Diseases of the oral mucosa

The oral mucosa lines the oral cavity and comprises a surface squamous epithelium with underlying lamina propria. Below the mucosa is the submucosa, which is composed of fibrous tissue and adipose tissue, and contains lobules of minor salivary glands and neurovascular bundles. In places, there is no submucosa and the lamina propria is continuous with periosteum, forming the resilient mucoperiosteum that covers the maxilla and mandible. The squamous epithelium is composed of keratinocytes arranged in lay­ers: there is a basal cell layer that rests on the basement membrane, a prickle cell layer, and usually a keratinized layer. The keratino­cytes are attached to each other by desmosomes and the basal keratino­cytes are attached to the basement membrane by hemi- desmosomes. The basement membrane is important in maintaining the integrity of the mucosa by sticking the squamous epithelium to the underlying lamina propria. There are two patterns of keratinization: par­akeratosis and orthokeratosis. In parakeratinized epithelium the surface keratinocytes become flattened and the nucleus becomes dark and shrunken (pyknotic). These terminally differentiated squamous cells are eventually lost at the surface by desquamation. In orthokeratiniza­tion, there is a granular cell layer (containing numerous keratohyaline granules) between the prickle cell layer and the keratinized layer. The surface squames become flattened and do not contain any discernible nuclear material. Whilst the majority of cells in squamous epithelium are keratinocytes, there are also accessory cells such as melanocytes, Langerhans cells, and neurosensory cells (Merkel cells and taste buds). The lamina propria is the connective tissue that lies immediately below the epithelium. It is divided into the superficial papil­lary layer (sometimes referred to as the corium) and the deeper reticu­lar layer. The lamina propria is composed of fibrous tissue with a rich neurovascular supply and contains fibroblasts that elaborate collagen and elastin fibres along with other extracellular matrix proteins. The lamina propria also contains components of the mucosal immune defence system such as Langerhans cells, macrophages, mast cells, and lymphocytes. The clinical appearance of the oral mucosa is dependent on the thick­ness of the epithelium, the amount of surface keratinization, melanin (and other) pigmentation, and the vascularity of the lamina propria.