Into this chapter are grouped the soft tissues of the mouth and the tooth-supporting tissues. The oral mucosa varies from the thin, fragile lining of the floor of the mouth to the rugged masticatory mucosa of the tongue and hard palate. These tough mucosal surfaces may have to withstand the rigors of masticating hard food. The periodontium which includes the structures supporting the teeth is of great importance to dentists. When it is infected and the tissue destroyed, the teeth may literally fall out. And notwithstanding all the progress in treating periodontal disease, it remains resistant to treatment in many patients. The junction between the tooth root and the supporting tissues provides a potential route of entry of bacteria into the body, which is unusual; all other external openings of the body are lined with epithelium with the exception of the fallopian tubes. The teeth are not held rigidly in their sockets like reptilian or fish teeth. They are able to move slightly in function and have the capacity to reposition as they erupt and drift when unsupported by neighbors or opposing teeth. An understanding of the dynamic structures of tooth support is essential to understanding the response of the periodontium to infection.
3.1 Structure of Oral Mucosa
The oral mucosa is the tissue lining the mouth. The two major layers of the oral mucosa, the oral epithelium and the lamina propria, are equivalent to the epidermis and dermis of the skin.
3.1.1 Oral Epithelium
The oral epithelium is a stratified layer of squamous cells which may either be keratinized or nonkeratinized. The characteristics of the individual layers (i.e., basal, prickle, granular, and keratin) are similar to those seen in the skin. Most of the cells of the epithelium are keratocytes. As they mature and are pushed to the surface by dividing cells in the basal layer, they will fill with keratohyalin granules and finally keratin.
There are three other types of cell in the epithelium.
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The melanocytes produce pigment and transfer it to the keratocytes around them. The number of melanocytes is no greater in heavily pigmented epithelium, but their activity is increased.
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The Langerhans and other dendritic cells are active in the immune response of the epithelium. They act as sentries, detecting the presence of foreign antigens on the surface of the oral epithelium. They then migrate from the epithelium to local lymph nodes where they present information about surface antigens to T (CD4) lymphocytes. The Langerhans cells do not have desmosome attachments, and so during histological processing the cytoplasm shrinks down around the nucleus producing a clear halo. Hence, these cells are referred to as clear cells.
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The Merkel cell is a mechanical receptor for tactile sensations.
The superficial layers of the epithelium may be both keratinized and nucleated. Keratinized epithelium is almost impermeable, due to a glycoprotein intercellular cementing substance, special cell junctions (desmosomes), and the keratin within the cell. Keratin (Greek, kera = horn) is a fibrous protein which is the main constituent in hair, hide, horns and hooves, claws, scales, feathers, and beaks. It is made up of a triple helix in a left-handed coil (contrary to the right-hand coil of collagen). The keratin fibers form a meshwork around the nucleus of the cell and attach to the desmosome plates inside the cell wall. Unlike collagen fibers, they remain within the cell (intracellular), where they provide a scaffolding joining one cell junction to another. The components for synthesizing the keratin molecules come from the constituents of the cytoplasm of the cell itself. As the content of keratin increases, the cell shrinks, shrivels up, dries out, and becomes lifeless but very tough.
The surface ultrastructure is characterized by microinvaginations on the surface side of the flattened squamous cell and microprojections on the deep side (▶ Fig. 3.1). The projections of the cell above interlock like a press stud with the invaginations on the surface of the cell below, providing a strong bond between the cells. Nonkeratinized epithelium is in general thicker than keratinized epithelium. It is slightly more permeable, but only to low-molecular-weight compounds such as glyceryl trinitrate (used to relieve an attack of angina pectoris). The surface ultrastructure lacks a robust interlocking mechanism with the adjacent cells which are readily detached from each other by light mechanical scraping.
Fig. 3.1 A diagrammatic representation of the components of oral mucosa. (a) The lining mucosa has a relatively thick epithelium which is not keratinized (E), supported by thin lamina propria (LP). The submucosa (Sm) contains blood vessels and minor salivary glands, in a loose connective tissue. The submucosa may be attached to muscle (M) or the periosteum (Po) covering bone. (b) Masticatory mucosa has a keratinized epithelium (K) and a dense lamina propria of collagen fibers, which attach the epithelium directly to the periosteum covering bone (B).
