Digestive System



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  • Digestive System

    • Components:

      • Alimentary canal: Oral Cavity, pharynx, esophagus, stomach, sm/lg intestine

      • Associated Organs: tongue, teeth, salivary glands, pancreas, liver, gallbladder

    • Fx:

      • Mechanical fragmentation & propulsion

      • Chemical digestion

      • Nutrient absorption

      • Protective barrier (stratified epithelium or tight jxn)

      • Immunological protection (lymphatics & IgA)

      • Lubrication (mucous)

  • Oral Cavity

    • Components:

      • Mouth & associated structures: tongue, teeth, salivary glands, tonsils

    • Divisions:

      • Vestibule: space b/t lips, cheeks, teeth

      • Oral Cavity Proper: space behind teeth before entrance to oropharynx & b/t hard/soft palate & mouth floor

    • Oral Mucosa Lining:

      • Moist stratified squamous epithelium—Thicker than skin

        • Mostly Non-keratinized

        • Includes keratinocytes, langerhans cells, merkel cells, melanocytes

        • s. basale, s. spinosum, s. superficiale (flat but nucleated)

      • Lamina Propria—Layer of LCT w/ blood vessels & nerves underlying mucosa

      • TYPE=MASTICATORY MUCOSA

Contains Keratinized epithelium

Contains Parakeratinized stratified squamous epithelium

(Superficial cells don’t lose nuclei instead highly condensed nuclei=Pyknotic)


      • TYPE=LINING MUCOSA

        • Found on lips, cheeks, mouth floor, inferior tongue surface, soft palate

        • Epithelium:

Generally nonkeratinized although some areas parakeratinized

        • Specialized mucosa

Dorsal tongue surface only

Keratinized epithelium w/ papillae & taste buds



      • TYPE=SUBMUCOSA

        • Coarser CT underlying mucosa

        • Provides attachment to muscle (cheeks & lips) or bone (palate, dental arches)

        • Contains Salivary glands

Named by location & can be purely mucous, or mixed serous and mucous

  • Lip: Alimentary canal entry point

    • Characteristics:

      • Point where think keratinized face epidermis changes to thick parakeratinized or nonkeratinized epithelium of oral mucosa—a mucocutaneous jxn

    • Zones:

      • OUTTER CUTANEOUS SURFACE

        • Thin skin (keratinized stratified squamous) w/hairs and sweat glands

      • RED (VERMILION) BORDER

        • Transition zone b/t outer skin & inner oral mucosa

        • Epithelium:

Keratinized epithelium w/ deep stromal papillae that bring capillaries to surface

      • INNER ORAL MUCOSAL SURFACE

Moist stratified squamous nonkeratinized (or parakeratinized) w/ dense lamina propria & submucosa, bound to underlying skeletal muscle

  • Tongue: Muscular organ

    • Skeletal muscle organization

      • Arranged in bundles that run in 3 planes (longitudinal, vertical, & horizontal)

      • Each bundle at right angles to other two

      • Allows for flexibility and precise movements essential for speech

    • Minor Salivary glands: Lingual Glands

      • Mucous, serous, or mixed

      • Embedded in muscle

    • Two different surfaces

      • Ventral

        • Thin mucosa w/ smooth, non-keratinized epithelium

      • Dorsal

        • Specialized oral mucosa: thick w/ keratinized epithelium & lingual papillae

          • ant. 2/3 and post. 1/3 by V-Shaped depression SULCUS TERMINALIS

          • Apex of V point posteriorly and is location of foramen cecum (remnant site of embryonic thyroid gland formation)

    • Four Lingual Papillae cover anterior 2/3 of dorsal surface

      • Filiform

        • Pointed epithelial projections w/ CT core

        • Smallest and most numerous in humans

        • Distributed over entire surface w/ tips pointing backward

        • Serves only mechanical role: no taste buds

      • Fungiform

        • Mushroom shaped projections

        • Scattered singly across surface among filiform papillae

        • May contain taste buds

        • Vascularity of secondary CT papillae shows through, visible to eye as small red dots

