Digestive System

• Digestive system


  • Gastrointestinal tract
  • Alimentary tract - the organs where food passes through
  • Accessory organs - associated organs which includes glands that produce secretions important in the process of digestion
• Functions of the digestive system


  • Ingestion - introduction of food and liquid into the oral cavity
  • Mastication - chewing, which divides solid food into digestible pieces
  • Motility - muscular movements of materials through the tract
  • Secretion - of lubricating and protective mucus, digestive enzymes, acidic and alkaline fluids, and bile
  • Hormone release - for local control of motility and secretion
  • Chemical digestion - enzymatic degradation of large macromolecules in food to smaller molecules and their subunits
  • Absorption - of the small molecules and water into the blood and lymph
  • Elimination - of indigestible, unabsorbed components of food
• Oral cavity


  • Lined with stratified squamous epithelium, which may be keratinized, partially keratinized, or nonkeratinized depending on the location
  • Flattened superficial cells of the oral epithelium undergo continuous desquamation, or loss at the surface but they retain their nuclei
• Oral cavity


  • Keratinized squamous epithelium in gingiva (gums) and hard palate, the lamina propria in these regions rests directly on the periosteum of underlying bone
  • Non-keratinized squamous epithelium lining mucosa over the soft palate, cheeks, the floor of the mouth, and the pharynx (or throat), the posterior region of the oral cavity leading to the esophagus
  • Lining mucosa overlies a thick submucosa containing many minor salivary glands, which secrete continuously to keep the mucosal surface wet, and diffuse lymphoid tissue
• Lips


  • Labia; well-developed core of striated muscle which makes these structures highly mobile for ingestion, speech, and other forms of communication
  • Internal mucous surface has lining mucosa with a thick, nonkeratinized epithelium and many minor labial salivary glands
  • Red vermilion zone of each lip is covered by very thin keratinized stratified squamous epithelium and is transitional between the oral mucosa and skin; lacks salivary or sweat glands
  • Outer surface has thin skin, consisting of epidermal and dermal layers, sweat glands, and many hair follicles with sebaceous glands
• Tongue


  • Mass of striated muscle (skeletal muscle) covered by mucosa, which manipulates ingested material during mastication and swallowing
  • Muscle fibers are oriented in all directions, allowing a high level of mobility
  • Connective tissue between the small fascicles of muscle is penetrated by the lamina propria, which makes the mucous membrane strongly adherent to the muscular core
  • Lower surface is smooth, with typical lining mucosa
  • Dorsal surface is irregular, having hundreds of small protruding papillae of various types on its anterior two-thirds and the massed lingual tonsils on the posterior third, or root of the tongue
  • Papillary and tonsillar areas of the lingual surface are separated by a V-shaped groove called the sulcus terminalis
• Lingual papillae


  • Filiform papillae - very numerous, have an elongated conical pointed in shape, and are heavily keratinized, which gives their surface a gray or whitish appearance and provide a rough surface and friction that facilitates movement of food during chewing
  • Fungiform papillae - much less numerous, lightly keratinized, and interspersed among the filiform papillae; mushroom-shaped with well-vascularized and innervated cores of lamina propria and found all over dorsal surface of tongue
  • Foliate papillae - consist of several parallel ridges on each side of the tongue, anterior to the sulcus terminalis, but are rudimentary in humans, especially older individuals, well developed in children
  • Vallate or circumvallate papillae - are the largest papillae, with diameters of 1-3 mm, 8 - 12 vallate papillae are normally aligned just in front of the terminal sulcus
• Vallate papillae


  Ducts of several small, serous salivary (von Ebner) glands empty into the deep, moat-like groove surrounding each vallate papilla and contains lipase
• Taste buds


  • Ovoid structures within the stratified epithelium on the tongue's surface
  • Each taste bud contains 50 – 100 cells
  • 250 taste buds are present on the lateral surface of each vallate papilla, with many others present on fungiform and foliate not restricted to papillae and are also widely scattered elsewhere on the dorsal and lateral surfaces of the tongue
  • The base of each bud rests on the basal lamina and is entered by afferent sensory axons that
• Taste bud cells


  • Gustatory Cells - elongated and half of the cells are this type life span is 7 – 10 days, the base of each bud rests on the basal lamina and is entered by afferent sensory axons that form synapses with these cells
  • Supportive Cells - slender supportive cells that are immature cells
  • Stem Cells - slowly dividing stem cells and give rise to other cells
  • Taste Pore - a 2-μm-wide opening that microvilli projects to from the apical end of then gustatory cells, Molecules (tastants) dissolved in saliva contact the microvilli through the pore and interact with cell surface taste receptors
• Types of taste


  • Salty - Sodium, Ion channels
  • Sweet - Sugar, Saccharine, G-protein coupled receptors
  • Sour - Hydrogen, Ion channels
  • Bitter - Alkaloids and toxins, G-protein coupled receptors
  • Umami "savory" - Amino acids – glutamate and aspartate, G-protein coupled receptors
• Types of teeth


