2.3 DENTAL DESIGN, FORM, AND FUNCTION

I. INTRODUCTION: WHAT TEETH DO

Teeth, like well-planned buildings, follow a simple rule: form follows function. When it comes to comminution (the trituration, pulverization, and reduction of food in chewing), the basic activities of teeth can be understood as machines to break up food.

In a classroom demonstration, three kitchen implements illustrate some basic activities of teeth.

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(1) Scissors with a serrated edge are ideal for cutting meat. Carnivores have specialized cheek teeth called carnassials that perform slicing functions with ruthless efficiency. Cats, including domestic felines live at the top of the food chain. Thanks to their meat-eating efficiency, they spend most of their day-asleep.

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(2) Kitchen graters nicely illustrate the action of the cheek teeth in large herbivores. Parallel ridges of enamel alternating with softer seams of dentin and cementum provide multiple cutting edges ideally suited for cutting cropped plants, preparing them for digestion in sacculated stomachs.

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(3) The mortar and pestle exemplify the action of cusps that 'chew into basins,' namely the fossae of antagonistic teeth in the opposing arches. Fruit-eating primates crush fruits like a fashionable orange juicer to express out the nutritious juices.

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II. INTRODUCTION: WHAT THEY USE TO DO IT

Teeth reflect both the opportunities and constraints afforded by their evolutionary origins. Nature makes do with what is at hand: jaw bones become ear ossicles, carpals become the panda's thumb, and in teeth-dentin, enamel, and cementum are creatively rearranged to suit the situation. In the following sections we will look at other animals, other possibilities which offer a degree of innovation beyond what we see in the human dentition.

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III. SOME CHARACTERISTICS OF VERTEBRATE DENTITIONS

(1) Fishes. Most fish have teeth, but in a few, teeth are absent. In most fishes, their function is to hold their prey.

The teeth of cartilaginous fishes, the sharks (and rays), are well known to movie fans who have seen the film Jaws. Those teeth are derivatives of placoid scales.

Bony fishes have a wide variety of teeth. Some even appear on the tongue, palate in extreme cases--the throat. The teeth of the fishes usually exhibit continuous succession (polyphyodonty). In a few cases, they are regionally specialized into classes (heterodonty).

(2) Amphibians. When present, the teeth in amphibians are small conical structures that are continuously replaced as in fishes. Toads don't have any teeth at all. Some amphibians have teeth only in one jaw.

(3) Reptiles. The typical reptilian dentition is a row of conical teeth of varying size and undergoing continuous succession. Some dentitions are more complex with piercing, cutting, and crushing teeth. All are single rooted. Teeth are absent in turtles. In some snakes elegantly specialized teeth function as poisonous fangs.

(4) Modern mammals. There is great variation on the primitive placental mammal model. A few sea mammals have an increased number of teeth. Most mammals show a reduction in number, some a reduction from two generations to one. At the extreme, the anteater has no functional teeth at all.

Usually there are two sets of teeth (diphyodonty) and regional specialization (heterodonty). In most species, teeth complete their growth early in life. Sometimes, as in rodents, the teeth are continuously growing.

In mammals, the molars are broadened for squishing, grinding, and cutting. This provides greater digestive efficiency to met the needs of homeothermic animals. The development of the temporomandibular articulation and the muscles of mastication increase the range of movements for chewing. Saliva and its enzymes facilitate chewing and initiate the digestive process. Prismatic enamel permits greater masticatory loads. Finally, development of the secondary palate permits breathing during extended periods of mastication

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IV. STRATEGIES FOR MAINTAINING TOOTH FUNCTION

One of the basic needs of the dentition is how to maintain the chewing surfaces presented by calcified dental tissues. A good starting point for this discussion is the human dentition. Our permanent teeth erupt with a fixed amount of enamel with the expectation that the teeth will serve for a lifetime. In some environments with grit in the diet, the teeth wear rapidly; in others, the teeth if not damaged by dental disease, can serve well into old age. The human dental strategy, therefore, is to start out with enough enamel to last a lifetime.

