Digestive System: Oral Cavity

Tongue:

• Filiform papillae are more numerous and the smallest. They are found on the entire anterior dorsal surface of the tongue and have tapered projections that point towards the rear of the tongue. They form rows that diverge and seem to run parallel to the sulcus terminalis. They are composed of keratinized stratified squamous epithelium and a core of lamina propria. The low rate of desquamation results in white coating, with no connection to poor health. It is the only type of papillae with no taste buds.
• Fungiform papillae are mushroom-shaped and are located on the dorsal surface. They are scattered among filiform and project above them. They are relatively translucent with nonkeratinized epithelium. This allows for the underlying capillaries to show through them, making them appear red. Taste buds are present in the epithelium on the dorsal surface.
• Circumvallate papillae are large, dome-shaped and are located just anterior to the sulcus terminalis. Humans have 8-12. Each papilla is surrounded by a moat-like space lined with stratified squamous epithelium. The lateral surface epithelium contains many taste buds. Ducts of the lingual salivary glands are Von Ebner’s glands, which deposit a serous secretion into the moat. Secretions clean debris from the moat to facilitate taste bud function.
• Foliate papillae consist of parallel ridges of mucosa. They are located near the base at the lateral margins of the tongue. They are separated by mucosal clefts that sit at right angles to the long axis of the tongue. Contain many taste buds in epithelium of the lateral walls of the papillae, and small serous glands empty into the cleft.

Taste Buds are oval with pale-staining that extend through the thickness of the epithelium. They have a small opening at the epithelial surface called the taste pore. There are three cell types:

• Neuroepithelial cells (taste or sensory cells) are chemoreceptors. They are elongated and most numerous, with microvilli on the apical surface. Apical cells are connected to each other or to the supporting cells by tight (occluding) junctions. At the base they synapse with processes (dendrites) of afferent sensory neurons (i.e. cranial nerves VII, IX, X). Turnover time is 10 days.
• Supporting cells are elongated and less numerous, and also contain microvilli at the apical surface. They have tight junctions but do not synapse. Turnover time is 10 days.
• Basal cells are at the basal portion of the bud. They are the stem cells for sensory and supporting cell renewal.

Acini of the Salivary Glands. There are three types:

• Serous acinus, have serous cells that secrete serous. They contain zymogen granules (which impart eosinophilia) in the apical cytoplasm, and large amounts of rER (which imparts basophilia), polyribosomes, and Golgi. The organelles are found in the basal cytoplasm. Serous cells are joined at the apex by junctions.
• Mucous acinus, have mucous cells that secrete mucin. They are usually the apical part of the cell and appear empty. The nucleus is typically flattened against the base of the cell. The Golgi and mucin granules are located apically. Golgi adds large amounts of carbohydrates to the protein base to make mucin. Junctional complexes are apically located.
• Mixed acinus, have both serous and mucous cells. The serous cells make a cap called demilunes, and canaliculi extend from the lumen between mucous cells to drain secretion from serous cells.

Duct systems have three parts:

• Intercalated ducts, which are located between secretory acinus and larger ducts. They are lined by low cuboidal cells and are active in serous and mixed glands.
• Striated ducts, which are simple cuboidal epithelium that become columnar near excretory ducts. They have numerous basal infoldings, which are seen in LM as strations. The infoldings contain elongated mitochondria. The basal infolds are always associated with fluid and electrolyte reabsorption (modification of secretion).
• Excretory duct

In serous glands, there are highly developed striated ducts, which modify secretions by removal or addition of components. In mucous glands, there are poorly developed striated ducts, which do not modify secretions.

Parotid glands are totally serous. They are the largest salivary gland with long, narrow intercalated ducts and large, conspicuous striated ducts. They may contain much adipose tissue. Secretions sometimes contain pieces of the facial nerve (CN VII). Presence of facial nerve is a diagnostic characteristic of the parotid glands. Viral infections of the parotid (ex. mumps) often damages the facial nerve

Submandibular glands are mixed glands with mostly serous secretions. Some mucous acini capped with serous demilunes can be found among the serous acini.

Sublingual glands are the smallest salivary glands, which empty directly into the floor of the mount or into submandibular ducts. Mucous acini predominate, and some acini have demilunes.