Digestive System I: Oral Cavity and Salivary glands Overview A. The digestive system comprises the oral region and alimentary canal (esophagus, stomach, small and large intestines), and several extrinsic glands. B. It consists of a hollow tube (highly modified in the oral cavity) of varying diameter, composed of a mucosa, submucosa, muscularis, and serosa (or adventitia). C. Function. The digestive system secretes enzymes and hormones that function in ingestion, digestion, and absorption of nutrients as well as in the elimination of indigestible materials.

Oral Region The oral region includes the lips palate teeth and associated structures Tongue major salivary glands lingual tonsils. It is covered in most places by a stratified squamous epithelium whose epithelial ridges interdigitate with tall connective tissue papillae (connective tissue ridges) of the subjacent connective tissue.

Lips The lips are divided into an external region, a vermilion zone, and an internal region. The first two regions are covered by stratified squamous keratinized epithelium, whereas the internal region is lined by stratified squamous nonkeratinized epithelium. A dense irregular connective tissue core envelops skeletal muscle. Sebaceous glands, sweat glands, and hair follicles are present in the external region; minor salivary glands in the internal region; and occasional, nonfunctional sebaceous glands in the internal region and vermilion zone.

The Palate It is divided into an anterior hard palate (possessing a bony shelf in its core) and a posterior soft palate (possessing skeletal muscle in its core). The hard palate is lined by stratified squamous parakeratinized to stratified squamous keratinized epithelium. The soft palate is lined on its oral aspect by stratified squamous nonkeratinized epithelium. It contains minor mucous salivary glands in the oral aspect of its mucosa.

Teeth Overview Teeth are composed of an internal soft tissue, the pulp, and three calcified tissues: enamel and cementum, which form the surface layer, and dentin, which lies between the surface layer and pulp. As in bone, calcium hydroxyapatite is the mineralized material in the calcified dental tissues. Teeth contain an enamel-covered crown, a cementum-covered root, and a cervix, the region where the two surface materials meet.

Tongue Overview The tongue is divided into an anterior two thirds and a posterior one third by the sulcus terminalis, whose apex ends in the foramen cecum. Its dorsal surface is covered by stratified squamous parakeratinized to keratinized epithelium, whereas its ventral surface is covered by stratified squamous nonkeratinized epithelium. Both epithelial surfaces are underlain by a lamina propria and submucosa of dense irregular collagenous connective tissue. The tongue possesses a core of skeletal muscle, which forms the bulk of the tongue.

Lingual papillae They are located on the dorsal surface of the anterior two thirds of the tongue. a. Filiform papillae are short, narrow, highly keratinized structures lacking taste buds. b. Fungiform papillae are mushroom-shaped structures interspersed among the filiform papillae; they contain occasional taste buds. c. Foliate papillae are shallow, longitudinal furrows located on the lateral aspect of the posterior region of the anterior two thirds of the tongue. Their taste buds degenerate shortly after the second year of life. d. Circumvallate papillae are 10 to 15 large, circular papillae, each of which is surrounded by a moat-like furrow. They are located just anterior to the sulcus terminalis and possess taste buds.

Filiform papillae Fungiform papillae

Circumvallate papillae Foliate papillae

Taste buds (a) Taste buds are intraepithelial structures located on the lateral surfaces of circumvallate papillae and the walls of the surrounding furrows. (b) Function. Taste buds perceive salt, sour, bitter, and sweet taste sensations. Glands of von Ebner are minor salivary glands that deliver their serous secretion into the furrow surrounding each papilla, assisting the taste buds in perceiving stimuli. The muscular core of the tongue is composed of bundles of skeletal muscle fibers arranged in three planes with minor salivary glands interspersed among them. Lingual tonsils are located on the dorsal surface of the posterior one third of the tongue.

Major Salivary Glands A. Overview The major salivary glands consist of three paired exocrine glands: the parotid, submandibular, and sublingual. Function. They synthesize and secrete salivary amylase, lysozyme, lactoferrin, and a secretory component, which complexes with immunoglobulin A (lgA) (produced by plasma cells in the connective tissue), forming a complex that is resistant to enzymatic digestion in the saliva. They also release kallikrein into the connective tissue. This enzyme enters the bloodstream, where it converts kininogens into the vasodilator, bradykinin.

Structure. The major salivary glands are classified as compound tubuloacinar (tubuloalveolar) glands. They are further classified as serous, mucous, or mixed (serous and mucous), depending on the type of secretory acini they contain. These glands are surrounded by a capsule of dense irregular collagenous connective tissue with septa that subdivide each gland into lobes and lobules.

Salivary gland acini a. Salivary gland acini consist of pyramid-shaped serous or mucous cells arranged around a central lumen that connects with an intercalated duct. Mucous acini may be overlain with a crescent-shaped collection of serous cells called serous demilunes. b. They possess myoepithelial cells that share the basal lamina of the acinar cells. c. They release a primary secretion that resembles extracellular fluid. This secretion is modified in the ducts to produce the final secretion. d. Salivary glands are classified according to their types of salivary gland acini.

(1) Parotid glands consist of serous acini and are classified as serous. (2) Sublingual glands consist mostly of mucous acini capped with serous demilunes. They are classified as mixed. (3) Submandibular glands consist of both serous and mucous acini (some also have serous demilunes). They are classified as mixed.

Salivary gland ducts a. Intercalated ducts originate in the acini and join to form striated ducts. They may deliver bicarbonate ions into the primary secretion. b. Striated (intralobular) ducts (1) Striated ducts are lined by ion-transporting cells that remove sodium and chloride ions from the luminal fluid (via a sodium pump) and actively pump potassium ions into it. (2) In each lobule, they converge and become the interlobular (excretory) ducts, which run in the connective tissue septa. These ducts drain into the main duct of each gland, which empties into the oral cavity.

Saliva 1. Saliva is a hypotonic solution produced at the rate of about 1 liter (L)/day. 2. Function a. Saliva lubricates and cleanses the oral cavity by means of its water and glycoprotein content. b. It controls bacterial flora by the action of lysozyme, lactoferrin, and IgA, as well as by its cleansing action. c. It initiates digestion of carbohydrates by the action of salivary amylase. d. It acts as a solvent for substances that stimulate the taste buds. e. It assists in the process of deglutition (swallowing).