Our dentition illustrates another strategy for maintaining the chewing surface: tooth replacement. Deciduous teeth are replaced by permanent teeth. The succession of teeth accommodates growth as well. In the elephant which we will meet later, a series of molars erupts, wear, are lost, and are replaced by situations teeth in a horizontal eruptive pattern.

Horses and others that wear their teeth during lifetimes of chewing grass have another strategy. They have long-crowned hypsodont teeth. Those teeth arrive on the scene with a vast reserve of tooth crown structure to survive a lifetime of wear brought on by abrasive silica in grass.

Another strategy, quite unlike anything in the human dentition, is the continuously growing incisor. The cutting surface is continuously maintained by an ever growing tooth. It has an apex which never closes, quite unlike the human dentition where each tooth goes on to completion.

With a bit of humor, I propose these terms to illustrate the two growth strategies of teeth. Our own teeth go on to completion. We can term that goal-seeking strategy as teleodont (Gr telos = goal seeking). When teeth retain the juvenile condition of persistent growth throughout life, I suggest the term anathatodont (Gr athanatos = undying), namely never ending.

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V. PERSISTANT INCISOR GROWTH IN RODENTS

All rodents have a similar dental configuration with incisors with sharp edges designed for gnawing. Maintaining the sharp cutting incisor edges is achieved by ongoing delivery of new tooth substance by continuing growth of the incisor teeth. In this section, we specifically discuss the laboratory rat.

Rodents, usually rats and mice, have been used extensively for dental research. They are cheap, small, grow quickly, and present many research opportunities.

The rat dentition is monophyodont; there is just one set of teeth. There are a conspicuous, continuously growing (and erupting) incisors that are constantly worn to a chisel-shaped edge. The upper incisors are more curved than the lowers; their enamel is limited to the labial surfaces.

Rodent incisors are often brightly colored in red or orange. It is due to an iron pigment. Rats fed an iron-poor diet fail to form the pigment.

The dental formula is:

The upper incisors overhang the lowers, which makes it necessary for the mandible to be brought forward in gnawing.

The incisors are separated from the molars by a diastema. The intrusion by special folds of the upper lip effectively separates the anterior gnawing and posterior grinding compartments.

The molars exhibit only limited growth; enamel does not form on the cusp tips. As the molars wear in function, they expose widening areas of dentin surrounded by rings of enamel. The differential wear of enamel and dentin on the occlusal surface improves chewing efficiency.

The dentition of the mouse is similar to, but smaller than that of the rat. Mice, unlike the rat, also have deciduous incisors.

Other rodents include beavers, squirrels, rabbits, and hares. The dental formula varies among the species, but characteristically the canines are absent creating a diastema. The functional design is rather consistent in rodents.

Rodents and their incisors are widely used for experimentation, owing to their continuous growth which makes all phases of odontogenesis, eruption, function, and wear in a single tooth.

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VI. BLADE-LIKE CUTTING TEETH: THE CARNASSIALS

 

The teeth in carnivores are adapted for catching, killing, and slicing their prey. All carnivores, except for the bear, have powerful blade-like cheek teeth known as carnassials.

The name tells of their singular dietary specialization: they are primarily meat eaters. Familiar carnivores are the dog, cats, bear, seal and the walrus.

Meat-eating demands hunting and killing prey. The enlarged canine teeth are accompanied by small incisor teeth for prehension. Canines are powerful, elongated, and sharp teeth. In a few species, they have achieved dramatic proportions as in the now-extinct saber tooth tigers of La Brea, Los Angeles County in California. Several dramatic specimens are on display.

The dental formula for the domestic cat is:

The last maxillary premolar and the mandibular first molar are specializing to form the elongated, blade-like carnassial teeth. The teeth immediately anterior to the carnassials are smaller, but they have a carnassial-like function.

