Digestive System Part I PDF

Digestive System Part I Digestive System Overview of digestive system Functions of digestive organs Relevance Microanatomy of digestive organs General structure of tubular organs Oral cavity Teeth Tongue Salivary glands Esophagus Stomach Small int...

Digestive System Part I Digestive System Overview of digestive system Functions of digestive organs Relevance Microanatomy of digestive organs General structure of tubular organs Oral cavity Teeth Tongue Salivary glands Esophagus Stomach Small intestine Large intestine Ano-rectal junction Functions of the gastrointestinal system: - Prehension: ingestion followed by fragmentation - Mastication - Digestion - Storage (forestomach [ruminants] and cecum [horses]) - Absorption - Expulsion - Unabsorbed food - Substances added by accessory glands Digestive System General structure of tubular organs 1. Tunica mucosa 2. Submucosa 3 Muscularis externa 2 Tunica submucosa 3. Muscularis externa 1 Tunica mucosa 4 Tunica Serosa 4. Adventitia/serosa General structure of tubular organs 1. Tunica mucosa or mucous membrane: Surface epithelium – varies from stratified squamous to simple columnar. Lamina propria – loose connective tissue rich in fibroblasts, macrophages, capillaries, lymphatic vessels and lymphoid tissue; may contain mucosal glands. Muscularis mucosae or lamina muscularis – smooth muscle layer that may or may not be present General structure of tubular organs 2. Submucosa Connective tissue with many blood and lymph vessels and ganglia and nerve networks of the submucosal nerve plexus (called Meissner’s plexus in the gastrointestinal tract). Submucosal glands and lymphoid tissue may also be present  General structure of tubular organs 3. Tunica muscularis: Usually 2 layers of smooth muscle; inner are circularly arranged, outer are arranged longitudinally. Striated muscle is present throughout the esophagus of dogs and ruminants; in the cranial esophagus in horse, pig and cat; and in external anal sphincter of all species. Between the two muscle layers are ganglia and nerves of the myenteric nerve plexus (called Auerbach’s plexus in the gastrointestinal tract), blood and lymph vessels  General structure of tubular organs 4. Tunica adventitia or Tunica serosa: Adventitia consists of connective tissue which blends with the surrounding fascia; no mesothelium Serosa consists of connective tissue with a covering of mesothelium (layer of simple squamous epithelium)  Tunica mucosa Meissner’s plexus Tunica submucosa Tunica muscularis Auerbach’s plexus Tunica serosa  1 2 3 4 General structure of tubular organs 1. Tunica mucosa 2. Submucosa 3. Muscularis externa 4 4. Adventitia/serosa 2 3 I Mesentery Colon Canine Oral cavity Tongue, teeth and salivary glands Mouth/cheek lining, hard & soft palate, and gingiva Mucosa Non-keratinized and keratinized stratified squamous epithelium (keratinized in dental pads of rum.) Lamina propria-submucosa Connective tissue (abundant type 1 collagen with elastic fibers) Minor salivary glands (mucous, serous, mixed) Labial, buccal, palatine and lingual glands (scattered throughout the oral cavity) Distinctive species feature: Dental pad in ruminants TOOTH PAD – DENTAL PAD in Ruminants TEETH Hypsodont teeth are much longer than brachydont teeth and continue their growth throughout a portion, if not all, of the adult life of the animal. They do not have a crown and neck but, instead, have an elongated body. Hypsodont teeth include all those of horses, the cheek teeth of ruminants, and the canine teeth of pigs. Brachydont teeth are short and cease to grow after eruption is completed. They have crown, neck and roots TOOTH TEETH STRUCTURE Brachydont teeth Hard structures: Enamel Dentin Cementum Soft tissues Pulp Peridontal ligament TEETH STRUCTURE ENAMEL Hard structures Enamel Produced by ameloblasts covers CROWN the external surface. DENTIN It is the hardest substance in the body, composed of 99% mineral NECK Neck (hydroxyapatite) and 1% organic TOOTH PULP matrix by weight. Dentin Alveolar bone Beneath the enamel produced by odontoblasts Odontoblasts Periodontal forms the wall of the pulp cavity ligament Cementum ROOT Gingiva ▪ Acellular