L12 GI Tract

What are the parts of the oral cavity?

• lips, teeth, tongue

What are the parts of the stomach?

Cardia, fundus, body, pylorus

What are the parts of the small intestine?

• duodenum

• jejenum

• ileum

Accessories of the GI system

salivary glands, live, gallbladder, pancreas

Digestive system function:

• ingest food and water

• absorb water and nutritive substances

• expel solid wastes

• mucosa: secretion, barrier, immunologic protection

Oral cavity structures function

• Structures: lips, teeth, gingiva, gum, palate, tongue, salvary glands, lymphoid tissue

• Function

  • receives, chews and moistens food

  • enzymes begin the digestion process (process food in mouth)

Oral mucosa

• lining mucosa: nonkeratinized stratified squamous

• masticatory mucosa: keratinized stratified squamous (resist abrasion)

• specialized mucosa: nonkeratinized stratified squamous with taste buds

oral cavity components

• epithelium

• lamina propria → minor salivary glands (always secrete)

• muscle → voluntary manipulation

• secretory products

• lymphatic tissue - diffuse, nodules, tonsils

The lip

Internal region: stratified squamous nonkeritinized (has minor salivary glands)

Vermilion zone: stratified squamous, slightly keritinized (has blood vessels)

External region: stratified squamous, keritinized (has hair follicles and sebaceous glands)

Vermilion zone of lip

• no sweat or sebaceous glands (need moisturizer)

• underlying CT is rich in sensory innervation and capillaries (pink colour)

Can enamel be recplaced?

no

Enamel

• 96-98% calcium hydroxyapatite

• integrity maintained by saliva

• secreted by ameloblasts

• acellular mineralized material

Dentin

• dense, calcified tissue, forms bulk of tooth

• secreted by odontoblasts

• 70% calcified hydroxyapatite, 30% collagen type 1

• harder than bone

• support enamel

Pulp cavity

• consists of delicate connective/supportive tissues

• highly vascular

• rich nerve supply (myelinated pain fibers get unmyelinated in pulp, extend into dentinal tubules, contact odontoblastic processes - sensory receptors for dentin

Cementum

• 65% mineralized, similar to bone

• covers root, secreted by cementoblasts

• reacts to compressive forces against alveolar bone and reacts by reabsorbing old bone and depositing new bone

• keeps root in close contact with socket and alveolar bone

• site of attachment for periodontal ligament/membrane to alveolar bone

What is the function of the periodontal ligament?

• permits limited amounts of movement → shock absorber

• derived by mesenchyme

• dense collagenous tissues (collagen fibers called sharpey’s fibers → run obliquely down from cementum to alveolar bone)

What happens if there is a problem with the periodontal ligament?

• issues = tooth exfoliation

What type of gland is a salivary gland?

• exocrine gland

• retain connection with surface epithelium by ducts that have glandular secretions reach the surface

Major salivary glands

• pair of parotid, submandibular, and sublingual glands

• regulated secretion in response to neural stimulus

  • parasympathetic activity (rest and digest)

Minor salivary glands

• numerous in mucosa of oral cavity, in tongue, cheeks, lip, soft palate

• continuous secretion

How are salivary glands organized?

• acinus (sac of cells) → make secretions

• myoeptihelial cell → can contract, squeeze contraction into duct

• intercalated duct, striated duct, excretory duct

Gland development

1. epithelium sits on a basement membrane supported by mesenchyme

2. basement membrane breakdown and epithelial cells migrate to underlying mesenchyme (mesenchyme cells and epithelial cells communicate with eachother)

3. epithelial cells de-differentiate into duct and secretory cells

4. lumen is constructed and differentiation continues

Secretory portion of gland structure

• either sac like (acinar/alveolar) or tube-like (tubular)

• also intermediate configurations (tubuloacinar or tubuloalveolar)

Structure of excretory duct

Simple: unbranched duct system (ex: sweat gland)

Compound: branched duct system (ex: salivary glands and pancreas)

Classification of acinar glands

• serous cells: protein rich secretory granules, pyramidal shape, round nuclei, eosinophilic

• mucous cells: synthesize and store mucinogen granules, cuboidal to columnar, flattened nuclei, pale colour washed out stain

• Mix of serous and mucous cells → serous demilune

Organization of ducts in glands

• lobe → salivary glands surrounded in a capsule of CT and divided into lobes

• lobules → CT divides lobes into lobules

• intercalated discs → join ducts together to form larger ducts, connect to acinar

• striated duct → cuboidal to columnar epithelium

Intercalated ducts

• secretion of the acinus enters the intercalated duct, lined by simple squamous to low cuboidal epithelium

Striated duct

• lined with tall cuboidal or low columnar cells

• nuclei towards the center of the cell

• basal portion present cell membrane interdigitations (foldings) and mitochondria in between the foldings

  • increase SA of pumps and mitochondria

Production of hypo-osmotic saliva

• acinar cell makes saliva and dumps it into the lumen continuous with the intercalated duct and it goes to striated duct

