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Digestive System Overview

• The digestive system includes the alimentary canal, its associated organs like the tongue and teeth, and exocrine glands, such as salivary glands, the liver, and the pancreas.

Lips and Cheeks

• Lips and cheeks consist of tissue folds with skin on the external surface and a mucous membrane adjacent to the oral cavity.
• The mucous membrane features stratified squamous epithelium, keratinized in ruminants and horses.
• The underlying connective tissue contains serous or mixed glands and skeletal muscle.

Hard and Soft Palates

• The hard palate presents a keratinized stratified squamous epithelium on its surface.
• The underlying connective tissue blends with the periosteum of the palate bones.
• The dental pad, which replaces upper incisors in ruminants, has very thick keratin.
• The soft palate, located caudally in the oral cavity, is a fold of mucous membrane with respiratory epithelium on the nasal side.
• The oral side of the soft palate contains stratified squamous epithelium.

Tongue

• The dorsal epithelium of the tongue is stratified squamous epithelium with varied keratinization degrees, while the ventral epithelium is thin and non-keratinized.
• The tongue has papillae, small outgrowths located mainly on the upper surface.
• Papillae vary considerably in size and appearance, with some (filiform) having threadlike projections or spines.
• Other papillae are cushion-shaped, like circumvallate and fungiform, whereas foliate papillae form a series of folds.
• Taste buds are in the epithelium of circumvallate, foliate, and fungiform papillae.

Teeth

• Enamel, the hardest substance in the body, makes up the outer surface of the crown of brachydont teeth.
• Dentine, composed of collagen fiber tubules and minerals, is deep to the enamel.
• The neck region of the tooth, which narrows the crown, sits just below the gingival surface.
• The gingival sulcus is the space between the tooth's neck and the surrounding gingiva.
• Cementum, made of collagen and mineral, covers the tooth root's outer surface.
• Odontoblasts produce dentine with roughly 90% type I collagen and 70% wet weight hydroxyapatite.
• Ameloblasts, tall columnar cells produce enamel prior to tooth eruption.
• Each ameloblast has a Tomes process, an elongated tip secreting the organic matrix of enamel rods.
• Enamel consists of 96% mineral content, of calcium phosphate crystals or prisms.
• The enamel layer cannot be repaired, as ameloblasts die when the tooth erupts.
• Cementoblasts produce cementum, which is slightly softer than dentin.
• Approximately 45% to 50% of cementum is inorganic material (hydroxylapatite) by weight.
• Approximately 50% to 55% of cementum is organic material and water by weight.
• All equine teeth, the porcine canine tooth, and some bovine teeth are hypsodont type.
• Hypsodont teeth have the outer surface formed by cementum overlying the enamel.
• Equine teeth feature cementum invagination into enamel and dentine, forming the infundibulum in the center.

Salivary Glands

• Secretory adenomeres classify into serous, mucous, or mixed types based on the epithelial cell composition.
• Serous cells feature apical, acidophilic zymogen granules, which contain stored protein and polysaccharide precursors of saliva.
• Mucous cells have light-staining cytoplasm and a basal nucleus.
• Mixed adenomeres feature either intermixed regions of serous and mucous cells or mucous cells with surrounding serous demilunes.
• Myoepithelial cells surround secretory units, contracting to aid saliva discharge into the duct system.
• The intralobular portion of the salivary gland duct system commences with a short intercalated duct lined by simple cuboidal epithelium.
• Epithelium continues as simple columnar with extensive basal plasma membrane infoldings in the striated duct.
• Striated ducts are unique to the salivary gland duct system.
• The striated ducts empty into interlobular ducts, followed by lobular ducts.
• Epithelium height increases from simple to stratified columnar in larger ducts.

Overview of the digestive system, lips, cheeks, and hard and soft palates, including their tissue composition and epithelial features. Focus on the mucous membrane of the oral cavity and the structure of the hard and soft palates.