ANATOMY & PHYSIOLOGY A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z                    HOME ABOUT CATEGORIES SITE MAP COPYRIGHT ADVERTISE CONTACT entire Web this site teeth Mouth and jaws of an adult human showing the position of the teeth. Top left: structure of a molar tooth Hard bonelike structures in the jaws of vertebrates which are used for biting and chewing or for attack and defense. An adult human normally has 32 teeth – 16 in each jaw. The upper jaw has on each side two incisors, one canine, two premolars and, at the back, three molars. The same is true of the lower jaw. Dental formula The number of teeth in an animal can be expressed conveniently by using the dental formula. This gives the number of teeth of each type in one half of each jaw reading from the front (incisors) to back (molars). Thus the dental formula for humans is 2123/2123. For a rabbit, which has no canine teeth at all in its jaws the formula is 2033/1023. The primitive placental condition (i.e. that found in the early placental mammals) is 3143/3143 but few animals retain the full set of 44. Arrangement and function of teeth The tooth arrangement depends to a large extent upon the feeding method and the nature of the food. Humans are omnivorous (eating all types of food) and consequently our teeth are not highly specialized when compared with the grinding teeth of herbivores and the cutting teeth of carnivores. Incisors are retained in most mammalian groups as chisel-shaped biting or cutting teeth. The canine is highly developed in hunting carnivores as a stabbing and tearing tooth. In humans it is rather reduced and functions almost as another incisor. Many herbivorous animals have lost the canine tooth altogether. The premolars and molars of humans are the grinding and chewing teeth. The surface of these teeth is covered with triangular or conical ridges (cusps) which fit into hollows of the opposing teeth when the jaws are brought together. Chewing movements then cause the teeth to act as millstones and grind up the food. Structure of teeth Although our teeth are modified for various purposes, they are all constructed to a definite pattern. Projecting from the gum is the crown of the tooth. The part embedded in the gum and reaching into a socket in the jawbone is known as the root. The body of the tooth is made up of a hard, pale yellowish, bonelike substance called dentine. Inside this is a cavity – the pulp cavity – which contains blood vessels and nerves. Branches from these, together with fine cytoplasmic threads, penetrate the maze of fine canals which spread throughout the dentine. When the latter is damaged, by decay or in some other way, the nerve endings are stimulated and we feel pain. Covering the crown of the tooth is layer of enamel of varying thickness. It is made up almost entirely of apatite crystals with calcium phosphate filling. Apatite crystals are composed of calcium phosphate plus calcium fluoride or calcium chloride. Calcium phosphate is also mainly responsible for the hardness of dentine. The enamel layer crystals are elongated and are all arranged with their ends toward the surface of the enamel. Around the root of the tooth, enamel is replaced by cementum (the cement), another bonelike material which fixes the tooth firmly in the socket of the jaw. However, between the bone of the jaw and the cementum layer there is a layer of tissue, called the periodontal membrane, which is in contact with the tissues of the gums and with the pulp cavity. Incisor and canine teeth have a single root, premolars have a double root, and molars have three branches to the root. The deciduous (milk) teeth The face of a six-year-old child showing the milk teeth in position and the developing permanent teeth (in red) Whereas an adult human has 32 teeth, a child up to about the age of six has only 20 teeth. These are the milk teeth or deciduous teeth which are gradually replaced by the permanent teeth after the age of about six years. There are no molars in the milk set but the teeth corresponding to the premolars are known as the milk-molars and perform the same grinding function. The first milk teeth to erupt (break through the gums) are usually the two central incisors in the lower jaw. These normally appear between eight and ten months after birth. Then, in a fairly regular order which takes 18 months to 2 years, a further 18 teeth push their way through the gums. By the time a child is 2 or 2½ years old it should have a full set of 20 teeth. Although quite small, the deciduous teeth are very strong and well able to deal with the hardest foods that the child may come across. The teeth are formed with the substance of the jawbones; the maxilla above and the mandible below. As they erupt they have to push their way through the tissues which cover the bones and in this process can cause marked inflammation. In a baby this is enough to cause a temperature and considerable disturbance of appetite and well-being. The permanent teeth The first permanent teeth appear at the age of about six years. These are the first molars, sometimes called the "six-year molars." They are the first permanent to erupt. Thereafter, during the course of 5 or 6 years, the milk teeth are all pushed out by permanent teeth which grow up beneath them. Incisors are replaced first, then premolars and then canines, the latter appearing as permanent teeth at eleven years or so. When all the milk teeth have been lost the 4 second molars appear immediately behind the first molars, at about 12 years. Only the third molars are now needed to make a complete set of permanent teeth. These four teeth, often called – wisdom teeth, seldom appear before the 15th year. In some people they may not erupt until much later in life; in others, their eruption may be impeded by the second molars. This condition is known as an impacted wisdom tooth, and because it is a painful condition it frequently requires the removal of the tooth. Origin and development of teeth Although teeth are composed of a hard bone-like tissue, they are in fact derived from various skin tissues in much the same way as the placoid scales (denticles) of the dogfish and