3.1.2 Lamina Propria
The lamina propria is a layer of interlocking fibers, which gives strength to the epithelium above. It consists mostly of tough collagen fibers, some elastic fibers, and reticulin. In between the fibers are fibroblasts and other connective tissue cells. Beneath the lamina propria of the mucosa is usually a layer called the submucosa. It is a loose connective tissue containing fat, blood vessels, nerves, and lymphatics. In some areas such as the hard palate, the submucosa is also fibrous and binds the overlying mucosa quite firmly. However, there is no submucosa at all beneath the gingival mucosa. The lamina propria of the gingiva is bound directly onto the periosteum. It is therefore often referred to as a mucoperiosteum (▶ Fig. 3.2).
Fig. 3.2 A scanning electron microscope (SEM) image of the surface epithelial cells from oral mucosa (magnification × 2,000). (a) The surface membrane of each cell has invaginations. (b) These interlock with projections in the adjacent cell. (c) Nonkeratinized epithelium has microplications, and each cell is less firmly attached to its neighbor than keratinized cells.
3.2 Function of Oral Mucosa
The oral mucosa has a protective, secretory, and sensory function.
The protective function is served by its resistance to tearing and compression, which is provided by the tough and yet resilient lamina propria. The oral mucosa is also mostly impervious to the penetration of bacterial toxins. Protection from microorganisms is also afforded by the shedding (desquamation) of the surface layer of cells. Bacterial colonies attached to these surface cells are thus regularly carried away when the surface cells are sloughed off. The cells are themselves flushed away in the saliva and swallowed.
Minor salivary glands in the submucosa secrete via ducts passing through the mucosa. These secretions help to keep the mucosa moist and free of excessive accumulations of bacteria. The secretions of minor salivary glands contain the same antibacterial elements as those from the major salivary gland and contribute to the control of bacterial growth on the oral mucosal surfaces. There are also sebaceous glands sometimes seen on the inside of the cheek (also called Fordyce’s granules). They have no function but are important to recognize as being normal.
The mucosa provides a suitable site for sensory nerve endings, such as those associated with pain, touch, temperature, and the taste receptors of the tongue and palate. Some of these receptors are important in the initiation of reflexes like swallowing or jaw opening.
3.2.1 Rates of Turnover of Oral Mucosa
The rate of cell division (mitosis) of the basal layer of cells normally keeps pace with the rate of desquamation from the surface. The time taken for a recently divided basal cell to reach the surface and exfoliate is the turnover time for one cell. The rate of mitosis is reduced with increasing age but increased by stress, infection, and changes in the levels of female hormones. Apart from these influences, the rates of turnover vary according to skin and mucosa types. The cells of a keratinized epithelium exfoliate after about 60 days for skin and after 45 days for gingiva. In comparison, the nonkeratinized lining mucosa turns over in 25 days. However, even this is a short period compared to the gut epithelium, where cells only last 10 days. The junctional epithelium around the cervical margin of the tooth is very fragile and turns over in 4 to 6 days. The rate of turnover of epithelium is related to its functions. Those which are primarily protective (skin) have a tough layer of impervious keratin and turn over relatively slowly. Mucosa, which must be flexible and stretch during function, is not protected by keratin but is quite thick. The shedding of the surface cells is an important protective device which compensates for the lack of a protective keratin layer. Mucosa which must be permeable to allow for food absorption (e.g., gut) or to allow secretions of fluid to combat bacteria (e.g., junctional epithelium) must be very thin and is therefore easily damaged. Its turnover rate is high.
3.3 Regional Variation of Oral Mucosa
The oral mucosa may be divided into three types, each of which has a different structure related to its function. The three types are masticatory mucosa, lining mucosa, and gustatory mucosa (▶ Fig. 3.3). The gingiva, while forming part of the masticatory oral mucosa, is also intimately related to the periodontium or supporting structures of the tooth. The periodontium consists of the gingiva, periodontal ligament, cementum, and alveolar bone.
