2.1 ORGANIZATION OF THE DENTITION

I. INTRODUCTION.

The very early history of teeth is not fully known. Teeth are calcified structures derived long ago from dermal denticles during the time in vertebrate history when jaws first evolved. Structurally a tooth consists of a calcified collagenous tissue known as dentin which is, at some stage of the life history of the tooth is covered by a highly calcified layer of enamel.

The size, shape, number, construction, location and life span of teeth reflect their function and their evolutionary history. We retain many of the early patterns from the ancient past. The order of eruption, the interdigitation of the teeth, the regional specializations of teeth into classes, and the replacement of deciduous teeth with permanent successors are among a few of those patterns. Our teeth reflect their evolutionary history.

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II. FEATURES OF DENTITIONS; THE CHARACTERISTICS OF TEETH.

In ourselves, we can identify the crown, neck and root regions. The pulp cavity of our teeth narrows to form a pulp chamber, as is true for most mammalian teeth. The continuously growing incisor of the rat is an exception. In reptiles, amphibians, and fishes, the pulp chamber is generally open at its base. Cementum, a form of bone, covers the root of mammalian teeth. Cementum is absent in the teeth of many lower vertebrates.

In sharks, there is more than one row of teeth. As the functional teeth wear, they are replaced by successional teeth supplied from the lingual aspect. The teeth are essentially similar to the placoid scales that cover the bodies of sharks. The progressive replacement of the teeth is a functional adaptation. In the bony fishes, teeth may appear additionally on the tongue, roof of the mouth, or in the throat. The teeth may occur on any of the jaw bones of dermal origin. The teeth of bony fishes vary greatly in shape and arrangement.

In amphibians and reptiles the teeth are usually of a simple conical form, are generally numerous, and often occur on bones of the palate as well as along the jaw margins. Among the reptiles ancestral to mammals (Therapsid reptiles), palatal teeth disappear, the teeth become limited to a single row along the jaws, and they begin to differentiate into distinct classes of teeth as in mammals.

Teeth in mammals (including ourselves) are confined just to the jaw bones. There are at the most two, and sometimes only one, functional generations. Toothless whales and certain edentates, (the anteaters) have no functional generations of teeth at all. There is great species diversity of form amongst mammals compared with the teeth of reptiles. The teeth of mammals are always socketed. In some species, the teeth are continuously growing.

The dental formula for primitive placental mammals is: There is a total of total of 44 teeth in the permanent dentition of primitive placental mammals.

There are many specializations. Some are mentioned here for reference only.

In carnivores (cats and dogs), the fourth upper premolar and the lower first molar have become a shearing apparatus known as carnassial teeth. They show their greatest development in the lion and tiger. Carnassials are longer and larger than the other teeth; they are adapted to cutting instead of tearing.

Several mammalian orders are herbivorous and hoofed such as horses, cattle, and elephants. The cusps of their molars are often crescentic in outline or are connected to form ridges. Grass is very abrasive and a very long-crowned hypsodont tooth has evolved. The molars present a very convoluted surface of ridges of enamel with the clefts filled in by a special tissue, coronal cementum. As a tooth wears away, grinding occurs on a complex surface of dentin, cementum and deeply enfolded enamel. In horses, the wear on incisors reveals complex internal anatomy that can be used to estimate the age of the animal.

In mammals in general, wherever the skull is elongated, there is spacing. Only in humans are all the teeth normally in proximal (no spacing) contact.

The canine teeth in many primates, such as the larger monkeys and baboons, are large and form powerful dagger-like teeth. They are especially prominent in the males. Some describe the large canine as a 'social tooth' since it is used for threats and intimidation by male baboons.

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III. THE ATTACHMENT OF TEETH.

The teeth of all mammals are anchored in the alveoli of the jaw in a peg-in-socket joint configuration known as a gomphosis.

Many other types of attachment are found in other vertebrates. A few are described here for reference only.

In reptiles, attachment of the teeth is commonly by ankylosis either to the lingual side of the jaw, pleurodont condition, or to the crest of the jaw, acrodont condition. In living crocodiles and alligators the teeth are not ankylosed but have roots which are situated in deep sockets, thecodont condition similar to the mammalian condition. It is different from mammals in one crucial way: the socket is not drastically remodeled during tooth succession as it is in our dentition.

In many reptiles, amphibians, and fish, the teeth are firmly attached to the bone. In rare instances, the teeth of some snakes and large predatory fish are hinged to accommodate ingestion of prey.

