Structure of the Oral Cavity 1621 The Mucosa

Selection of drug candidates for oral transmucosal drug delivery (TMD) requires cognizance of the physicochemical properties of the drug and likely effect on absorption. The anatomy and physiology of the delivery site also needs to be recognized and evaluated as structural and regional variations in oral mucosa can influence drug permeation characteristics.

All covering and lining tissues of the body consist of a surface epithelium supported by fibrous connective tissue [4]. Comparing the structure of skin and oral mucosa to that of the GI tract, major differences are apparent in the organization of the epithelium. These reflect the different functions of the regions. The linings of the stomach and small and large intestine comprise a simple epithelium. Skin and oral mucosa, in contrast, are covered by a stratified epithelium composed of multiple layers of cells showing various patterns of differentiation between the deepest cell layer and the surface (Fig. 16.1). Such differentiation reflects the functional purposes and demands required of such tissue, such as mobility or rigidity and resistance to mechanical or other damage.

In general, three different types of oral mucosa are recognized (Table 16.1):

• Masticatory mucosa: This covers the gingiva and hard palate, regions that are subjected to the mechanical forces of mastication. It comprises keratinized epithelium resembling the epidermis of the skin and is usually tightly attached to underlying structures by a collagenous connective tissue.

• Specialized mucosa. This has characteristics of both masticatory and lining mucosa and is found on the dorsal region of the tongue. Its surface consists of areas of both keratinized and nonkeratinized epithelium, tightly bound to the underlying muscle of the tongue.

Oral Mucosal Medications

Fig. 16.1 Mucosal regions of the mouth with insets showing the compositions of (a) keratinized and (b) nonkeratinized mucosal epithelium lining various mucosal regions: 1. Mucus layer, 2. Parake-ratinized layer, 3. Epithelium, 4. Lamina propria, 5. Stratum basale, 6. Basal lamina

Table 16.1 Regional variation of mucosal tissue within the oral cavity

Mucosa type

Characteristics

Masticatory

Keratinized epithelium

Hard palate, gingival

25% of total surface area of oral cavity

Lining

Nonkeratinized epithelium

Cheek, sublingual, alveolar

60% of total surface area

Specialized

Both keratinized and nonkeratinized

Tongue

15% of total surface area

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