 

The dental formula for the dog reflects the more omnivorous diet of this animal. It is:

Compared with the cat, there are more teeth: cats have 30 and dogs, 42. The presence of extra molar teeth provide additional grinding surfaces. The familiar carnassials are well developed.

In long-muzzled dogs, the premolar teeth of both jaws are separated by spaces, while in dogs with short muzzles these teeth are crowded together and often irregular in position. The malocclusion is a consequence of selective breeding.

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The teeth of seals are modified for the seizing and holding the slippery bodies of fish. They, like the walrus, have lost the carnassial functional adaptation. The teeth of seals interdigitate to pierce their prey.

The walrus lives on a specialized diet of shellfish. All teeth except the huge upper canines are reduced to blunt, crushing cone-like structures. Its large tusks are continuously growing specialized canine teeth. When first erupted, they have a small cap of enamel, but this is quickly worn away. The tooth thus formed is primarily dentine invested in a layer of cement. The tusks are continuously growing. Lower incisors and all molars have been altogether lost. Growth of the cheek teeth is continuous, but it gradually ceases later in life. The walrus tusk has long been prized for its ivory.

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VII. THE ODD-TOED UNGULATES

There are two orders of ungulates, the so-called hoofed mammals. The Perissodactyla is "odd toed" because the third digit of each limb is developed into a single hoof. Perissodactyla, the odd-toed ungulates (Latin nail; hoofed mammals) includes horses, zebra, tapirs, and the rhinoceroses.

 

Horses present the following dental formula:

Horses exhibit an unusual sexual dental dimorphism: in the females, the canines are rudimentary or may be absent altogether.

The premolars and molars are similar in form. Their crowns are covered by cementum; with progressive wear, the grinding surfaces present a complicated pattern of enamel, dentin, and cementum.

In horses, freshly erupted teeth show a central pit. With wear, the enamel becomes worn off so that the central pit (called the mark) becomes isolated from the enamel by an area of exposed dentin. In old animals, the central pit area is made of up secondary dentin. These patterns reflect the age of the horse, hence the old phrase "looking a gift horse in the mouth."

Horse molars are described as hypsodont (Gr. height) for their tall teeth adapted to accommodate a lifetime of occlusal wear. The cheek teeth have exhibit infolding and lophs of enamel that form continuous rows. Grasses contain silica which makes tooth wear a problem in large herbivorous animals. (Vocabulary hint: hypsodont describes tall teeth; brachyodont (Gr. short) describes short teeth.)

Horse teeth are continuously erupting, and become 'rooted' (acquire roots) late in life. Thus, they represent an intermediary condition between teleodont teeth like our own that go onto to completion quickly, and anathatodont teeth such as the continuously growing incisors in rodents.

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VIII. THE EVEN-TOED UNGULATES

Artiodactylia (even-toed ungulates) are mostly large herbivores (only a few are omnivores). They have cloven hooves; there is on hoof on each of the third and fourth digits. This order includes pigs, camelids, deer, cattle, and sheep.

Sheep have a dental formula that is surprising for the practicing dentist who is used to teeth always occluding with teeth:

The sheep has proved a useful model for the study of inflammatory periodontal disease. In them, periodontal disease occurs naturally. It is a condition known as 'broken mouth.' There are no anterior teeth in the upper arch; instead there is a horny pad. The lower anterior teeth are very procumbent and form a row of eight teeth that help to crop the grass upon which the animal feeds.

The selenodont-shaped posterior teeth have cusps that are crescent-shaped, facing inward in the lower and outward in the upper.

The cheek teeth are hypsodont, having long crowns and short roots. The cusps are selenodont (cresent-shaped (Gr. moon), so named for the moon-like configuration of the cheek teeth enamel lophs.

The height of the teeth allows for slow, long term eruption to compensate for dental wear.

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Pigs are even-toed ungulates, with the addition of second and fifth digit to their limbs. Domestic pigs have reduced molars and tusks (canines).