  • Primary Teeth/Deciduous Teeth/Milk Teeth - 20 in number, shed
  • Adult Teeth/Permanent Teeth - 32, arranged in two bilaterally symmetric arches in the maxillary and mandibular bones
  • Incisor, canine, pre-molars, molar (wisdom tooth is 3rd molar)
• Primary teeth


  • Upper Jaw: 4 incisors, 2 canines, 4 molars
  • Lower Jaw: 4 incisors, 2 canines, 4 molars
• Permanent teeth


  • Upper Jaw: 4 incisors, 2 canines, 4 pre-molars, 6 molars
  • Lower Jaw - Upper Jaw: 4 incisors, 2 canines, 4 pre-molars, 6 molars
• Parts of a tooth


  • Crown - exposed above the gingiva or gums
  • Neck - a constricted portion at the gums
  • Roots - one or more roots that fit firmly into the bony sockets in maxilla and mandible
  • Dental alveoli - bony sockets in the jaws
• Tooth tissues


  • Enamel - hard, acellular that covers the crown
  • Cementum - bone-like tissue that covers the roots
  • Enamel and cementum meet at the neck
  • Dentin - calcified material that makes up the bulk of the tooth
  • Pulp Cavity - an internal cavity surrounded by dentin, extends to the neck and composed of mesenchyme CT
  • Dental Pulp - highly vascular and well-innervated and consists largely of loose, mesenchymal connective tissue with much ground substance, thin collagen fibers, fibroblasts, and mesenchymal stem cells
  • Root canal - a narrowing of the pulp at the root
  • Apical foramen - an opening at them tip of each root for the blood vessels, lymphatics, and nerves of the pulp cavity
  • Periodontal ligaments - fibrous connective tissue bundles of collagen fibers inserted into both the cementum and the alveolar bone
• Dentin


  • A calcified tissue harder than bone, consisting of 70% hydroxyapatite, organic matrix contains type I collagen and proteoglycans
  • Odontoblasts - tall polarized cells derived from the cranial neural crest that line the tooth's pulp cavity, secretes proteoglycans, mineralizes the pre-dentin matrix, continue pre-dentin production into adult life, gradually reducing the size of the pulp cavity, and are stimulated to repair dentin if the tooth is damaged
• Odontoblast processes


  • Long apical extend from the odontoblasts within dentinal tubules which penetrate the full thickness of the dentin, gradually becoming longer as the dentin becomes thicker
  • Along their length, the processes extend fine branches into smaller lateral branches of the tubules
  • The odontoblast processes are important for the maintenance of dentin matrix
• Enamel


  Hardest component of the human body, consisting of 96% calcium hydroxyapatite and only 2%-3% organic material including very few proteins
• Dentin


  A calcified tissue harder than bone, consisting of 70% hydroxyapatite, organic matrix contains type I collagen and proteoglycans
• Odontoblasts


  • Tall polarized cells derived from the cranial neural crest that line the tooth's pulp cavity, secrete proteoglycans, mineralize the pre-dentin matrix, continue pre-dentin production into adult life, gradually reducing the size of the pulp cavity, and are stimulated to repair dentin if the tooth is damaged
• Odontoblast processes


  • Long apical extend from the odontoblasts within dentinal tubules which penetrate the full thickness of the dentin, gradually becoming longer as the dentin becomes thicker, along their length the processes extend fine branches into smaller lateral branches of the tubules, important for the maintenance of dentin matrix
• Enamel


  Hardest component of the human body, consisting of 96% calcium hydroxyapatite and only 2%-3% organic material including very few proteins and NO COLLAGEN
• Enamel rods


  • Uniform, interlocking columns about 5 μm in diameter surrounded by a thinner layer of other enamel, each rod extends through the entire thickness of the enamel layer, which averages 2 mm, the precise, interlocked arrangement of the enamel rods is crucial for enamel's hardness and resistance to great pressures during mastication
• Enamel organ


  • Specialized epithelium in the tooth bud that contains ameloblast
• Ameloblast


  • Tall, polarized cells and their apical ends face those of the odontoblasts, ameloblast process/Tomes process - apical extension from each ameloblast contains numerous secretory granules with the proteins of the enamel matrix, amelogenin - the main structural protein of developing enamel and guides formation of matrix by elongating an enamel rod, the secreted matrix undergoes very rapid mineralization
• Teeth development


  Ameloblasts - derived from the ectodermal lining of the embryonic oral cavity, odontoblast - derived from neural crest, pulp tissues - derived from mesoderm, produce a series of 52 tooth buds in the developing oral cavity, 20 for the primary teeth and 32 for the secondary or permanent teeth, primary teeth complete development and begin to erupt about 6 months after birth, development of the secondary tooth buds arrests at the "bell stage," until about 6 years of age
• Periodontium


