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.
• 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.
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).
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.
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.