In mammals (and humans) the tooth socket or alveolus is subject to continuous remodeling. This adaptation allows for the eruption and shedding of the deciduous teeth and provides for eruption of permanent teeth. After the permanent teeth come into a functional relationship, the alveolar process constantly remodels itself in life to accommodate the secondary eruption of the teeth, itself a compensation for occlusal wear. Also, remodeling facilitates physiological mesial drift to compensate for proximal wear.

The remodeling response of the alveolar bone to functional change is the basis of orthodontics: apply prolonged steady pressure to a tooth and it will move.

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IV. HETERODONTY, THE REGIONAL SPECIALIZATION OF TEETH.

In alligators and crocodiles, the teeth have a simple crown form, conical and sharp. When the teeth in the tooth row are similar in size and shape, we describe that condition as homodont.

Many species of animals have teeth that are regionally specialized. The teeth of some reptiles show a tendency for the "regionalization" of the tooth row. In humans as in other mammals, we recognize classes of teeth such as incisors, canines, premolars, and molars. A dentition with such regional specialization is described as heterodont.

The most spectacular specialization occurs in an arctic whale, the narwhal.

In the male narwhal, a single extremely elongated spirally grooved left single tusks comprises the whole dentition. A pulp chamber extends for the entire length of the tooth, making it somewhat fragile. There is no enamel, which renders the tusk somewhat elastic. That single-left tusk in the male can achieve a length of 12 feet. The right incisor grows up to 29 cm in length, but usually does not erupt.

In female narwhal, the upper teeth achieve a length of 23 cm, but do not erupt. The non-eruption of teeth is normal is some species; we will cover that issue in another article.

Another illustration of extreme specialization is the elephant. Elephants have a proboscis, a trunk that functions like a fifth extremity. Their intelligence may correlate with this apparatus. The robust skulls are notable for their size and heavy construction to service the long trunk and the dentition.

The tusks of elephants are incisors. Why aren't they classified as canines instead of as incisors? The answer is that the definition of tooth class in the maxillary arch is defined by supporting bone and the succession of teeth.

Upper incisors by definition arise in the premaxilla, a bone that is distinct in most mammals, but is merged into the maxilla in humans. The ancient division between these two bones reasserts itself in clefts of the upper lip involving the alveolar ridge. In such an anomalie, incisors are mesial to the cleft, canines and posterior teeth are distal to the cleft.

Upper canines are defined as those teeth in the maxilla adjacent to the incisor. Premolars are defined as cheek (posterior) teeth with deciduous predecessors. Molars are cheek teeth distal to premolars that DO NOT have deciduous predecessors.

Lower teeth are defined by their relationship to the upper teeth--a stable relationship in the evolution of teeth. Alternation is the general rule that one lower tooth relates to the upper tooth of its own number and the one in front. An illustration of this for humans is in shown here.

The dentition in elephants consists of the tusks (only in the upper) and the distinctive lophodont (a crown form with transverse ridges) molar teeth. The mature adult tusk is preceded by a 'milk' tusk which is shed after one year. The adult tusk in continuously growing. Enamel occurs only at the tip; it is soon worn off. Fossil intermediaries suggest that the tusks are lateral incisors. The tusk, homologous to dentin in humans, is known commercially as ivory. Due to massive destruction of elephants by poachers, the international trade in ivory is now prohibited.

In the elephant, canines and premolars are missing. Elephants are herbivores with large cheek teeth. In each quadrant (half of a jaw) there are three deciduous molars replaced by three permanent molars. Functionally, only one and part of a successor molar are present in the oral cavity at any one time. The crowns are arranged in lamellae, presenting alternating layers of dentin, enamel, and cementum. Each tooth is worn down to the last remains. Succession is horizontal, not vertical as it is in our mouth.

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V. CONCEPT OF THE MORPHOGENETIC FIELD.

Heterodonty, the specialization of teeth into classes has raised a number of theoretical questions. In experimental embryology, the early embryo is viewed as a mosaic of 'organizational fields' each of which pursues its own unique developmental path. Butler has applied this idea to teeth. It works like this: the tooth row is conceived of as three regions, the incisive, canine, and molariform regions. In each region there is a 'best copy', the flagship of the group. It is best illustrated with the canine since there is only one in its class, and it is extremely stable. Dentists find that the canine is very consistent in form and is rarely missing.