its relatives (the Elasmobranch fish). It is fairly certain that the placoid scale and the teeth have a common origin in the bony plates that covered the bodies of primitive fish. The placoid scales, like teeth, have a pulp cavity surrounded by dentine and enamel, and, in only slightly modified form, function as teeth in the mouths of elasmobranchs. More concrete evidence of the origin of teeth is obtained by studying their development in the gum. Human teeth are formed in the following way. In the early embryo the skin along the future line of the jaw-bones thickens and is known as the dental lamina. The edge of this extends into the tissues of the jaw and forms bud-like thickenings at intervals along the jaw. There are, at first, ten of these thickenings in each jaw. They are the "buds" of the first set of teeth. The dental lamina later extends beyond the last deciduous tooth bud and slowly forms the buds of the permanent molars. When the embryo is about three months old the dental lamina forms further tooth buds on the inside of the developing milk teeth. These are the buds of the permanent teeth. They develop in the same way as the milk teeth but much more slowly. The epithelial tissues of the tooth buds grow inward and form a bell-shaped structure in which a group of cells shows up densely and is termed the enamel knot. Under this knot, cells of the connective tissue become dense, forming the beginnings of the tooth body, the tooth papilla. The cells of the papilla grow and multiply and push up under the enamel knot, forming a simple tooth-shaped structure. The cells of the enamel knot get larger and begin to produce enamel while some cells of the papilla start to release dentine. For the laying down of good hard material and its impregnation with calcium and other minerals, salts and vitamins – especially vitamin D – are needed in the blood. The hard layers are first deposited at about 20 weeks old by which time the bone of the jaws has started to form as a cup surrounding the developing teeth. More enamel and dentine are produced until the crown of the tooth is complete. The time required depends on upon the type of tooth but when the crown is complete the tooth erupts (i.e. breaks through the gum surface) by growth of the root. The latter continues to grow for a while until it is completely enveloped in the jawbone which has grown up around it. Cement is produced by the tissues of the papilla when the root begins to grow. When the root is fully formed the opening of the pulp cavity closes so that very little nutrient transfer can occur. Growth then ceases although the tissues still receive enough nourishment to stay alive. The permanent teeth continue to develop slowly under the milk teeth. When the crown of the permanent tooth is fully formed, its root begins to grow. This causes an increase in pressure on the base of the milk tooth. The result is that the periodontal membrane and the cement and even part of the milk tooth root are broken down by enzymes and by special scavenging cells, called macrophages, which absorb the material in the manner of a feeding amoeba. When the cement and the membrane have gone there is no firm attachment of the tooth to the jaw – the tooth in fact becomes loose and eventually falls out, leaving a clear path for the permanent tooth which now rapidly grows up through the gum to take its place in the adult tooth row. When it has reached full size and is firmly embedded in its socket, the tooth ceases to grow because the opening of the pulp cavity closes just as in the milk teeth. The gum is a mass of dense, fibrous tissue attached to the jaw bones. It is continuous with the periodontal membrane of the tooth socket which it supplies with food and oxygen via its rich blood supply. Teeth in other animals Only in the mammals do we find the various types of tooth described above. In all lower vertebrates the teeth are simple conical structures, usually without roots, and are present in greater numbers than in mammals as a rule. They are attached to the jaws by cement or by fibrous tissue (ligaments) but apart from crocodile and some fish teeth, they are not placed in sockets. Frequently, especially in fish, teeth are found in the roof and base of the mouth as well as in the jaws. Although the size of teeth may vary in an individual animal – the crocodiles have both large and small teeth – all are of the same general shape. Some modifications are found in fish that feed off molluscs. The teeth are often flattened and may be joined together in plates in order to crush the shells. The poison fangs of snakes are modified teeth. They are elongated and grooved to carry the venom from the poison glands into the wound made by the teeth. In contrast with mammals, where there are usually the two sets of teeth described for humans, reptiles and most other lower vertebrates replace their teeth many times, new ones replacing worn ones regularly throughout life. Modern birds are without teeth although fossils show that the earliest birds had simple conical teeth similar to those of their reptilian ancestors. All mammalian teeth can be classed by the four types described earlier but, as we have seen, they may be modified according to the feeding habits and diet of the animals concerned. The molars of cows and other grass or foliage-feeding animals are very large and have a complex pattern of ridges and grooves to ensure adequate grinding of the food. Teeth such as these, which are subject to hard wear, continue to grow throughout life as the root canal remains open. The dog and related carnivores have very sharp, pointed carnassial teeth. These are formed by the last pair of premolars in the upper jaw and the first molar of the lower jaw. The chewing motion of the jaws causes the teeth to shear through meat. Other tooth modifications include the tusks of elephants, which are greatly enlarged upper incisors. Whalebone whales and some ant-eating mammals have no teeth at al. Related category    • ANATOMY AND PHYSIOLOGY Also on this site: Encyclopedia of Alternative Energy & Sustainable Living Encyclopedia of History Transport Concepts & Designs (partner site) BACK TO TOP