Chapter 8: Gastrointestinal System
Oral cavity
The major structures of the oral cavity are the lips, teeth, tongue, oral mucosa and salivary glands. The primary function of the oral cavity is in prehending, masticating, and moistening of food. The latter two functions prepare food for deglutition (swallowing).
The oral mucosa (labial and gingival) offers protection during mastication and deglutition, and is composed of stratified squamous epithelium that may be keratinized. Below the mucosa is a dense layer of vascular collagen-rich stroma, the lamina propria. Mobile regions of the oral cavity, such as the soft palate, may also contain skeletal muscle that aids in mastication and deglutition.
Tongue
The tongue is a muscular organ that serves two major roles: mechanical manipulation of ingesta and sensing (taste). The tongue is covered by typical oral mucosa (stratified squamous epithelium) overlying a lamina propria and large amounts of organized skeletal muscle (discussed below).
The dorsal surface of the tongue is decorated by specialized structures called papillae. These structures serve diverse functions, ranging from facilitating taste (e.g. circumvallate) to grooming (e.g. filiform papillae in cats). There are four distinct papillae: filiform papillae, fungiform papillae, foliate papillae, and circumvallate papillae. The filiform papillae generally predominate in the rostral portion of the tongue. Species such as cats have well-developed filiform papillae which contribute to the “sandpaper-like” feel of their tongues. Circumvallate and fungiform papillae are partially lined by taste buds, specialized chemoreceptors that transmit the sensation of taste. These papillae are often located caudally on the dorsal surface of the tongue and are bilaterally symmetric in distribution. They may be prominent structures and should be not be misinterpreted as pathologic lesions.
The core of the tongue is composed of abundant skeletal muscle that is arranged in three distinct layers of alternating orientations/planes (vertical, horizontal, and longitudinal). This robust musculature facilitates the extreme mobility of the tongue.
Salivary Glands
Salivary glands, which include parotid, submaxillary, sublingual, zygomatic (carnivores) and accessory glands, contribute to the production of saliva. Saliva is a complex watery secretion that acts to lubricate, moisten and digest ingesta and is composed of a mixture of mucus, electrolytes, antibodies and digestive enzymes (e.g. amylase).
Structurally, salivary glands are composed of lobules of exocrine tubuloacinar glands that secrete into a duct. Secretion is mediated, in part, by contraction of myoepithelial cells that surround acini. Salivary glands are composed of serous and/or mucus secretory epithelial cells. Serous secretory cells have an eosinophilic, slightly granular cytoplasm, whereas mucus cells have abundant foamy, lightly staining cytoplasm. The parotid gland is entirely composed of serous acini, the sublingual primarily mucous acini, and the submaxillary a combination of serous and mucous acini.