cementum is composed of lamellae oriented parallel to the Root canal surface of the tooth. CEMENTUM ▪ Cellular cementum has Apical foramen cementocytes, which occupy lacunae and canaliculi similar to those of bone. TEETH STRUCTURE Odontoblasts Dentin Soft tissues Pulp Composed of connective tissue containing blood vessels and nerves Periphery is a layer of odontoblasts which produce predentin (unmineralized dentin) which becomes mineralized Apical cytoplasmic processes of odontoblasts extend into dentinal tubules Peridontal ligament Bundles of collagen fibers anchor cementum of tooth root to bone Pulp Predentin Cells that form the Toot Hard Tissue Ameloblast enamel Odontoblast dentin Cementoblasts cementum Soft tissue Toot pulp loose connective tissue and nerves(root mesenchymale is derived from the neural crest cells Periodontal ligament fibroblast Tooth Development B. The cells of the epithelial tooth bud develop C. Odontoblasts begin to produce A. Early stage of tooth into the enamel organ, differentiating into a predentin and this stimulates the formation from both bell-shaped structure with a central core of production of enamel by the ectoderm (epithelial) loosely arranged stellate cells (epithelium) and ameloblasts. Calcification of the and neural crest derived a peripheral layer of cuboidal or low columnar predentin and pre-enamel begins mesenchyme. external and internal enamel epithelium. The immediately and dentin and enamel internal enamel epithelium differentiates into an continue to be laid down until the form outer layer of tall columnar ameloblasts and a 2- of the tooth is complete. The dental papilla becomes enclosed by the dentin 3 cell thick inner layer the stratum intermedium. to form the dental pulp. Non-ameloblast In the concavity of the enamel organ, the components of the enamel organ mesenchyme continues to condense to form become reduced and eventually atrophy. the dental papilla and a row of odontoblasts When the enamel formation is complete, develops at its junction with the enamel organ, the ameloblasts degenerate. in contact with the ameloblasts.  Enamel and dentin formation during tooth development. Odontoblasts will Enamel cover the surface of the mesenchymal papilla, and produce predentin and dentin. Ameloblasts Ameloblasts are tall columnar cells that Dentin produce enamel Odontoblasts Oral cavity Tongue Muscular organ (skeletal muscle in multiple directions) bundles of striated muscle are arranged longitudinally, transversely and vertically Mucosa covered by keratinized and non-keratinized stratified squamous epithelium (keratinized in some types of papillae) Lamina propria-submucosa (no muscularis mucosae) Lingual salivary glands (minor salivary glands; serous and mucous secretion) are scattered within lamina propria-submucosa and muscle. The dorsal surface contains many macroscopic papillae (elevations of surface epithelium and lamina propria) which vary in shape and have mechanical or gustatory function Tongue Papillae Projections of tissue from tongue surface Modifications of mucosa With or without taste of buds Functions ❖ Mechanical Assist in prehension, movement and breakdown of food ❖ Sense of taste Types of lingual papillae: Filiform Fungiform Most numerous type, have a heavily keratinized Scattered amount filiform papillae, keratinized epithelium. Mechanical function. epithelium, taste buds in upper surface epithelium. Gustatory function. Conical Found on root of tongue in dogs, cats and pigs; Vallate (Circumvallate) cheek and floor of oral cavity in other species. Nonkeratinized epithelium, taste buds in lateral Keratinized epithelium. Mechanical function. surface epithelium. Gustatory function. Lenticular Foliate Found mainly in ruminants. Keratinized epithelium. Nonkeratinized, taste buds in lateral surface Mechanical function. epithelium. Gustatory function.  