3.3.1 Masticatory Mucosa
Masticatory mucosa is a descriptive term for the mucoperiosteum, which forms most of the gingiva and covers the hard palate. It is hard enough to resist the abrasion of coarse and rough food particles. It is also firmly attached to the alveolar bone and teeth so that little displacement occurs when a tough food bolus slides down the tooth. Edentulous patients, who have no dentures, are often able to chew directly onto their masticatory mucosa covering the residual alveolar ridges without causing any damage; they also make such frequent use of the tongue against the palate that the tongue becomes larger and more muscular than usual.
Gingiva: The part of the masticatory mucosa which covers the alveolar bone around and in between the teeth is firmly attached to the underlying bone and therefore referred to as attached gingiva. The part of the gingiva which encircles the teeth but is not attached to either tooth or bone is known as the free gingiva. The attached gingiva is usually keratinized, but if it becomes inflamed, it may become nonkeratinized. The more effectively the teeth and gingiva are cleaned, the greater is the tendency for the gingiva to be keratinized. The width of the attached gingiva varies between 4 and 8 mm but is on the average wider in older people. The gingiva is paler pink in color than the mucosa lining the mouth, due to its opacity, but also may be more heavily pigmented with melanin. The junction between the gingiva and the mucosa lining the vestibule and cheeks is noticeable. This mucogingival junction forms a scalloped line around the root eminence of each tooth (▶ Fig. 3.4).
There is a shallow sulcus (about 2-mm deep) between the free gingiva and the enamel of the tooth, and this sulcus is lined with nonkeratinized epithelium. A probe can normally be inserted into this sulcus to measure the depth without causing bleeding. This sulcus epithelium is continuous with the junctional epithelium, which is a physical attachment or junction between the tooth and its gingiva. When viewed in histological sections, the gingival epithelium has long rete pegs, which represent long and tall ridges penetrating deeply into the lamina propria (▶ Fig. 3.5).
Fig. 3.5 A diagrammatic representation of the epithelial attachment to the tooth. The junctional epithelium is a continuation of the sulcus epithelium but is physically attached to enamel by hemidesmosomes. Note the relationship between the cementoenamel junction (CEJ) and the crest of the alveolar bone. C, cementum; D, dentin; E, enamel.
Most of the fibers of the oral mucosa are irregularly arranged, but there are some recognizable gingival fiber groups associated with the fibers of the periodontal ligament (▶ Fig. 3.6). They are named simply by their orientation. Thus, there are a group of fibers connecting the gingiva to the tooth, the so called dentogingival fibers. The ends of the fibers are anchored into the root by being included into cementum. The alveolar crest fibers bind the gingiva to the crest of the alveolar bone surrounding the tooth socket. Another circular group surrounds the tooth and the interdental fibers run between the buccal and lingual papilla between the teeth.
Fig. 3.6 A diagrammatic representation of the gingival fiber attachment to the tooth and alveolar bone. The dentogingival fibers are anchored into the root by cementum. The alveolar crest fibers secure the gingiva to the alveolar bone. Circular gingival fibers provide a firm cuff around the tooth, holding the attached gingiva to the tooth. C, cementum; D, dentin; E, enamel.
The attached gingiva has a finely pitted or stippled surface due to the insertion of fiber bundles which help bind the lamina propria to the fibers of the periosteum and periodontal ligament.
Collagen fibers are the most essential component of the gingival fiber complex. Reticulin fibers are numerous in the tissue adjacent to the basement membrane and in the tissue surrounding large blood vessels. Oxytalin fibers are present in the connective tissue of the periodontium and appear to be concerned with vascular support. Elastic fibers are only found in gingiva and the periodontium in the connective tissue associated with blood vessels. In the lining mucosa, however, elastic fibers are numerous.
Hard palate: The mucosa of the hard palate is continuous with the gingiva on the palatal side of the maxillary teeth. A pattern of ridges with varying size and shape occurs toward the anterior half of the palate. These ridges are known as rugae and may be as characteristic in their pattern as finger prints. The most anterior feature is the incisive papilla which indicates the position of the incisive foramen. The palatal mucosa has no submucosa and is tightly bound to the palatal periosteum except for an area on either side of the midline, where the submucosa contains fat and more posteriorly, the palatine mucous glands. It also contains the palatine arteries, veins, and nerves.
