Tooth Development, Loss, Replacement, and Decay
Teeth are required for the mastication process, during which food is ground up and swallowed.
The development of teeth in humans, which begins prior to birth, is a highly orchestrated process, with over 20 genes known to be involved. At birth, the developing teeth are usually still hidden beneath the surface of the gums. But, beginning at around six months of age and extending until about 24 months of age, a succession of teeth appears. These so-called deciduous (or milk) teeth are designed for grinding (the molars located in the back of the mouth) and for cutting (incisors) and tearing (canines). A normal set of deciduous teeth comprises 10 upper teeth (central incisor pair, lateral incisor pair, canine (or cuspid) pair, first molar pair, and second molar pair). This first set of teeth is successively replaced by the adult (permanent) teeth, from seven up to about 20 years of age.
In the replacement process, the anchorage of a deciduous tooth is gradually lessened until the tooth "falls out." The replacement of the lost tooth need not occur immediately. Months may go by until the space occupied by the tooth is filled with an adult tooth. A normal adult set of teeth consists of 32 teeth, arranged as 16 upper and lower pairs of incisors, canines, premolars (or bicuspids), first molar, second molar, and third molar.
The loss of an adult tooth is a permanent event. In contrast to reptiles such as alligators, where tooth loss and replacement is ongoing, humans do not generate a third set of teeth.
Teeth consist of a portion that projects above the gum line and the roots, which are below the gum line and serve to anchor the tooth in position. Each tooth is similar in construction, being composed of multiple layers and components. Reflecting the role of teeth in grinding and slicing food, the outer surface is hard. The material, called enamel, is the hardest substance in the human body. Underneath the enamel lies the dentine, a layer that is hard but not quite as hard as the overlaying enamel. Dentine extends down into the roots. Within the dentine are millions of tiny tubes that run into the next layer, called the pulp. The pulp is the central portion of the tooth, and consists of blood vessels and nerves. It is the layer that nourishes the tooth. Despite its appearance, teeth are living structures. A bone-like substance called cementum covers the roots. Finally, each tooth is held in position and anchored to bone by a peridontal ligament.
The health of teeth can be adversely affected by several factors. Environmental compounds can harm teeth. For example, it is now known that the compound dioxin can cause malformation in developing teeth, because of the interference with epidermal growth factor. Teeth can also e mechanically damaged, such as by a blow. Overwhelmingly, however, the health of teeth is compromised by decay (also called caries). Tooth decay is the second most common disorder in humans, next to the common cold. The basis of tooth decay is the bacteria that inhabit the mouth. A great number of species of bacteria live in the mouth, where they convert foods--especially sugars and starches--into acids. The bacteria combine with food debris, saliva and other compounds in the mouth to form a coating on teeth. The coating is referred to as plaque. Normally, plaque is removed by brushing and flossing. However, plaque that is not removed because of improper dental hygiene, or because the bacteria are able to hide in crevasses, can harden into a structure called tartar. Within tartar the acids from the metabolic activity of the bacteria can dissolve the enamel. If left untreated, the resulting cavity (literally a hole in the tooth) can reach the interior dentine and enamel, and can kill the tooth (tooth abscess). Initially, tooth decay is not painful and so can escape detection. However, once the nerves are involved, a toothache ensues, and can be very painful.
Depending on the severity of a cavity, treatment can be minimal (sealing the hole; often referred to as a filling) or more drastic (complete removal of the tooth). Preventative measures can be taken to ward off the development of dental caries. Proper tooth hygiene is important. In urban settings, drinking water often is supplemented with fluoride to retard the onset of cavities. The issue of fluoridation of drinking water continues to be debated, as some evidence exists that too much fluoride can damage tooth enamel and in fact promote tooth decay.
This is the complete article, containing 751 words
(approx. 3 pages at 300 words per page).
