teeth

teeth have a distinctive anatomy and physiology that is different to the biology of the skeleton. The great eighteenth-century zoologist and anatomist Baron Georges Cuvier was once quoted as saying, ‘show me your teeth and I will tell you who you are.’

Our teeth tell us something about our ancestry, ethnic background, and age, our environment and our health. Teeth are one of the hardest tissues in our body and a valuable source of evidence in understanding the biology of ancient communities throughout the evolution of mankind. Teeth can be used as part of our repertoire of behaviour, by showing or ‘baring’ one's teeth as a means of aggression, or by adding them to a perfect smile. The movement of teeth during eating can stimulate reflex salivation or can limit damage if excessive force is applied. Teeth can therefore serve a variety of functions in our lives from the time that the first deciduous (milk or primary) teeth appear, a process commonly known as teething. By the age of 3 the deciduous teeth are fully formed and by the age of 6 the first permanent teeth appear by displacing their predecessors. A complete permanent dentition (set of teeth) is present at or around the age of 18 years. In the complete deciduous dentition there are 20 teeth, 10 in each jaw. In the complete permanent dentition there are 32 teeth, 16 in each jaw. There are different classes of teeth each with a particular role in eating; incisors (Latin dentes incisivi; cutting teeth), canines (Latin dentes canini; dog teeth), premolars, and molars (Latin dentes molars; grinding teeth).

Each tooth is divided into a crown that projects into the mouth and a root that is embedded into the jaws. The crown is coated with heavily mineralized enamel and the root with a thin layer of cement. The visible crown surface is usually smooth, translucent, and white but may show defects in thickness, mineralization, and colour. During development these defects can be attributed to an interruption of the normal growth processes due to malnutrition or diseases (such as rickets and measles) or to high levels of fluoride in drinking water. Betel nut chewing and smoking can also lead to discolouration.

The bulk of the tooth consists of the bony substance dentine, surrounding the soft inner pulp that contains blood vessels and nerves. In the crown, the pulp has small conical extensions (pulp horns) into the cusps of the tooth, and in the root it extends along one or more canals to the tooth apex, where the nerves and blood vessels enter. The density of nerve fibres in dental pulp is very high; in the mid crown of the average tooth there may be as many as 3000 axon branches with approximately 2000 nerve endings per mm², the highest density in the body. The role these nerve fibres play in the normal sensations from teeth has been a point of conjecture for dentists for over a century, but whatever the stimulus it invariably leads to the pain usually described as toothache which, according to Robert Burns was, ‘the hell o' a' disease’.

Sufferers are well aware that a tooth can become acutely sensitive. Yet there are no nerve endings on the exposed surface. The mystery of the process that activates the nerve endings inside the teeth has only recently been solved. The dentine contains a honeycomb of dentinal tubules that radiate out from the pulp chamber, tapering along their length and increasing in spacing. At the pulp end of dentinal tubules is a layer of cells called odontoblasts. These cells secrete the initial predentine matrix and mineralize it to produce mature dentine. At the dentinal surface of each odontoblast is a process arising from the cell, which tapers as it penetrates up to half the thickness of the dentine. Some of the dentinal tubules containing odontoblast processes also have nerve fibres and most, if not all, also contain dentinal fluid that is formed from the blood capillaries in the pulp. Under normal circumstances the whole system is in equilibrium, with little movement of dentinal fluid, and the tooth is relatively insensitive. Only when the dentinal tubules are open to the atmosphere through damage, disease such as caries, or wear of the enamel (through the excessive use of a toothbrush or acidic drinks) can the dentinal fluid flow out through the tubules into the mouth (see figure of exposed dentine, showing movement of dentinal fluid). Despite the lack of nerve endings at the exposed surface, the area is now acutely sensitive to thermal, mechanical, and osmotic stimuli. An explanation for this apparent paradox involves the nerve fibres at the pulpal end of the dentinal tubules, and the dentinal fluid, which forms a hydraulic bridge. Most investigators now agree that the movement of fluid due to changes in pressure or in surface tension at the open end of exposed dentinal tubules is transferred through the odontoblast process to nerve endings at the base of the tubules. The dentinal fluid in the tubules can now move when a stimulus is applied, and it is this movement that is picked up by the nerve endings — the sensory receptors. All manner of stimuli would then cause fluid movement; temperature — heat greater than 45°C, cold less than 27°C; mechanical — use of a dental bur, a probe, or a toothbrush; removal of fluid by an air stream, absorbent materials, or hyperosmotic solutions such as strong sugar solutions. Blocking the dentinal tubules using dental resins restores the equilibrium and the tooth will once again become relatively insensitive. Thus the hydraulic or hydrodynamic theory of movement of fluid through the dentinal tubules goes some way to explaining this exquisitely sensitive and unique system of receptors that occasionally gives rise to the ‘sensitivity’ of teeth.

Duncan Banks

Bibliography

Bradley, R. M. (1995). Essentials of oral physiology. Mosby, St Louis.

See also dentistry.