3.3.2 Lining Mucosa
The lining mucosa covers the cheeks, vestibular sulcus, inner surface of the lips, floor of the mouth, and undersurface of the tongue. Most of the lining mucosa lies directly over muscle, and so there is little space for any submucosa. Where the submucosa is more substantial, there are minor salivary glands, vessels, and nerves. The lining mucosa of the floor of the mouth is particularly thin. It provides a useful route of entry for some drugs. Patients who suffer from angina (heart pain) may get relief by placing a tablet of nitroglycerine under the tongue. Recall that it is common practice to place a thermometer under the tongue as it is in close contact with blood vessels.
3.3.3 Gustatory Mucosa
The dorsal surface of the tongue is covered with a specialized mucosa which has both taste and masticatory function. Taste buds lie within the epithelium over the surface of the tongue, the soft palate, and epiglottis. The tongue is divided into two parts, an anterior two-thirds and a posterior third, by a V-shaped groove, the sulcus terminalis. The epithelium of the tongue is highly keratinized with pointed projections of keratin with a core of lamina propria. The keratin at the tip is extended into a short spiky process. These fine projections are known as filiform papillae. The tongue may appear fury if during illness this keratin is not worn away. The purpose of these papilla in man is uncertain. They probably serve a similar function as in other mammals where they protect the surface of the tongue from the abrasive action of rough foods. They also serve to provide a grip on slippery foods in order to form a bolus and to cleanse the oral surfaces of food debris. There are larger, less thread-like papillae of the tongue called fungiform papillae from their resemblance to a mushroom. They are scattered singly over the tongue but concentrated near the tip. The epithelium of fungiform papillae contains taste buds. The vallate papillae are prominent features, just anterior to the sulcus terminalis. There are only about 12 vallate papillae and they do not project above the surface. The furrow contains numerous taste buds and the openings of the serous glands of von Ebner. The foliate papillae are projections around the side of the tongue and have no special function.
Taste buds are oval structures within the epithelium of the tongue. They consist of elongated cells packed together whose ends open out into a pit below an opening in the epithelium. Not all the cells are active, but some may be either supportive to, or precursors of, active cells. The number of taste buds diminishes with age. The sensory supply to the anterior two-thirds is via the lingual branch of the mandibular nerve. The taste sensations are carried via the chorda tympani branch of the facial nerve. The posterior third receives its sensory supply from the glossopharyngeal nerve. The motor supply to all muscle of the tongue except the glossopharyngeus is the hypoglossal nerve.
The periodontium is a term used to describe the supporting and investing structures of the tooth. These comprise the alveolar bone, periodontal ligament, cementum, and the gingiva, an important component of which is the gingival attachment to the tooth by the junctional epithelium. We have reviewed the structure of the gingiva and gingival attachment, so what follows are the other components of the periodontium, the alveolar bone, cementum, and periodontal ligament.
A removable dental prosthesis may have to derive occlusal support from the residual alveolar ridge. The thickness, and resilience to displacement, of the mucosa overlying the residual ridge exhibits regional variation. This variation may limit the ability of the mucosa in certain regions to resist compression caused by masticatory forces. These limitations may determine the patient’s ability to tolerate the prosthesis.
3.4 Alveolar Bone
The roots of the teeth are held in a ridge of bone which is called the alveolus. Dense bone (cortical plate) covers the outer surface and lines the interior surface of each tooth socket. The dense appearance when seen on a radiograph, of the cortical plate around the tooth root, gave rise to the term lamina dura. Foramina (holes) in the outer cortical plate and the lamina dura allow for the passage of blood vessels and nerves. Within the dense bony plates covering the alveolar bone is a less dense network of bone trabeculae. These trabeculae are like inner girders, which are frequently remodeling in response to the changes in directions of the stresses and strains occurring in the bone during forces applied by the teeth during function. A thin septum of bone separates adjacent teeth and roots of multirooted teeth. The level of alveolar bone around each tooth is surprisingly constant in the unworn dentition at 1 to 2 mm below the level of the cement-enamel junction (CEJ). If this distance is greater than 1 to 2 mm, it is an indication that the bone has been resorbed or remodeled due to disease of the periodontium (chronic periodontitis). This distance does, however, increase quite normally in individuals who experience tooth wear, reflecting another compensatory mechanism, continued eruption (see ▶ Fig. 8.14). Radiographs of the teeth reveal the difference in density between the cortical plates and trabeculae and the level of bone around the roots of each tooth. If for any reason the teeth should not develop, the alveolar ridge of bone is absent. The alveolus gradually resorbs following the loss of teeth due to extraction. There is some evidence that alveolar bone can be distinguished from the rest of the bone of the mandible or maxilla, which is then described as basal bone. Elephants’ teeth erupt surrounded by a shell of alveolar bone, which is quite separate from the jaw bone until eruption occurs.