Vallate (Circumvallate) Conical Conical Fungiform Foliate Filiform Lingual Papillae of a canine tongue Filiform papillae Fungiform papillae Taste buds are intraepithelial structures which are sensitive for taste. Sensory gustatory cells of the taste bud convert chemical molecules into neural impulses which travel along neurons to higher centers in the brain for interpretation as sweet, sour or salty tastes. The sustentacular cells of the taste bud are supportive, while the basal cells are thought to be precursor cells Esophagus: Muscular tube” Function: Transports food bolus and liquid from oral Esophagus without Large intestine submucosal glands cavity to stomach (also, opposite direction in ruminants) Mucosa Stratified squamous epithelium (keratinized or nonkeratinized varies with species) Lamina propria Muscularis mucosae of smooth muscle (may or may not be present). Submucosa Loose connective tissue contains seromucous glands (presence varies with region). Main components of secretory product: Pepsinogen (protease) Lysozyme (bactericidal) Mucous (lubricant) Muscularis Skeletal and smooth muscle except all skeletal in dog and ruminants. Auerbach’s myenteric plexus (between inner circular and outer longitudinal layers) Adventitia or serosa Adventitia of loose connective tissue containing blood and Esophagus with Small intestine with and lymph vessels, and nerves in the cervical region submucosal glands without submucosal Serosa in the thorax and abdomen, lined by simple Epithelium (E), lamina propria (F), lamina muscularis (G). Tela submucosa (H). Tunica muscularis: circular layer (I), longitudinal layer (J). Tunica serosa (K). Tunica adventitia (L) squamous epithelium (mesothelium)  Comparative histology of esophageal wall Horses Pigs Cattle Goats Sheep Dogs Cats Stratified Keratinized Keratinized Keratinized Keratinized Keratinized Nonkeratinized Nonkeratinized squamous epithelium (a) Lamina b Absent in b b b Absent in cranial b muscularis cranial part part Interrupted in Highly middle part developed in caudal part Submucosal c Present only in c c c Present throughout c glands cranial half and extend into stomach Tunica Cranial two Cranial part Striated throughout and extends into the Striated throughout Cranial part muscularis thirds striated striated reticular sulcus striated Caudal Caudal one Middle part one third to one third smooth mixed Caudal part smooth fifth smooth Tunica Loose connective tissue cells and fibers with blood and lymph vessels and nerves surround the esophagus. A tunica serosa may adventitia be present in the thoracic cavity (mediastinal pleura) or near the stomach (visceral peritoneum). (a) Related to character of food: coarse food, highly keratinized; soft food, slightly keratinized to nonkeratinized. (b) Isolated bundles of smooth muscle near pharynx, increases in thickness near stomach. (c) Present only at the pharyngoesophageal junction. (d) Inner circular muscle layer becomes thicker at the cardiac ostium of the stomach (forming the cardiac sphincter muscle), especially in horses. Esophagus Esophagus Mucosa Lamina propria Epithelium Salivary Glands Major (separated from oral cavity, long ducts): ▪ Parotid ▪ Mandibular ▪ Sublingual ▪ Zygomatic. Minor (within oral cavity) Functions: Moisten oral cavity Lubrication of food Initiate digestion of carbohydrates and lipids Antibacterial activity Stimulate hound healing Classified by type of secretion produced (species variability) 1) Serous - Watery, low viscosity - Secretory product high in protein (enzymes) Example: Parotid 2) Mucous - High viscosity for lubrication - Secretory product high in sugar 3) Mixed - Serous & mucous secretory components Dog Horse Cow Rodent Man Mandibular Mixed Mixed Mixed Serous Mixed (most mucous) Sublingual Mixed Mixed Mixed Mucous Mixed (most mucous) Parotid Serous Serous Serous Serous Serous  Salivary Glands Structure Grossly: Multilobular (lobules separated by septa of connective tissue) Histo: Secretory units Acini (round) or tubules (elongate) *Secretory units and ducts are surrounded by myoepithelial cells (contraction and facilitates movement of product)  Mixed salivary gland Note the lobules separated by septa of connective tissue. The larger, round to elongated, “clear” spaces are ducts. Myoepithelial cells Contractile Surrounds base of secretory cells Facilitates movement of secretory product Myoepithelial cells are rich in actin filaments and myosins Ducts of a salivary gland A series of channels that transport secretory product to