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Chapter: Anatomy and Physiology for Health Professionals: Digestive System

Humans have two sets of teeth that form during their lifetimes. The primary teeth, also called ­deciduous teeth, erupt through the gums between the ages of six months and four years. These are also known as the baby teeth or milk teeth.



Humans have two sets of teeth that form during their lifetimes. The primary teeth, also called ­deciduous teeth, erupt through the gums between the ages of six months and four years. These are also known as the baby teeth or milk teeth. There are a total of 20 deciduous teeth, 10 in each jaw, which is made up by the mandible and maxilla. Each tooth is clinically described as a dentition. The lower central ­incisors are the first teeth to appear, usually at the age of six months. In one to two-month intervals, additional pairs of teeth erupt. In most cases, all 20 primary teeth have emerged by the age of two years. They are eventu-ally shed, usually in the order they appeared.

The secondary teeth or permanent teeth push the primary teeth out of their sockets called alve-oli, and usually consist of 32 teeth (16 in each jaw; ­FIGURE 24 -7A). They lie more deeply than the primary teeth and absorb the roots of the primary teeth from below. The secondary teeth usually begin to appear at six years of age, but the final teeth, the third molars, may not appear until between ages 17 and 25. The third molars are commonly referred to as wisdom teeth. However, in some individuals, wisdom teeth either never emerge or are totally absent. The primary and deciduous teeth are summarized in TABLE 24-1.

Teeth are classified by their shapes and functions, and include incisors, canines, premolars, and molars. Incisors are shaped like chisels and are ideal for cut-ting food (chopping) or for slicing off pieces of food. The canines, also known as the cuspids or eyeteeth, are cone-shaped and resemble fangs. The cuspids are pointed teeth that are adapted for tearing and shred-ding. The teeth best used for crushing or grinding are the premolars or bicuspids and molars. Both have broad crowns and rounded tips called cusps. Extra grinding ability is found with the molars because they have four to five cusps. Great crushing forces are developed as the upper and lower molars lock together during chewing.

Tooth Structure

Each tooth consists of a crown that projects beyond the gum, also called the gingiva, and a root that is anchored to the jaw’s alveolar process. The alveolus is the pink ridge that surrounds the bases of the teeth. The crown of each tooth surrounds it tightly, like a collar. A gingival sulcus, which is a shallow groove, surrounds the neck of each tooth. Its mucosa is thin and not tightly connected to the periosteum. The epithelium is attached to the tooth at the base of the sulcus, preventing bacte-rial access to the lamina propria of the gingiva and to the cementum of the root, which is relatively soft. These attachments are strengthened and the epithelial cells stimulated when you brush and massage your gums.

The crown of each tooth is covered in glossy, white enamel that mainly consists of calcium salts. The enamel is the hardest substance in the body (­FIGURE 24- 7B), even harder than bone. However, it is brittle and similar to the substance called ceramic. It is only about as thick as a dime and bears the force of chewing directly on it. Enamel contains force-­resisting columns of dense hydroxyapatite crystals, which are minerals aligned perpendicular to the surface of each tooth. When a tooth erupts, the cells producing the enamel degenerate. Therefore, when damaged by abrasion or injury, enamel is not replaced, and it wears away with age. This requires artificial fillings to replace the lost areas of enamel.

The neck of the tooth is constricted, connecting the crown and the root. The root is enclosed in thin, bone-like cementum surrounded by a periodontal ligament, meaning the root is actually embedded in the jawbone. The cementum is a calcified connective tissue. The periodontal ligament contains blood vessels and nerves along with thick collagenous fibers passing between the cementum and bone of the alveolar pro-cesses. The periodontal ligament anchors the tooth in its alveolus, and a fibrous joint or gomphosis is formed.

One root is found in each of the canines, incisors, and premolars. However, the first upper premolars usually have two roots. There are three roots to the first two upper molars, but the corresponding lower molars only have two. Although the roots of the third molars vary, a fused single root is most commonly seen.

The bulk of a tooth below the enamel is the ­dentin, which is harder than bone but similar in structure. It surrounds the tooth’s central pulp cavity that contains blood vessels, connective tissue, and nerves. Collectively, these components are called pulp. Blood vessels and nerves reach the pulp cav-ity through root canals extending into the root. The dentin is rich in proteins, acting as a shock absorber when chewing and biting. It contains unique radial striations known as dentinal tubules . Dentin, cemen-tum, and enamel are calcified. They differ from bone in that they are avascular. Also, dentin and cemen-tum contain collagen, whereas enamel does not and is almost all mineral in content. The pulp supplies nutri-ents to the tissues of the tooth and provides sensation. At the point where the pulp cavity extends into the tooth root, it becomes the root canal. The proximal end of each root canal has an apical foramen, which allows blood vessels, nerves, and other structures to enter into the pulp cavity. The superior and inferior alveolar nerves, which are branches of the trigeminal nerve, serve the teeth. Blood is supplied by branches of the maxillary artery called the superior and infe-rior alveolar arteries.

Tooth Disease

When bacteria eventually demineralize enamel and dentin, cavities or dental caries result. The decay starts when a film of bacteria, sugar, and various debris adheres to the teeth. This is known asdental plaque. Acids are produced, which dissolve the calcium salts in the teeth. Bacterial enzymes can then easily digest the rest of the organic teeth matrices. Plaque can be removed by frequent brushing and daily flossing.

Gum Disease

When plaque is not removed from the gums, it calci-fies, forming a calculus, which is also known as tartar. This condition is more serious than tooth decay. The extremely hard calculi disrupt the seals between the gums and teeth. The sulci are deepened and the gums may become infected by pathogenic anaerobic bacte-ria, a condition called gingivitis. The gums become red, swollen, sore, and often bleed. If the calculi are removed, gingivitis is reversible. If not, inflamed pock-ets of infected tissue develop. Immune system cells and neutrophils then attack the pathogens as well as the gum tissues. Deep pockets develop around the teeth, the periodontal ligament is destroyed, and bone is dis-solved by activated osteoclasts. This condition, known as periodontitis (periodontal disease), affects nearly 95% of people over the age of 35. In adults, periodontal disease causes between 80% and 90% of tooth loss.

However, intervention is still possible. The teeth are scraped, the infected pockets cleaned, and the gums are cut to shrink the size of the pockets. Anti-­ inflammatory and antibiotic therapies are initiated. The surrounding tissues can then reattach to the teeth and bone. Home regimens follow, including consistent brushing, flossing, and rinsing with hydrogen peroxide mixtures. It is also believed that untreated periodon-tal disease may be linked to heart disease and stroke. This is because the existing, chronic inflammation promotes atherosclerotic plaques, and bacteria that enter the blood from the infected gums result in clots to form, clogging cerebral and coronary arteries. Peri-odontal disease is commonly linked to diabetes melli-tus, smoking, and piercings of the tongue or lips.

1. Describe the time periods involved in the eruption of the primary and secondary teeth.

2. Explain the tooth substance that is harder than bone yet can become permanently eroded.

3. Why is gum disease more serious than tooth disease?

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