The 'best copy' of the incisors is the central incisor. It is very stable in form and is seldom missing. Lateral incisors are variable to the point of being peg-shaped. They are sometimes congenitally absent.

In the molariform group, the first permanent molar is the 'best copy' of the group. In modern humans, second and third upper molars are progressively smaller, the distolingual cusp tends to disappear and the tooth retreats from being rhomboidal in shape to heart shape. Going forward in the tooth row, the pattern is less clear.

Paleontologists tell us that our 'first' and 'second' premolars are actually the third and fourth in the tooth row. Remember the primitive mammalian tooth formula with four premolars per quadrant? We've lost the first two premolars. So in a sense, the premolars most distant from the first permanent molar have disappeared from their field altogether.

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The reaction to the field theory by dental students is usually a yawn. To do it justice, think of it as a scientific hypothesis and consider this implication: events at one place along the tooth row should have an effect elsewhere along the tooth row. As we will see, this is frequently the case.

On one hand, teeth near each other in the human dentition do, in the main, look alike. They share a similar morphology, so that fits with the hypothesis. Now, try a prediction: if a tooth is missing in the tooth row, then others should be missing, or at least smaller. In a future section we will look at work by Garn and associates, and indeed it is true. If one or more third molar teeth are missing, then there is a significantly greater chance of other teeth being missing or at least smaller.

The 'field theory' is one of the old war horses in dentistry, but its implications are important for work in developmental biology and molecular biology.

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VI. SUCCESSION IN TEETH/GENERATIONS OF TEETH.

In humans, as well in most mammals, there are two functional generations of teeth. When the deciduous (also called the milk or lacteal) teeth are replaced by permanent teeth, this condition is called diphyodont.

Amongst toothed whales and certain seals, there is only one functional generation of teeth; this condition is called monophyodont. In those creatures, a rudimentary deciduous dentition is formed, but is resorbed without erupting. An extreme reduction of teeth is achieved in the South American anteater. It is without teeth all together, a condition described as anodontia.

Reptiles, amphibians and fishes have not one or two, but many generations of teeth. They are polyphyodont. The succession of teeth occurs throughout the life of the animal and the total number of replacements for each tooth position may be very large, for example fifty replacements per tooth position in the crocodile.

The replacement teeth erupt from the lingual (tongue) side. The system is most elegant in the shark where the simultaneous replacement teeth progress in a 'revolver' fashion with the newly formed teeth arising from the lingual and the exfoliated 'spent' teeth shed from the external or labial side.

Dental development begins first at the anterior end of the jaws and proceeds caudal (distally). Teeth erupt, are shed, and replaced in a similar anterior to a posterior pattern. Functionally, this prevents a large section of the jaw from being denuded at any one time. We retain a trace of this ancient replacement in three aspects: Our succession of teeth is reduced to two generations, and that replacement usually arrives from a slightly lingual position.

Our tooth eruption and shedding reflects another ancient pattern, that is the in growth and development, maturation is from the cephalic (head) to caudal (tail) end. We see it as the teeth first to erupt--and shed are the anterior teeth at the midline; the succession is posteriorly, away from the midline.

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VII. CONCLUDING REMARKS.

One major section, the history of cusps has been omitted here. That will be developed in a future unit.

Two major themes have been stressed. One is the remarkable variation in tooth morphology that exists among the vertebrates. On the other hand, basic features commons to dentition have been emphasized. These have included the succession of teeth, the order of their replacement, and their arrangement according to the functional needs of the oral apparatus.

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Resources

Gaunt, W. and Miles, A. 'Fundamental Aspects of Tooth Morphogenesis' in Miles, A. ed Structural and Chemical Organization of Teeth. New York: Academic Press, 1967.

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

Hillson, S. Dental Anthropology. New York: Cambridge University Press, 1996.

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

Miles, A. and Poole, D. "The History and General Organization of Dentitions" in Miles, A. ed Structural and Chemical Organization of Teeth. New York: Academic Press, 1967.

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

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

This is a wooly mammoth skull unearthed at Ilford, near London. It has the distinctive twist of the mammoth's enormous tusks. Today, elephants are the last survivors of a formerly widespread and diverse group of mammals that included the mammoth and the mastodon. Mammoths grazed in open grassland and the tundra during the Ice Ages. Mastodons were forest dwellers. Their skeletal remains are uncovered frequently here in the Midwest.