      • Foliate

        • Deep mucosal folds on lateral tongue surface

        • Contain taste buds on lateral surface of folds

        • Less prominent in human than other species

      • Circumvallate (vallate)

        • 10-12 large (1-3mm diameter)

        • dome shaped papillae surrounded by moat-like invagination

        • Located in V-shaped boundary = sulcus terminalis

        • Contain taste buds on lateral surface that faces moat

        • Surrounded by circular trench into which serous (posterior lingual) glands of von Ebner empty to flush material from cleft

    • Taste buds

      • Oval groups of sensory cells found primarily w/in the epithelium of papillae

      • Taste pores-openings to surface

      • Cell types

        • Neuroepithelial (sensory) cells:

          • Have apical microvilli (taste hairs)

          • Synapse on afferent nerve terminals

          • Transduce taste impulses

        • Supporting cells:

          • Have apical microvilli

          • Contain secretory granules

          • Less numerous

        • Peripheral or basal stem-type cells:

          • Small & undergo rapid renewal and replace other cell-types every 10-14 days

      • Recent studies challenge taste as a simple classification of sweet, sour, salt, or bitter

      • Taste perception may include transduction systems like

        • Direct action on ion channels

        • Interaction w/ G proteins via secondary messenger systems

      • Free nerve endings ramify b/t & around taste buds providing general chemical sensation of taste




  • Major salivary glands: Compound tubulo-alveolar glands

    • 3 sets of large paired salivary glands

      • Parotid glands-

        • Largest

        • Located below and in front of ear

        • Ducts enter oral cavity opposite 2nd upper molar

        • DISTINGUISH: serous acini only, often adipose cell infiltration, long intercalated ducts and numerous striated ducts

      • Submandibular glands

        • Located under either side of the floor of the mouth

        • Ducts run along floor of mouth

        • Ducts empty just lateral to tongue frenulum (midline fold f/ bottom of tongue to floor)

        • DISTINGUISH: mixed secretion (mostly serous acini, some mixed acini, few mucous acini) shorter intercalated ducts than parotid

      • Sublingual glands

        • Smallest

        • Located in floor of mouth anterior to submandibular glands

        • Ducts empty into submandibular ducts as well as directly onto floor of mouth

        • DISTINGUISH: mixed secretion (mostly mucous acini, some mixed acini, but purely serous acini rarely present), ducts poorly developed and not prominent

    • Secrete approx. 1 L of saliva/day in response to mechanical, chemical or psychic stimuli

    • Organization

      • Secretory unit—(acini, intercalated ducts, & striated ducts)

        • Secretory cells organized into spherical or tubular clusters called acini

        • Secretory product is collected from acini by intercalated ducts

        • Intercalculated ducts are drained by straited ducts

        • Product modified to form saliva (final product) in intercalated ducts and striated ducts

        • Drained by excretory ducts and delivered to oral cavity

      • Lobular organization

        • Secretory unit (acini, intercalated ducts, & striated ducts) organized into lobules

        • Lobules surrounded by CT

        • Multiple lobules are collected into lobe

        • Multiple lobes are combined to form a gland

      • Surrounding stroma

        • CT w/ plasma cells which

          • Produce IgA which is modified by serous acinar cells to form secretory IgA

          • sIgA is released into lumen

          • immunological protection similar to sIgA in distal GI tract

      • Ducts

        • Intralobular ducts: Modify salivary fluid by secreting HCO3-,K+ and reabsorb Na+, Cl-

          • Intercalated ducts

initial portion of duct

small w/ squamous to simple low cuboidal epithelium



          • Striated (secretory, salivary) ducts

Simple cuboidal to simple columnar epithelium

Have basal infoldings of plasma memb. (striations)