One fearsome wild pig of Malaysia, Sus babirussa can be mentioned here for its unique upper canines. They are tusks that erupt extraorally. In some animals, if the growth is left unchecked, they can go on to kill the animal. Some authors ascribe their function to sexual combat.

The upper tusks of this rare species can achieve a length of seventeen inches in the male. In the female, they are just nubs.

 

The male wart hog (the wild boar) canines of the upper jaw may turn upward and attain a length of eight to 10 inches. The tusks are used for digging roots and are formidable weapons in combat. Castration in the male stops canine growth in the wild boar. On the Hebrides Islands, aboriginal peoples will break out the upper canines, causing the lower ones to grow into fanciful spirals that are used for body adornment. The canines are used to dig up roots.

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IX. SOME UNUSUAL TOOTH FORMS AND FUNCTIONS 

(1) Lemurs have comb-like lower incisors adapted for grooming. This is a specialized paramasticatory (non-chewing) adaptation of teeth. Grooming has particular social significance in the great apes.

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(2) Elephants and their extinct brethren, the Ice Age mammoth and mastodon have the largest incisors.

In elephants, the tusk incisors grow from persistent pulps and continue to grow throughout life-a condition they share with rodents. The tusks have an enamel tip present at the time of eruption; however, cementum covers the remainder of the crown portion of the tusk and extends onto the root. The adult tusks have deciduous predecessors. Ivory obtained from elephants is indistinguishable from that of mammoth or mastodons. Another historic source of ivory is from the tusks of the walrus.

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(3) Smilodon, the saber-tooth cat is now extinct.

The most extreme canine development was seen in Smilodon of North and South America. Well-known from La Brea on Wilshire Boulevard in Los Angeles County, this spectacular animal had massive maxillary canines over 6 inches in length. This carnivore died out 10,000 years ago. It may have been encountered by the First Americans who came from Siberia. The action of the remarkable canines is debated. They seem to have been used to attack large, thick-skinned animals such as the mammoth and sloth.

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(4) The walrus is a pinniped related to seals and sea lions. It lives on a specialized diet of shell fish. All of its teeth are reduced to blunt, cone-like structures except for the tusk-like continuously growing canines.

They are used to scoop up seaweed and for locomotion on ice when out of the water. Inuit refer to walrus as 'toothwalkers.' There are no incisors or mandibular canines. At the time of eruption there is a small cap of enamel which is quickly worn away. The canine tusks can reach 30" in length with a circumference of 8" at the base. The tusk in females achieves a length of about 24".

You may ask, why are these tusks defined as canines? The definition of tooth class in the maxillary arch is defined by the supporting bone and the succession of teeth. Upper incisors by definition arise in the premaxilla, a bone that is distinct in many mammals, but is merged into the maxilla in humans. Canines are defined as those teeth in the maxilla just distal to the incisors. Premolars are cheek teeth with deciduous predecessors. Molars are cheek teeth distal to premolars that DO NOT have deciduous predecessors.

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(5) Sexual dimorphism is present to a greater or lesser extent in nearly all mammal species. This has been carried to the extreme in the single tusk of the male narwhal Monodon. Thus, the narwhal is unique for its single tooth and its extreme development in the male, and non-eruption in the female.

Horses present an unusual dental sexual dimorphism. In females, the already reduced canines may be rudimentary or may be absent altogether.

In mammals the greatest sexual dimorphism is seen in the canines. The degree of dimorphism gradually diminishes on either side of the canine-suggesting a canine 'field' for sexual dimorphism.

The canine achieves its greatest sexual dimorphism in the gelada baboon with a difference of 100% larger canines in males than in females. In humans, the difference in size of canines is approximately 7%. That dimorphism varies in different populations. In humans, there is only a low correlation between tooth size and body size.

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(6) Some bats have pointed, hooked deciduous teeth to help the infant hang on to its mother during flight. They are also reported to use those teeth to cling to the mother's teats-a though that would make any nursing mother cringe!