  • Cementum - covers the dentin of the root and resembles bone, avascular, cementocytes - cells that reside near the root tip in lacuna, maintain their surrounding matrix, react to stresses by gradually remodeling
  • Periodontal ligament - fibrous connective tissue with bundled collagen fibers (Sharpey fibers) binding the cementum and the alveolar bone, highly cellular and has a rich supply of blood vessels and nerves, giving the periodontal ligament sensory and nutritive functions, permits limited movement of the tooth within the alveolus and helps protect the alveolus from the recurrent pressure exerted during mastication, thickness decreases in aging
  • Alveolar bone - lacks the typical lamellar pattern of adult bone but has osteoblasts and osteocytes engaging in continuous remodeling of the bony matrix, surrounded by the periodontal ligament, which serves as its periosteum, collagen fiber bundles of the periodontal ligament penetrate this bone, binding it to the cementum
  • Gingiva - gums; keratinized oral mucosa firmly bound to the periosteum of the maxillary and mandibular bones, gingival sulcus - a groove between the enamel and the gingival epithelium surrounding the neck, junctional epithelium - bound to the tooth enamel by means of a cuticle, which resembles a thick basal lamina to which the epithelial cells are attached by numerous hemidesmosomes
• Salivary glands


  • Parotid glands - located in each cheek near the ear, are branched acinar glands with exclusively serous acini, serous cells secrete abundant α-amylase that initiates hydrolysis of carbohydrates and proline-rich proteins with antimicrobial and other protective properties
  • Submandibular glands - produce two-thirds of all saliva, are branched tubulo-acinar glands, having primarily serous acini, but with many mixed tubulo-acinar secretory units, within the mixed units grouped serous cells occur distally on short mucous tubules and often assume a crescent-shaped arrangement called a serous demilune, in addition to α-amylase and proline-rich proteins, serous cells secrete lysozyme for hydrolysis of bacterial walls, mixed, but more serous than mucous
  • Sublingual glands - the smallest of the major glands, are also considered branched tubulo-acinar glands, but here secretory tubules of mucous cells predominate and the main product of the gland is mucus, the few serous cells present add amylase and lysozyme to the secretion, mixed, but primarily mucous
• Salivary gland structure


  • Connective tissue capsule surrounds the gland, the parenchyma of each consists of secretory units on a branching duct system arranged in lobules, separated by septa of connective tissue, the secretion of each gland is either serous, seromucous, or mucous, depending on its content of the glycoprotein mucin
• Serous cells


  • Polarized protein-secreting cells, usually pyramidal in shape, with round nuclei, well-stained RER, and apical secretory granules, joined apically by tight and adherent junctions, form a somewhat spherical unit called an acinus, with a very small central lumen, secrete enzymes and other proteins
• Mucous cells


  • Somewhat more columnar in shape, with more compressed basal nuclei, contain apical granules with hydrophilic mucins that provide lubricating properties in saliva but cause poor cell staining in routine preparations, most often organized as cylindrical tubules rather than acini
• Myoepithelial cells


  • Found inside the basal lamina surrounding acini, tubules, and the proximal ends of the duct system, small, flattened cells extend several contractile processes around the associated secretory unit or duct and their activity is important for moving secretory products into and through the ducts
• Intralobar duct system


  Secretory acini and tubules empty into intercalated ducts (cuboidal epithelial cells) and several of these ducts join to form a striated duct (reabsorb Na+ ions from the initial secretion and their folded cell membranes present a large surface area with ion transporters, facilitating rapid ion transcytosis and making the secretion slightly hypotonic), ducts from each lobule converge and drain into interlobular excretory ducts
• Pharynx regions


  • Nasopharynx - the posterior part of the nasal cavity, lining epithelium: pseudostratified columnar ciliated epithelium
  • Oropharynx - caudally continuous to the nasopharynx, throat, posterior part of the oral cavity, lining epithelium: stratified squamous epithelium, non-keratinized
  • Laryngopharynx - most inferior part of the pharynx, posterior to the larynx, lining epithelium - pseudostratified columnar ciliated epithelium, stratified squamous epithelium, non-keratinized
• Tunica Mucosa (Mucous Membrane)


  • Epithelium
  • Lamina propria
  • Muscularis mucosae
  • Mucosal folds
• Tunica Submucosa


  • Dense irregular connective tissue
  • Glands may be present
  • Meissner's plexus

  
• Tunica Muscularis


  • Variable arrangement
  • Inner circular, outer longitudinal
  • Inner oblique, middle circular, outer longitudinal
  • Auerbach's plexus
• Tunica Serosa / Adventitia


  May be lined by serous membrane or not
• Pharynx


  • Fibromuscular tubular organ
  • Part where digestive & respiratory tract merge & cross each other
  • Divisions: Nasopharynx, Oropharynx, Laryngopharynx