• acinar cell actively pumps Na+ and Cl- which are resorbed in the striated duct

• Kallikrein is a protease released into striated duct that modifies proteins in saliva

• bicarbonate is released into striated duct to neutralize the acid and create buffering capacity

Striated (intralobular) duct

• striations at basal portion

• sodium ions are reabsorbed from raw saliva to produce hypotonic saliva (less sodium and chloride, more potassium and bicarbonate)

Parotid gland

• tubuloalveolar gland

• serous acini produce thin water secretion rich in enzymes and antibodies

• 25% of alkaline salivary volume is secreted by parotid gland

Sublingual gland

• branched compound tubulo-acinar gland

• mixed, MOSTLY mucous

• secrete lysozymes to hydrolyze cell walls of bacteria

sublingual gland

Submandibular gland

• branched compound tubolaracinar gland

• mixed, mostly serous

• mucous acini with serous demilune in a mixed gland

submandibular gland

What is Waldeyer’s ring?

• in tonsils (first organ of defense in oral cavity)

• an interrupted circle of protective lymphoid tissue at the upper ends of the respiratory and alimentary tracts

Taste buds structure

• specialized epithelial structure

• pale staining, barrel shaped, lie perpendicular to epithelium

• each taste bud has receptors for all 5 tastes

Where are taste buds found?

• mainly on dorsal surface of tongue

Neuroepithelial/sensory cells in taste buds

• elongated cells, microvilli

• tight junctions with neighbouring cells

• at bases, they form synpases with CN VII, IX, and X (7, 9, 10)

Sustentacular/supporting cells of taste buds

• slender, microvilli

• do not synapse with nerve cells

Basal cells of taste buds

• small cells located near basal lamina

• stem cells for the supporting and sensory cells

How do we taste?

1. tastant binds with a TR1 or TR2 receptor

2. G proteins and secondary messengers are activated and bind to G complex

3. depolarization

4. calcium is released and triggers neurotransmitters from taste cell

Foliate papillae

• disappear as we get older (may be vestigial), well defined in infants and youth

• occur on lateral edge of tongue

• taste buds found along lateral wall of furrows

Circumvallate papillae

• largest, 8-12 found anterior to sulcus terminalis

• each is surrounded by a gutter/moat/rampart

• taste buds on lateral wall (about 250 on each)

• serous glands of Von Ebner found in CT around papillae

Fungiform papilla

• look like mushrooms

• concentrated in anterior portion of tongue

• narrow stem, broad round top that projects above surface, red colour

• taste buds are on stratified squamous on dorsal surface

  • only a few taste buds on dorsal

Filiform papillae

• most numerous, conical structures on anterior 2/3 of tongue

• NO taste buds

• keritinized simple squamous

• mechanical function (ex: licking)

• white in colour

Hairy tongue

• patient has dark hypertrophied filiform papilla with bacterial colonies on it

Dorsal surface of tongue

• bottom of tongue

• circumvallate papillae (on v-shaped teminal, Ebner’s glands)

• foliate papillae (not lot in human)

• fungiform papillae (on margin of tongue)

• filiform papillae (no taste buds, increase friction between tongue and food)

The tongue

• skeletal muscle

• helps in mastication, swallowing, speech, taste, temp, pain and touch perception

• dorsal and ventral surface

How is the tongue divided?

Dorsal surface: anterior 2/3 and posterior 1/3

V-shaped sulcus terminalis

Tongue histological features

Core: formed by intrinsic, very vascularized skeletal muscle

Mucosa: stratified squamous on doral and ventral

Lamina propria: CT, many blood vessels, lymphatics and nerves, lingual salivary glands

Mumps

• swollen, painful salivary glands

• fever and sometimes orchitis, pancreatitis, etc.

• 1/3 infected people do not show symptoms

• effective vaccine since 1967

Salivary duct cysts

Epithelial lined cavity filled with mucus and fluids

Sialolithhiasis

• salivary stones

• calcium salts deposited around debris

Sialodentitis

• inflammation of salivary glands because of mumps, influenza, coxacki virus, or systemic disease (Sjogren’s)

Pleomorphic adenoma

• large tumour from salivary gland

Saliva functions

• protection (fluid lubricant with imunoglobulins and calcium binding proteins))

• antimicrobial (lysozymes, antibodies, lactoferrin)

• Buffering (HCO3 and PO4 and saline maintain pH and neutrolize)

• Tooth integrity: source of calcium and phosphate → increases enamel hardness in new erupted tooth

• Digestion: moisten bolus of food, amylase, lingual lipase

• Taste: solubilizes molecules sensed by taste buds

Myoepithelial cells

• at base of acinar cells

• between acinar cells and their basal lamina

• have actin filaments that contract

What is the main cell type in the parotid gland?

• serous

• has myoepithelial cells

• numerous intercalated discs

• moderate striated ducts

What is the main cell type in the submandibular gland?

• serous mainly (some mucous)

• has myoepithelial cells

• medium intercalated discs

• abundant striated ducts

What is the main cell type in the sublingual gland?

• mixed, mostly mucous

• has myoepithelial cells

• not many intercalated discs

• not many striated ducts

Does the palate have an oral and nasal surface?