Characteristic of ion-pumping activity

        • Excretory Ducts: interlobular and larger

          • Surrounded by CT

          • Drain to primary or main ducts that enter the oral cavity

          • Epithelium increases in height f/ simple columnar through pseudostratified columnar to stratified columnar

    • Cells

      • Secretory cells

        • Serous cells: protein producing cells w/ prominent rER and Golgi

        • Mucous cells: secrete mucin

      • Myoepithelial cells

        • Contractile cells surround base of acini, straight ducts, & whole gland

        • Enclosed w/in the basal lamina of epithelial cells

        • Help move secretory product out

      • Acinar cells

        • Contacted by nerve endings

        • Parasympathetic stimulation results in watery secretion

        • Sympathetic stimulation yields a viscous enzyme rich secretion

    • Secretory acini

      • Secrous acini

        • Only serous cells

        • Generally spherical

        • Watery secretion w/ α-amylase which breaks 1-4 glycoside bonds for carbo digestion

        • Produce peroxidase and antimicrobial agents (lysozyme, cystatin, hystatin)

      • Mucous acini

        • Only mucous cells

        • More tubular

        • Produce sialomucin & sulfomucin (glycoprotiens)

        • H&E pale cells due to loss of mucinogen during tissue prep

      • Mixed

        • Contain serous & mucous cells

        • H &E serous cells are seen as serous demilunes (crescent shaped groups of serous cells surrounding mucous acini)

        • Product flows via intercellular canaliculi to lumen

        • Recent studies suggest both cell types may border similarly on lumen

  • Teeth

    • Divisions

      • Crown

        • Anatomic-part of tooth covered by enamel

        • Clinical- part of tooth that extends above gums

      • Root: part of tooth embedded in an alveolus in the bone and is covered w/ cementum

    • Bone-Like components

      • Enamel: Covers anatomic crown

        • Hardest substance in body

        • 96-98% inorganic w/ organic enamelin (a unique glycoprotein)

        • Acellular

        • Prisms or rodsof calcium hydroxypatite phosphate crystallites (.1µm diameter)

        • Rods (4µm diameter), run perpendicular to tooth surf.

        • Rods surrounded by interprismatic substance

        • Rods fit together in keyhole pattern

        • Rods produced daily during development by ameloblasts they give rise to incremental lines of Retzius (when mineralization changes) and degenerate after enamel fully formed (around time of tooth eruption)

      • Dentin: yellowish, semi-transparent material forming bulk of tooth

        • Harder than bone

        • 70% inorganic w/ organic collagen fibers surrounded by ground substance and are masked after calcification

        • Produced throughout life by odontoblasts

        • Layer of odontoblast retreats as dentin is laid down, leaving processes embedded in cavities in dentin (dentinal tubules) which extend to dentinoenamel jxn

      • Cementum: Covers the root

        • Composition similar to bone

        • 50-65% inorganic salts w/ organic collagen and ground substance

        • Secreted by cementocytes that resemble osteocytes

        • Aneural and avascular no haversian system!

        • Cellular cementum (lower part of root near apical foramen contains cementocytes in lacunae w/ canaliculi)



    • Dental Pulp

      • LCT inside pulp cavity

      • Branches of nerves and blood vessels that enter the tooth at apical foramen

      • Since dentin secreted throughout life, volume of pulp cavity decreases w/age

    • Gingiva- epithelium surrounding teeth (gums)

      • Keratinized stratified squamous epithelium (masticatory mucosa)

      • Forms gingival attachment to tooth, enamel or cementum via hemidesmosomes

      • Fx as seal to prevent foreign material entrance

    • Peridontal membrane or ligament

      • D Fibrous CT b/t cementum of tooth and alveolar bone

      • Contains blood vessels, lymphatics, nerves (esp. proprioceptive nerves)

      • Collagen fibers emedded in cementum or bone are called Sharpey’s fibers (attach ligament to cementum or bone)

      • Fx

        • Helps attach tooth to bone

        • Serves as suspensory ligament and prevents crushing of soft tissues near apex


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