7.2. The Oral Cavity

The cheeks, tongue, and palate frame the mouth, which is also called the oral cavity or buccal cavity. At the entrance to the mouth are the lips, or labia (singular = labium). Their outer covering is skin, which transitions to a mucous membrane in the mouth proper. The lips cover the orbicularis oris muscle, which regulates what comes in and goes out of the mouth. The labial frenulum is a midline fold of mucous membrane that attaches the inner surface of each lip to the gum.

Vestibule

The anterior and lateral portions of the mouth form a U-shaped cavity between the teeth, lips and cheeks. This area is called the vestibule.

Cheeks are muscular in humans and cover the teeth, forming the lateral aspect of the face. The inside of the cheek is lined with a mucous membrane (buccal mucosa) which is part of the oral mucosa. During mastication (chewing), the cheeks and tongue between them keep the food between the teeth.

The cheeks are covered externally by hairy skin, and internally by the mucous membrane. This membrane is comprised by a sheet of stratified squamous epithelium and a sheet of areolar connective tissue. The epithelium is mostly smooth, but may have caudally directed papillae (for example, in ruminants). The mucosa is supplied with mucous or salivary secretions from the buccal glands located between the mucous membrane and the buccinator muscle. In carnivores, a superior buccal gland is large and conspicous. It is called the zygomatic gland, because it is found above the zygomatic arch. Between the skin and mucous membranes are connective tissue and buccinator muscles. During chewing, food that gets crushed by premolars and molars tends to fall on the tongue or into the vestibule. Contraction of the buccinator muscle pushes the cheeks against the teeth. This helps to direct food that would fall into the vestibule back in between the teeth for further reduction of particle size. The buccinator muscle may also help in suckling by babies.

Some mammals such as squirrels and hamsters have highly distendable cheeks. Their vestibules form buccal (cheek) pouches that are used for temporary storage of food or during foraging. These pouches are most common in rodents and monkeys but they are also present in the platypus and the marsupial koala. The cheek pouches of chipmunks can reach the size of their body when full. The females of some species of hamster are known to hide their young in their cheek pouches to carry them away when they fear danger. Other species of hamsters are known to fill their pouches with air to increase bouyancy while they swim.

Oral Cavity Proper

The part of the oral cavity (mouth) that is not the vestibule is called the oral cavity proper. It is delimited by the teeth, the pharynx, the roof of the mouth and the tongue, or floor of the mouth.

Simultaneous chewing and breathing is made possible by the roof of the mouth, which is called palate. The anterior region of the palate serves as a wall (or septum) between the oral and nasal cavities as well as a rigid shelf against which the tongue can push food. It is created by the maxillary and palatine bones of the skull and, given its bony structure, is known as the hard palate. If you run your tongue along the roof of your mouth, you’ll notice that the hard palate ends in the posterior oral cavity, and the tissue becomes fleshier. This part of the palate, known as the soft palate, is composed mainly of skeletal muscle. We move the soft palate subconciously to yawn, swallow, or sing. During swallowing, contraction of two muscles that attached to the soft palate move it upward to close the passage into the nasal cavity and direct the food into the oropharynx.

A fleshy tissue called uvula extends down from the center of the posterior edge of the soft palate. It has been suggested that the uvula is a vestigial organ, but it may have a role in preventing unintended early swallowing of items in the mouth during chewing. The space between the uvula and the base of the tongue is the fauces and it marks the limit between the oral cavity and the pharynx. Two muscular folds extend downward from the soft palate, on either side of the uvula. Anteriorly, the palatoglossal arch lies next to the base of the tongue. Posterior to it, the palatopharyngeal arch forms the superior and lateral margins of the fauces. Between these two arches are the palatine tonsils. These are clusters of lymphoid tissue that protect the pharynx from pathogens. The lingual tonsils are located at the base of the tongue.

Oral mucosa

The oral mucosa is the mucous membrane lining the inside of the mouth and consists of stratified squamous epithelium termed oral epithelium and an underlying connective tissue termed lamina propria. Some alterations in the oral mucosa lining the mouth can reveal systemic conditions, such as diabetes, vitamin deficiency, or chronic tobacco or alcohol use.

Classification

It can be divided into three main categories based on function and histology:

• Masticatory mucosa, keratinized stratified squamous epithelium, found on the dorsum of the tongue, hard palate and attached gingiva.

• Lining mucosa, nonkeratinized stratified squamous epithelium, found almost everywhere else in the oral cavity, including the:

  • Buccal mucosa lining of the cheeks.

  • Labial mucosa lining of the lips internally.

  • Alveolar mucosa refers to the mucosa between the gums and the buccal/labial mucosa.

• Specialized mucosa, specifically in the regions of the taste buds on lingual papillae on the dorsal surface of the tongue that contains nerve endings for general sensory reception and taste perception.

Structure

The epithelium of the oral mucosa may be keratinized or nonkeratinized. Keratinization is the differentiation of keratinocytes in the stratum granulosum into nonvital surface cells or squames to form a stratum corneum very rich in keratin.

Keratinized squamous epithelium is present in the attached gingiva and hard palate as well as areas of the dorsal surface of the tongue. Nonkeratinized squamous epithelium covers the soft palate, inner lips, inner cheeks, and the floor of the mouth, and ventral surface of the tongue.

In a keratinized oral mucosa, the epithelium consists of four layers:

• Stratum basale (basal layer)

• Stratum spinosum (prickle layer)

• Stratum granulosum (granular layer)

• Stratum corneum (keratinized layer) 

In nonkeratinized epithelium, the two deep layers (basale and spinosum) remain the same but the outer layers are termed the intermediate and superficial layers.

Deep to the epithelium, the connective tissue layer (lamina propria) has two layers: papillary and dense. The papillary layer is superficial and consists of loose connective along with blood vessels and nerve tissue. The tissue has similar amounts of fibers, cells, and ground substance. The deeper dense layer contains a larger amount of fibers. Between the papillary layer and the deeper layers of the lamina propria is a capillary plexus, which provides nutrition for the all layers of the mucosa and sends capillaries into the connective tissue papillae.

A submucosa may or may not be present deep to the dense layer of the lamina propria, depending on the region of the oral cavity. If present, the submucosa usually contains loose connective tissue and may also contain adipose tissue or salivary glands, as well as overlying bone or muscle within the oral cavity.

A variable number of Fordyce spots or granules are scattered throughout the non keratinized tissue. These are visible as small yellowish bumps on the surface of the mucosa. They correspond to deposits of sebum from sebaceous glands in the submucosa.

Summary

The oral cavity is formed by the vestibule and the oral cavity proper. The vestibule is delimited by the teeth, lips and cheeks. The oral cavity proper is delimited by the teeth, fauces, roof and floor of the mouth. The oral cavity is lined with oral mucosa, which can be keratinized or nonkeratinized and contains mucous and salivary glands.

Key terms

Vestibule, mouth, oral cavity, buccal cavity, palate, tongue, teeth, gum, gingiva, oral mucosa, cheek pouch, palatopharyngeal arch, palatoglossal arch, tonsil, zygomatic gland, fauces, uvula, masticatory mucosa, buccal mucosa

Figure credits

Figure 1 by Lusb - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=12839749

Figure 2 by Duncan Kenneth Winter - originally posted to Flickr as 65-5390-1 by Otis Historical Archives Nat'l Museum of Health & Medicine. See Wired article for more information., CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=6266235

Figure 3 by OpenStax College - Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013., CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=30148422

Figure 4 by OpenStax College - Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013., CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=30148358

Figure 5 by Wiki-minor - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=17201584