oral cavity Begins at opening of acinar or tubular lumen Ends at opening to oral cavity STOMACH Distention of digestive tube with sphincters at entry (CARDIA) and at exit (PYLORUS). Tunica mucosa may be Glandular (with simple columnar epithelium). Non-glandular (cutaneous) Glandular stomach Structure Gastic folds (plicae or rugae) Facilitate expansion of stomach as food is ingested Gastric pits Depressions into mucosa (with openings of mucosa glands into gastric pits) Lamina muscularis mucosae Gastric glands Extensions of gastric Tunica mucosa pits, lined by Tunica submucosa epithelial cells of Tunica muscularis various types Tunica serosa Stomach: Storage and digestion Mucosa ► Glandular in carnivores ► Glandular & nonglandular (proventricular or esophageal) regions in herbivores Man Dog ► Nonglandular mucosa has stratified squamous epithelium which Ruminant may be keratizined ► Lamina propria is loose connective tissue with smooth muscle and lymphoid cells ► Muscularis mucosae is composed of several layers of smooth Abomasum muscle Submucosa ► Loose connective tissue with no glands Muscularis ► Usually has inner oblique, middle circular and outer longitudinal ► Myenteric plexuses between middle and outer layers Serosa ► Connective tissue covered with mesothelium Pig Horse In Horses No glandular stomach separated from glandular stomach by margo plicatus (mp) Glandular mucosa Three regions: ► Cardiac ► Fundic (gastric) ► Pyloric. Each region of glandular mucosa has: ► Surface epithelium of tall columnar mucous cells that secrete an alkaline mucus ► Mucus lubricates ingested food and protects against corrosive effects of acid and enzymes ► Surface epithelium invaginates into the lamina propria to form gastric pits which are continuous with tubular glands ► Lamina propria is loose connective tissue with smooth muscle and lymphoid cells ► Muscularis mucosae (lamina muscularis) consists of several layers of smooth muscle ► Carnivores may have a stratum compactum (a layer of densely packed collagen fibres) between the base of glands and the muscularis mucosae ► Abomasum is the true stomach in ruminants a simple stomach exists in other species  Three types of Gastric Glands Cardiac glands Short, branched coiled tubular glands consist of neck region, body and base Composed of: ► Mucous cells are the main cell type also some: ► stem cells ► mucous neck cells ► enteroendocrine cells Glandular mucosa Fundic glands Straight, branched tubular glands consist of neck region, long body and base Composed of: ► Mucous neck cells ► Chief (zymogen) cells ► Parietal (oxyntic) cells ► Enteroendocrine cells, small, round shape. Identified by special staining methods. ► within glandular regions of stomach and throughout the intestines ► at least 12 different cell types produce gastrointestinal peptide hormones (e.g. somatostatin in fundic glands) ► Stem cells capable of differentiating into any cell type of the mucosal epithelium; found in the neck region of gastric, cardiac and pyloric glands  Glandular mucosa ► Mucous neck cells ► Irregular shape ► Secrete different type of mucus than surface mucous cells ► Found in the neck region of gastric, cardiac and pyloric glands  Glandular mucosa ► Chief (zymogen) cells ► Cuboidal to pyramidal shape ► Basophilic cytoplasm ► Secrete pepsinogen  Glandular mucosa ► Parietal (oxyntic) cells ► Large, round to pyramidal shape ► Eosinophilic ► Secrete hydrochloric acid  Parietal versus chief cells Parietal (oxyntic) cells or oxyntic cells (secrete hydrochloric acid) are distributed along the length of the gastric glands but are more numerous in the mid section. These cells are large, round, have an extensive eosinophilic (acidophilic) cytoplasm and centrally located nucleus. Chief (zymogen) cells or peptic cells, and zymogenic cells (secrete pepsinogen), are basophilic (blue) with granular cytoplasm and dense basally located nuclei. PI Parietal cells, Pc Peptic cell, E Neuroendocrine cell  Glandular mucosa Pyloric glands Short, branched coiled tubular glands consist of neck region, body and base Composed of: ► Mucous cells are the main cell type also some: ► stem cells ► mucous neck cells ► enteroendocrine cells