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I INTRODUCTION  Teeth, hard, bony structures in the mouths of humans and animals used primarily to chew food, but also for gnawing, digging, fighting, and catching and killing prey. Teeth are the body’s hardest, most durable organ—long after bones and flesh have dissolved, archaeologists find well-preserved teeth from humans and other animals that lived thousands of years ago. Humans use teeth to tear, grind, and chew food in the first step of digestion, enabling enzymes and lubricants released in the mouth to further break down food. Teeth also play a role in human speech—the teeth, lips, and tongue are used to form words by controlling airflow through the mouth. Additionally, teeth provide structural support to muscles in the face and form the human smile. Like humans, most animals use their teeth to chew food, although many animals have evolved teeth that perform other specialized tasks. For example, many carnivorous (meat-eating) animals, such as tigers, have developed long, sharp teeth for clamping down on and killing prey. Beavers have chisel-like front teeth that they use to cut down large trees for building dams. II HUMAN TEETH   Human teeth are made of four distinct types of tissue: enamel, dentin, pulp, and cementum. Enamel, the clear outer layer of the tooth above the gum line, is the hardest substance in the human body. In human teeth, the enamel layer is about 0.16 cm (about 0.06 in) thick and protects the inner layers of the teeth from harmful bacteria and changes in temperature from hot or cold food. Directly beneath the enamel is dentin, a hard, mineral material that is similar to human bone, only stronger. Dentin surrounds and protects the pulp, or core of the tooth. Pulp contains blood vessels, which carry oxygen and nutrients to the tooth, and nerves, which transmit pain and temperature sensations to the brain. The outer layer of the tooth that lies below the gum line is cementum, a bonelike substance that anchors the tooth to the jawbone. The visible portion of the tooth is called the crown. Projections on the top of each crown, used primarily for chewing and grinding, are called cusps. The portion of the tooth that lies beneath the gum line is the root. The periodontal ligament anchors the tooth in place with small elastic fibers that connect the cementum in the root to a special socket in the jawbone called the alveolus. A Types of Human Teeth   Adult humans typically have 32 teeth—16 in the upper jaw and 16 in the lower jaw—that fit together and work in concert to chew food. Teeth on the right side of each jaw are usually identical to the teeth on the left side and matching teeth on opposite sides are referred to as sets, or pairs. Humans are heterodonts—that is, they have teeth of different sizes and shapes that serve different functions, such as tearing and grinding. In contrast, the homodont teeth found in many animals are all the same size and shape, and perform the same function. Humans have four types of teeth, each with a specific size, shape, and function. Adult humans have eight incisors, located at the front of the mouth—four in the upper jaw and four in the lower jaw. Incisors have a sharp edge that is used to cut food. On either side of the incisors are the canines, named for their resemblance to the pointy fangs of dogs. The upper canines are sometimes called eyeteeth. There are two canines in each jaw, and their primary role is to tear food. Behind the canines are the bicuspids, or premolars, flat teeth with pronounced cusps that grind and mash food. There are two sets, or four bicuspids, in each jaw. Behind the bicuspids are the molars, where the most vigorous chewing occurs. There are twelve molars—three sets in each jaw—referred to as the first, second, and third molars. Third molars are often called wisdom teeth; they developed thousands of years ago when human diets consisted of mostly raw and unprocessed foods that required the extra chewing and grinding power of a third set of molars. Today wisdom teeth are not needed for chewing and, because they can crowd other teeth, are often removed. B Tooth Development   Humans are diphyodont—that is, they develop two sets of teeth during their lives. The first set of teeth are the deciduous teeth, 20 small teeth also known as baby teeth or milk teeth. Deciduous teeth start developing about two months after conception. By the time a child is six years old, a second set of 32 larger teeth, called permanent teeth, start to erupt, or push out of the gums, eventually replacing the deciduous teeth. Human tooth development occurs in stages. The hard tissue of the deciduous teeth, or the dentin, forms while the fetus is in the womb. After the child is born, tooth enamel develops in stages. Front tooth enamel, for example, is usually complete around one month after birth, while the enamel on the second molars is not completely developed until a child is about a year and a half old. When the enamel is fully developed the tooth erupts. Front teeth usually erupt when a child is from 6 to 12 months of age, second molars between 13 and 19 months old, and canines usually erupt at 19 months or older. The final stage of tooth development is root completion, a slow process that continues until the child is more than three years old. Around the age of six, the roots of deciduous teeth slowly dissolve as the developing permanent teeth start to push them out. Deciduous teeth eventually fall out and are replaced by the erupting permanent teeth. This begins a transitional phase of tooth development that takes place over the next 15 years. As baby teeth are pushed out by permanent teeth, the entire mouth and jaw transform from their childhood shape to a more pronounced, adultlike structure. From age six to age nine, a child’s permanent incisors, canines, and first molars erupt. The bicuspids erupt from age 10 to age 12, and the second molars come in by age 13. The third molars, or wisdom teeth, usually erupt by the age of 21. When human teeth grow to a certain size, the root essentially closes and the teeth stop growing. Closed-rooted