GI System
Tubular Organ Terminology
-
-
-
Similar mucosal histology is found in the following anatomical structures
 Lip, buccal mucosa, tongue, hard palate, soft palate, pharynx
Lamina propria mucosa
 Constituent of mucous membranes (mucosa)
o Line various tubes in the body
 Respiratory, GI, urogenital tracts
 Thin layer of loose CT that lies beneath epithelium
o Lamina propria and epithelium form the mucosa
 Contains varies cell and tissue types
o Capillaries
o Central lymph vessel
o Lymphoid tissue
o Glands with ducts that open onto mucosal epithelium
 Mucous
 Serous
Lip
 Transition from skin to mucous membrane
 Remains cornified in ruminants and equids
 Lamina propria and submucosal layers blend together to form
the propria submucosal
 Minor salivary glands found in the submucosal
Tongue
 Lingual papilla (dorsal surfaces)
o Filiform papillae
 Most numerous
 Highly keratinized in cats
o Fungiform papillae
 Mushroom-shaped
 Few taste buds on upper surface
o Vallate (circumvallate) papillae
 Flattened surface
 Surrounded by deep sulcus
 Many taste buds in sulcus region
o Foliate papillae
Skin
Lip
Mucosa
Filiform papillae
Tongue
Fungiform papillae
-
-
Vallate papillae
Foliate papillae
Taste buds
 Taste cells are similar to olfactory cells
 “Taste” is a result of chemical sensation
o Sweet, salty, bitter, sour, umami
o Mediated through second messenger systems
 Bitter – intracellular Ca++ release
 Sweet – closing of K+ channels
 Sensory (neuroepithelial) cells
o Synapse with CN VII, IX, and X
o Regenerate every 10 days
 Supporting cells
 Basal (stem) cells
o Differentiate into sensory and supporting cells
Hard palate
Pseudostratified ciliated columnar
-
Soft palate
 Dorsal and ventral
surfaces are lined
with two types of
epithelium
Non-keratinized stratified squamous
-
Teeth

-
Brachydont
o Teeth that stop growing when eruption complete
o Exposed surfaces covered with enamel
o Composed of crown, neck, 1 or more roots
o Development
 Bud stage
- Ameloblasts (produce enamel) and odontoblasts (dentin) differentiate
 Bell stage
- Enamel and dentin clearly visible
- Dental alveolus forms
 Functional stage
- Periodontal ligament forms
o Incisors of ruminants and pigs, carnivore teeth
 Hypsodont
o Growth and eruption continues throughout adult life
o Exposed surfaces covered with cementum
o Composed of elongated body (no crown or neck)
o Molars (cheek teeth) of horses and ruminants
 These teeth have roots and though they continue to erupt for decades, they have a
finite life (can ultimately be worn down to the gumline)
o Incisors of horses, rodents, rabbits
 These teeth are “rootless” and never stop erupting
Salivary glands
 All are compound tubuloacinar glands
 Can produce mucous, serous, mixed (serous-mucous) secretions
 Two types of salivary glands
o Major type
 Parotid (serous)
- Looks like a pancreas
- Eosinophilic (pink) cytoplasm
- Granules
 Sublingual (mucous)
- Pale blue cytoplasm
- No granules
 Mandibular (mixed)
- Serous demilunes
Sublingual gland
Parotid gland


Zygomatic (carnivores; mostly mucous)
Molar (cats; mostly mucous)
Mandibular gland
o

