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.
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).
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.
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).
Fig. 3.3 A diagrammatic representation of the distribution of lining, masticatory, and gustatory mucosa.
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. 3.4 Healthy oral mucosa. The attached gingiva (AG) reveals the contour of the underlying alveolar bone. The broken line marks the mucogingival junction between the attached gingiva and the lining mucosa (LM).
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