisors modified as long tusks which are used for digging roots of plants, for removing barks from trees or for offence and defense. Premolars and molars are alike in appearance and they have broad and lophodont surface, in which cusps unite to form circular lophs of ridges with silica deposits in the depressions. Tusks of African elephant can reach a length of two metres and may weigh as much as 100 kg each.
CARNIVORES. Carnivores include cats, tigers, lions, dogs, wolves, jackals and bears and the aquatic seals, sea lions and walruses. Their canines are long and pointed, dagger like for tearing flesh of the prey. Canines of walruses are modified into long tusks. Carnassial teeth are enlarged teeth with pointed cusps. These teeth also have sharp cutting edges and are called secodont teeth used for shearing flesh.
CETACEA. There are no teeth in Mysticeti or baleen whales which possess baleen plates hanging from the palate that are suitable for straining planktons from sea water. In Odontoceti or toothed whales, teeth are homodont and monophyodont which are used to seize fish or other prey.
MARSUPIALS. Generally marsupials retain milk teeth except the last premolars that are replaced. Herbivores have a diastema and premolars and molars modified for grinding.