Yes

What structure shuts food off from going into the air tract pathway?

epiglottis

What type of palate is at the roof of the oral cavity

Hard palate

Hard palate tissue

• keritinized stratified squamous supported by LP

• Upper layers supported by spicular bone

Soft palate structure and function

• nasal and oral surface

• involved in talking, breathing and swallowing

Soft palate support tissue

• supported by skeletal muscle

• houses many secretory mucous glands

What type of epithelium is in the nasal surface of the soft palate?

pseudostratified ciliated columnar epithelium with mixed glands

What type of epithelium is in the oral surface of the soft palate?

• nonkeratinized stratified squamous

• wet kind

• interdigitated with LP under

Pharynx location and function

• extends back from oral cavity

• passage for food that goes through pharynx to esophagus

• nasopharynx, oropharynx, hypopharynx

Pharynx mucosa and submucosa tissue

• MUCOSA: wet nonkeratinized stratified squamous epithelium with LP rich in elastic fibers

• SUBMUCOSA: CT between LP and underlying skeletal muscle, occasional seromucous glands that can extend into muscle

What structure does stroke affect if issue in coordination of swallowing muscles?

Pharynx

What are the 4 layers of the GI tract?

• mucosa

• submucosa

• muscularis externa

• serosa/adventitia

What tissues are in the mucosa? (GI tract)

• epithelium lining

• lamina propria (CT)

• muscularis mucosae (smooth muscle)

What tissues are in the submucosa? (GI tract)

• glands

• vessels with nerve plexus

What tissues are in the muscularis externa? (GI tract)

• inner circular layer and outer longitudinal layers of smooth muscle

• nerve plexus in between

What tissues are in the adventitia/serosa (GI tract)?

• loose CT on outside

• simple squamous epithelium (mesothelium)

• adventitia - secures organs to surrounding tissue

• serosa - cover organs

Mucosa functoins

• epithelium → barrier + absorptive functions

• mucosa → secretory function (enzymes, hormones) and contains glands, BV, lymphatic tissue

• muscularis mucosa → allows fine adjustments in mucosa by making ridges and valleys (make longitudinal folds → elastic fibers recoil and move food)

Submucosa functions

• within CT there are nerves (enteric nervous system), blood vessels, glands and lymphatics

• loose CT allows folding of overlying mucosa to form folds

Muscularis externa function

• contracts, mixes, and propels contents

• can cause bowel movements

Esophagus function

• strong muscular tube, conveys food from oropharynx to stomach

How does swallowing work?

• initiation is a voluntary act with skeletal muscle of oropharynx

• followed by strong peristaltic reflex in upper part of esophagus → convey bolus of food and fluid to stomach

What are the diff muscles in each third of the esophagus?

• upper 1/3 = skeletal muscle

• middle 1/3 = smooth + skeletal

• lower 1/3 = smooth involuntary muscle

Esophagus lumen tissues

• thick, protective, non-keratinized stratified squamous epithelium

• lamina propria

• muscular mucosa under lp

Esophagus submucosa tissue

• very loose, fair amounts of elastic fibers, has seromucous glands

Esophagus muscularis externa tissue

• outer longitudinal and inner circular layer

• smooth muscle

Esophagus submucosa layer tissue

• peripheral ganglia between layers of muscularis

• very thin, hard to see

How does epithelium change from esophagus to stomach?

• drastic change from stratified squamous in esophagus to simple columnar in stomach

  • withstands acidity

• seromucous glands into stomach for short distance, then disappear

Barrett esophagus

• intestinal metaplasia (columnar replace squamous) of distal esophagus

• patches of red, velvety mucosa extending up from gastroesophageal junction

• increases risk of esophageal adenocarcinoma (cancer)

What are the 3/4 regions of the stomach?

• cardiac region (superior opening of stomach)

• fundus and body (major part of stomach)

  • combine bc same tissues histologically

• pyloric part (funnel-shaped outflow)

What are rugae?

• folding of mucosa in wall of stomach body

• folds when stomach is empty and disappears when stomach is full

What makes rugae possible?

The contractions of muscularis externa and loose submucosa

What kind of organ is the stomach?

exocrine-endocrine organ

Stomach function

• mechanically churn food and make chyme

• digestion

  • some absorption in stomach (water, ions, alcohol, aspirin, caffeine)

  • produces pepsin, HCL, and gastic lipase to help digest proteins, triglycerides

What does the stomach produce?

• pepsin

• mucous

• lysozyme

• gastrin

• somatostatin

Which structure has 3 layers in the muscularis externa?

The stomach, has additional oblique muscle layer

Gastric pits

• in the stomach and connect to gastric glands in mucosa

• lined by surface mucous cells

What makes up the gastric mucosa?

• gastric pits

• gastric gland isthmus

• gastric gland neck

• gastric gland base

What cells are in the gastric gland isthmus?

parietal cells

What cells are in the gastric gland neck?

• neck mucous cells

• parietal cells

• stem cells (replenish cells at surface constantly exposed to stomach acid)