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(7) Elephants and the beaver use their teeth for porterage-to carry things.

(8) Teeth that don't erupt sounds like pathology to the clinical dentist, yet this is the normal state of affairs in several mammals. In the male narwhal, the right upper incisor forms, but does not erupt. In female narwhal, both incisors form but don't erupt.

In certain seals, the deciduous dentition is still produced in anlage, but they do not reach a functional state and are instead resorbed largely before birth.

The baleen (toothless) whales acquire teeth in utero that cover the margins of both the maxilla and mandible. These calcify, but are soon resorbed. There are no adult teeth. In the maxilla, baleen plates have developed in place of teeth. They are homologous to our palatal rugae. Baleen in whaling days was known commercially as 'whale bone' and was used in women's corsets.

The aquatic mammal dugong has incisor teeth that never erupt into the mouth. They seem to support horny plates used for digging up food. This strange animal is probably the source of many stories about mermaids.

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X. NEW WORLD MONKEY

The diet of New World monkeys consists mainly of fruits and insects. The incisors are spatulate. Canines are large and tusk-like. Premolars are bicuspid, except for the lower first which is unicuspid. The mandibular premolars have two roots; the maxillary premolars have three roots.

There are variations, but the representative dental formula for the group is:

The molars decrease in size from mesial to distal. Upper molars have three roots, the lowers have two roots.

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XI. OLD WORLD MONKEYS

The dentition of Old World monkeys differ from those of New World monkeys in many respects. The number of premolars is reduced to two. The mandibular first premolar of Old World monkeys is a sectorial tooth. (My note: a sectorial tooth is one that is adapted for cutting.)

In Old World monkeys, the cusps are connected by distinct transverse ridges (lophs) and third molars are well developed. (My note: loph is from Greek, meaning crest. A bilophodont tooth has four cusps with a connecting ridge to form two lophs which you know as transverse ridges. Our lower second permanent molar is a bilophodont tooth.)

The dental formula is familiar to the practicing dentist:

In Old World monkeys, the molar teeth are more specialized. The upper and lower cheek teeth usually carry four cusps which are connected by transverse ridges, the ridges of the upper teeth occluding with the corresponding grooves of the lower teeth and visa versa. This type of ridge and groove occlusion is quite distinct from the more primitive cusp and fossa occlusion found in New World monkeys, apes and man.

The lower first premolar is a specialized tooth in the Old World monkeys. It is implanted by two roots, mesial and distal in position. The degree to which the canine teeth project beyond the plane of occlusion varies between sexes and among different species. The enlarged canine of males is often used in making threats and is described as a 'social tooth' by some authors.

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Resources

Berkowitz, B., Holland, G., and Moxham, B. Oral Antomy Histology and Embryology. St. Louis: Mosby Year Book, 1992.

Davis, S. The Archaeology of Animals. New Haven: Yale University Press, 1987.

Hillson, S. Teeth. New York: Cambridge University Press, 1996.

Hyman, L. Comparative Vertebrate Anatomy. Chicago: University of Chicago Press, 1949.

Brian Johnson, Personal Communication regarding Greek and Latin terminology.

Jordan, R., Abrams, L., and Kraus, B. Kraus' Dental Anatomy and Occlusion 2nd ed St. Louis: Mosby Year Book, 1992.

Jay Kelley, Personal Communication regarding tooth function, design, and evolution.

Krause, B. and Jordan, R. The Human Dentition Before Birth. Philadelphia: Lea & Febiger, 1965.

Peyer, B. Comparative Odontology. Chicago: University of Chicago Press, 1968.

Riplley, D. ed. Tropical Asia. New York: Time, Inc., 1964.

Romer, A. Vertebrate Paleontology 3rd ed.Chicago: University of Chicago Press, 1967.

Widdowson, T. Special or Dental Anatomy and Dental Histology Vol II, 7th ed. London: Staples Press Ltd., 1946.