teeth have narrow root openings that are only big enough for the periodontal ligament, blood vessels, and a nerve. C Disorders of Human Teeth   The three main diseases of human teeth are tooth decay, also called dental caries; gum disease, or periodontal disease; and problems with tooth alignment, called malocclusions. Human teeth problems are treated or prevented by dentists, professionals who are specially trained to practice dentistry. Tooth decay affects approximately 90 percent of all children by the time they are 14 years old. Tooth decay begins when bacteria are passed from mothers or caregivers to children between their first and second birthdays. When these bacteria are exposed to sugars commonly found in foods, the bacteria produce harmful acids that attack tooth enamel. Left unchecked, the acid eats holes in the enamel and forms cavities of tooth decay. Most tooth decay forms in the deep grooves on the chewing surfaces of the molars, called pits and fissures. Daily tooth brushing and proper dental care help prevent and reduce tooth decay. Dentists use preventive treatments to reduce the risk of tooth decay; clear plastic coatings painted on the teeth, called dental sealants, and applications of the mineral fluoride, which fortifies tooth enamel, are two such treatments. Fluoride is also added to public water supplies in a process called fluoridation, which benefits more than 150 million Americans. Gum disease, or periodontal disease, is a progressive condition that worsens with age. Gum disease occurs when bacteria eat away at gum tissue, causing it to pull away from the teeth. This space between the tooth and gum, called a periodontal pocket, traps even more bacteria. Gum disease develops in two stages. Gingivitis, the early stage, causes red, swollen gums that bleed easily. Gingivitis can be eliminated through good oral hygiene and dental care. If not treated, gingivitis can progress to periodontitis, when bacteria attack the bone supporting the teeth. To treat periodontitis, dentists may have to surgically cut out the infected portion of the gum so the bacteria can be removed. Malocclusions—teeth that are crowded, crooked, or out of alignment—make it more difficult to clean teeth, which can lead to other oral health problems such as tooth decay and gum disease. Many of these disorders start to appear between the ages of 6 and 12, when permanent teeth begin to erupt. Generally, malocclusions result when the jaw is too small to hold all of the teeth. Malocclusions are often genetic, tending to run in families. In other cases, dental injury or chronic thumb sucking may lead to poorly aligned teeth. Malocclusions are treated by dentists specially trained to correct them, called orthodontists. III TEETH IN OTHER ANIMALS   Animal teeth have the same four tissues that make up human teeth: enamel, dentin, cementum, and pulp. The composition and structure of each substance may differ in each animal. In horses, for example, enamel is found inside the tooth as well as on the outer surface, rather than simply encasing the dentin and the pulp as it does in human teeth. Some animals are monophydont, developing only one set of teeth that grow continuously throughout an animal’s lifetime. These animals have open-rooted teeth, which have wide openings at the root that permit dentin-forming cells to grow and multiply. Most rodents, for example, have open-rooted teeth. The gnawing habits of these animals wear down their teeth, otherwise the teeth would grow very long. The front teeth of beavers, for example, can grow up to 1.2 m (4 ft) a year. Sharks and some other fish are polyphydont—that is, they continuously lose their teeth and develop new ones. Most mammals that depend on catching, chewing, and digesting food for survival have developed teeth that meet these needs. Mammal teeth are classified by the type of food the animal eats. Insectivores are animals that eat only insects, such as bats, shrews, anteaters, and armadillos. These animals have square teeth with special V-shaped edges that efficiently grind the hard coverings of insects. Carnivores, such as dogs, cats, hyenas, and walruses, generally have large and well-developed teeth with long canines for clamping down on prey or fighting. A walrus also uses its canines, which grow up to 1 m (about 3 ft) long, as hooks in climbing on ice. Herbivores—cows, sheep, deer, and horses, for example—eat only plants and have sharp incisors for cutting vegetation and flat teeth with complicated ridges for grinding and mashing. Piscivores, or fish-eaters, have sharp teeth that angle backward to catch and hold their prey. Seals and dolphins swallow food whole without chewing it first; they are equipped with many identical, conical-shaped teeth that are used to catch and grasp their slippery prey before swallowing. Fish teeth have evolved to perform different functions in each species. For example, fish that feed on crab, shrimp, and other crustaceans have developed strong, blunt teeth for crushing and grinding the hard outer shells. Piranhas have serrated teeth that fit together like scissors, enabling the small fish to cut the flesh from their prey. Many fish have teeth on their tongues or gills. A hagfish uses the rasping teeth on its tongue to bore holes in its victims and drain their blood. Sawfish have long, flat beaks with a row of weaponlike tooth projections in each jaw that can cut their prey in half. Only some reptiles and amphibians have teeth. Salamanders have rows of small pointed teeth, but frogs and toads do not have teeth after infancy. Some snakes and frogs develop an egg tooth that enables a hatching young to chip its way out of its egg. This tooth eventually disappears. Many reptiles have teeth growing on the tongue or the palate, and some even have a second set in the throat. Some snakes, such as rattlesnakes, have prominent fangs for delivering injections of venom to their victims. Crocodiles have between 30 and 40 teeth in each jaw. These reptiles use their daunting teeth not for chewing, but to gradually tear food into bits as they thrash violently with their prey in the water. 쥁