There is a clinically important regional variation in shape and thickness of the alveolar bone which is determined by the size and shape of the tooth roots and their position in the arch.
Maxillary alveolar bone: In the maxilla, alveolar bone is thinnest around the labial aspect of the maxillary incisor roots. Here, the cortical plate and the lamina dura fuse together without any intervening trabecula bone. The bone becomes progressively thicker toward the molar teeth but particularly so on the palatal side of the roots. Sometimes, there is very little bone separating the apex of the roots of the maxillary posterior teeth and the floor of the maxillary antrum. The thinness of the labial/buccal alveolar bone covering the maxillary roots has important clinical application when anesthetizing the teeth to allow cavity preparation to be carried out painlessly, or a tooth to be extracted. If local anesthetic is injected under the lining mucosa, next to the thin buccal bone of the maxillary teeth, it infiltrates through to the periodontal ligament and dental nerve where it blocks nerve transmission to the tooth allowing painless restorative or surgical procedures. The thinner buccal bone of the maxillary teeth also permits the tooth socket to be expanded sufficiently to allow extraction of the tooth in a buccal direction. The proximity of the molar roots to the maxillary antrum may be a clinical hazard. During efforts to remove a fractured molar root fragment, it may be pushed into the antrum, with resulting clinical complications.
Mandibular alveolar bone: As in the maxilla, the mandibular alveolar bone is thinnest around the labial aspect of the mandibular incisors’ roots and thicker around the molar roots. The lingual plate of bone supporting molar roots is usually thinner than the buccal plate. This should not encourage the clinician to extract mandibular teeth by expanding the socket toward the lingual plate as the lingual nerve and artery may be damaged. The root apices of the third molar may be close to the inferior dental nerve which makes surgical removal of third molar roots potentially hazardous.
The cortical plate of bone of the mandible is thicker than that covering the maxilla. A clinical consequence of this thick cortical plate is that anesthetic solutions do not readily filter through it. Infiltration anesthesia for mandibular teeth is usually ineffective. The alternative is to block the mandibular nerve by depositing anesthetic solution close to the nerve before it enters the mandible on the mesial aspect of the ramus.
Histology of alveolar bone: The cortical plates and lamina dura of alveolar bone consist of circumferential and concentric (haversian) lamellae (see Chapter 7.3.4 Intramembranous Bone Formation). The bone intervening between the lamella has the same basic histology, but as a result of resorption, it has been remodeled into a honeycomb-like system of trabeculae (struts). The histological appearance of trabecula bone is misleading as it gives no indication of their three-dimensional structure (see▶ Fig. 7.6). The spaces between the trabeculae are occupied by red marrow (hematopoietic tissue) in the young, but this is replaced in the adult by fatty tissue. Fibers run through the alveolar bone, connecting the roots of neighboring teeth. Fibers embedded in the cementum of the roots are also embedded in the lamina dura of the tooth socket and are known as Sharpey’s fibers.
Alveolar bone, like all other bones, contains no sensory nerves except those conveying impulses along C fibers which are concerned with healing. The extraction of a tooth is painful due to damage to the nerves supplying the dental pulp, periodontal ligament, gingiva, and periosteum. When the osteotomy (bone removal) site for an implant fixture is prepared, the only tissue with a nerve supply is the periosteum, which may be anesthetized using a local infiltration. This is of clinical importance, as it allows an osteotomy to be prepared in the mandible, without administering a nerve block to the inferior dental nerve. This nerve therefore remains sensitive and reactive to any damage which might occur during the osteotomy procedure by the operator cutting too deep into alveolar bone. Inferior dental nerve damage is a serious complication of implant placement, which can be avoided by leaving the inferior dental nerve responsive, so that the patient may warn the operator before serious damage to the nerve occurs.