Minor type
 Labial (lips)
 Lingual (tongue)
 Gustatory (all are serous)
- Found with foliate and vallate glands
 Buccal (cheeks)
 Palatine (hard and soft palate)
 Pharyngeal
Secretory product is saliva
o Functions of saliva
 Mechanically protects mucosal surfaces by softening and coating ingest
 Lubricates mucosal surfaces
 Dissolves water soluble components
 Serves as a solvent for molecules that stimulate taste buds
 Aids movements of lips and tongue
 Facilitates mastication & swallowing
 Helps keep the mouth and teeth clean
o Constituents of saliva
 Water and electrolytes
- Secrete electrolytes (Na+, Cl-) and water follows via osmosis
- Produce protective mucous that is resistant to digestion (with mucins)
 Digestive enzymes
- Lipases initiate triglyceride (lipid) digestion
- Amylases initiate starch digestion
 Glycoproteins (mucins )
- Produce protective mucous that is resistant to digestion
- Lubricate ingesta and protect oral mucosa from abrasion
- Buffer the pH of the ingesta
 Bicarbonate (HCO3-, Acts as a buffer; protects enamel)
 Immunoglobulin A (IgA)
- Initial protection against bacteria and viruses
 Lysozyme (attacks bacterial walls)
 Lactoferrin
- Bacteriostatic (limits bacterial growth) iron-binding molecule
 Proline-rich Proteins (may protect enamel)
o Salivary gland ducts (smallest  largest)
 Intercalated duct (squamous epithelium)
 Striated duct
- Cuboidal to columnar epithelium
- Most prevalent in serous glands
- Many mitochondria b/w infoldings of cell membrane in basal region
- Biocarbonate, IgA, and kallikrein (protease) secretion
- Electrolyte (Na+, Cl-) reabsorption
 Intralobular duct (cuboidal to columnar epithelium)
 Interlobular duct (pseudostratified columnar epithelium)
 Lobar duct (stratified columnar epithelium)
 Main duct
o Secretory volume varies with species
 People (1 L/day)
 Sheep (6-10 L/day)
 Cattle (100-200 L/day)
Esophagus
- Pharyngoesophageal limen
 “Threshold” or division between pharynx and
esophagus
- Tunica mucosa
 Lamina epithelialis
o Prominent stratified squamous
epithelium that varies in thickness and
degrees of keratinzation with species
 Keratinized
- Ruminant, horse, pig
 Non-keratinized
Esophagus
- Carnivores
 Lamina propria
o Extracellular meshwork of CT fibers
 Heavy feltwork of collagen fibers
 Abundance of evenly-distributed elastic fibers
o Contains blood vessels, nerves, lymphatics, epithelial ducts, inflammatory cells, fibroblasts
Lamina epithelialis

-
Lamina propria
Lamina muscularis
Lamina muscularis
Lamina muscularis
o Longitudinally-oriented smooth muscle bundles
o Abundance and location vary with species
 Ruminant, horse, cat
- Isolated bundles near pharynx increase in thickness and become confluent
as they near the stomach
 Llama – Only a few, thin, scattered strands in distal esophagus
 Pig – Bundles absent from cranial esophagus and confluent at stomach
 Dog – Isolated bundles absent from cranial esophagus but found distally
Tunica submucosal
 Loose CT containing large longitudinally-oriented arteries, veins, lymph vessels, and nerve trunks
 Loose nature of the submucosa allows for longitudinal folding (rugae) of relaxed esophagus and
expansion during ingestion of a food bolus
 Contains seromucous glands with mucous acini and serous demilunes
o Location of glands varies with species
 Ruminant, horse, cat
- Present only at pharyngoesophageal junction
 Pig
- Abundant in cranial half of esophagus (do not extend to caudal half)
 Dog
-
Present throughout length of esophagus
Extend to cardiac region of stomach
Density of glands greater in caudal half
Stomach
- Esophagus-glandular stomach junction
- The gastric mucosal surface consists of mucosal surface cells that secrete various important substances, which
provide a physiological barrier and protective layer (thick, slimy) against HCl
o Mucin, HCO3- Gastric glands contain many cell types
 Mucous cells
o Found in surface and neck region
o Secrete mucous that maintains gastric-mucosal barrier
 Mucous traps protective HCO3- ions
 Parietal cells
o Lifespan ~120 days
o Maintain gastric pH (<1.0-2.0)
o Secrete HCl
 3 receptors types can activate HCl secretion
- Acetylcholine
- Gastrin
- Histamine (H2)
 Enteroendocrine cells
o Secrete gastrin (paracrine hormone)
 Chief cells
o Releases pepsinogen, a precursor enzyme that is converted to
pepsin in the presence of gastric acid (HCl)
- Lamina subglandularis
 Only within the stomachs of carnivores (especially prominent in cats)
 Located b/w the bases of glands and lamina muscularis
 Refractile and eosinophilic
 Composed of two layers
o Stratum Granulosum
Gastric mucosa
 Inner layer containing many fibroblasts
o Stratum Compactum
 Outer, dense sheet of collagen fibers
Lamina propria
Stratum
compactum
Statum
compactum
Pareital
cell
Lamina subglandularis
Muscularis
mucosa
Chief cell
-
-
Ruminant forestomachs
 All forestomach compartments are lined with stratified squamous epithelium
 All are histologically similar
Rumen
 Involved in fermentation
o Mechanical and chemical action
o Facilitated by resident bacteria & protozoa
 Rumen mucosa
o Outermost epithelial layers provide protection
o Deep layers are involved in the
absorption of volatile fatty acids
 Acetic
 Butyric
 Propionic
Rumen mucosa
-
Rumen mucosa
Camelid forestomach
 Vastly different from ruminant forestomach
 Contain mucigenous glands
o Saccules are lines by columnar epithelium
 May be involved with HCO3- secretion
 Has portion similar to ruminant abomasum
- Equine and rodent stomach
 Mucosa composed of ~ 50% squamous epithelium
Small Intestine
- Transition occurs at stomach-duodenum junction
- Characterized by three regions, each with special features
 Duodenum
Mucigenous glands
o Submucosal (Brunner’s) glands
 Secrete HCO3
 Neutralize HCl
 Jejunum
 Ileum
o Contains Peyer’s Patches
 Form of submucosal lymphoid tissue (GALT)


-
-
-
-
Covered with specialized epithelium
Contain dendritic and M cells
- Take up antigens from
intestinal lumen
- Interact with CD4 T
lymphocytes
 Not seen with H&E stain
Villi and crypts are present within all three regions
 Small intestine villus
o Epithelial cell brush border forms microvilli
o Villus lamina propria
 Consists of loose CT, blood, and
lymphatic vessels
Mucosal cells secrete digestive enzymes that break
Crypt of
down disaccharides into glucose
Liberkűhn
 Lactase, maltase, sucrase
Enteroendocrine cells
 Produce >20 peptide hormones
o Gastrin (stimulates parietal cell HCl secretion)
o Cholecystokinin (stimulates gall bladder and pyloric sphincter contraction)
o Secretin (stimulated insulin secretion)
o Gastric inhibitory protein (inhibits HCl secretion; stimulates insulin secretion)
o Motilin (stimulates gastric and SI motility)
o Somatostatin (stimulates gastrin secretion)
 Cell are also known as…
o Argentaffin cells
o Argyrophil cells
o Enterochromaffin cells
o Kulchitsky cells
Paneth cells
Large Intestine and Cecum
- Size and function varies greatly with species
- Contain straight tubular glands called crypts of Liberkűhn
- NO villi
- Prominent submucosal lymphoid tissue
- Function to absorb water, vitamins, electrolytes
- Large intestine and cecum have identical histology
Avian Digestive System
- Proventriculus (glandular stomach)
 Epithelial surfaces lined with simple cuboidal epithelium
 Contains gastric glands
o Oxynticopeptic cells secrete HCl and pepsinogen
- Ventriculus (gizzard)
 Epithelial surfaces lined with simple cuboidal epithelium
 Inner lining composed of koilin membrane
o Koilin is a secretory product of mucosal glands
 NO keratin in the gizzard
Large intestine and cecum
Brunner’s
gland
Acellular koilin
Mucosal glands
Proventriculus
Ventriculus