Small intestine

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Lecture 2
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Small intestine

• Primary action is to absorb nutrients into
  circulation
• Very metabolically active
• Highly adaptive organ
• Increase in absorptive surface area
• Enhance digestive and transport capacity

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Small Intestine cont

• Plicae Circularis
• Circular folds within the submucosa
• Contains many villi.
• Villi
• Finger-like mucosal projections within the lamina
  propria
• Each villus contains a capillary bed and a
  lymphatic vessel (lacteal)

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Gropper et al., 2005
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Villi Crypt
Gropper et al., 2005
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(No Transcript)
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Four layers to the intestine

• Mucosa
• Submucosa
• Muscularis
• Serosa

Gropper et al., 2005
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Mucosal Layer (Layer 1)

• Epithelium
• Lamina Propria
• Loose connective tissue
• Peyers Patches
• Lacteals
• Muscularis Mucosae

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Mucosal Epithelium

• Epithelium
• Enterocytes
• Goblet cells
• Paneth cells (small intestine only)
• Entero-endocrine cells
• M cells

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Enterocytes

• Absorptive Cells
• Columnar cells with basal nuclei
• Microvilli on apical surface (lumen side)
• Digestive enzymes in microvilli glycocalyx
• breakdown sugars proteins
• Form junctional complexes (barriers)

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Goblet Cells

• Interspersed among enterocytes
• Produce mucus
• Look transparent under a microscope
• More numerous in large intestine than small
  intestine

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Paneth Cells

• Located in Crypts of Lieberkuhn
• Provide host defense against microbes in the
  small intestine
• Functionally similar to neutrophils
• Secrete antimicrobical molecules into the lumen
  of the crypt
• Regulate bacterial flora of gut and barrier
  function

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Enteroendocrine Cells

• Secrete Hormones
• Gastric inhibitory polypeptide (GIP)/Glucagon-like
  peptides
• Inhibits gastric secretion and motility
• Cholecystokinin (CCK)
• Stimulates fat and protein digestion
• Secretin
• Regulates pH
• Controls gastric acid secretion

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M cells

• Differ from normal enterocytes
• Lack microvilli on their apical surface
• Possess broader microfolds
• Less abundant than enterocytes
• They sample antigen from the lumen of the small
  intestine and deliver it to antigen presenting
  cells and lymphocytes

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Mucosal Lamina Propria

• Thin layer of loose cellular connective tissue
• Peyers Patches
• Lymph nodules and immune cells
• Lacteals
• Lymphatic channels in the villi that uptake lipid
  droplets
• M Cells

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Mucosal Muscularis Mucosae

• Forms the boundary between the mucosa and
  submucosa
• Composed of two thin layers of smooth muscle

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Submuscosa (Layer 2)

• Layer of loose connective tissue
• Supports the mucosa
• Joins the mucosa to the underlying smooth muscle

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Muscularis (layer 3)

• Region of muscle below the mucosa and submucosa
• Responsible for intestinal peristalsis

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Serosa (Layer 4)

• Smooth membrane consisting of a thin layer of
  cells

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Large Intestine

• Similar four layers
• Reabsorbs water and electrolytes
• Excretes waste
• Crypts
• No villi
• Large number of goblet cells (mucus)

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Large intestine mucus

• Protects against bacterial activity
• Medium for holding fecal matter
• Barrier to keep fecal acids from attacking the
  intestinal wall

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Large intestine auxiliary digestion

• Bacteria
• Breakdown undigested carbohydrates and proteins
• Lactate and short-chain fatty acids
• Produce large amounts of absorbable vitamins

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Intestinal epithelium
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The Epithelial Barrier

• Critical for maintaining a physical barrier
  between the intestinal lumen (external) and
  internal environments
• Dependent on the junctional complexes connecting
  enterocytes together
• Control the movement of membrane proteins between
  the apical and basolateral surfaces of the cell
• Receptor-mediated endocytosis at the apical
  surface and exocytosis at the basolateral surface
• Preserves transcellular transport

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• Well organized intercellular array of molecules
  surrounding the apical region of epithelial cells
• Tight junctions
• Adhesion junctions
• Desmosomes

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Tight junctions/adhesion

• Regulated by the membrane spanning proteins
• Claudin
• Occludin
• Zonula occudens-1 and 2 (ZO-1, ZO-2)
• Cingulin
• E-cadherin

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Epithelial Cell Regeneration

• Occurs in the intestinal mucosa
• Deep epithelial cells in the crypts
• Continually undergo mitosis
• New cells migrate upwards and out of the crypts
  towards the villi tips
• Replace villus epithelium and differentiate into
  mature absorptive cells (enterocytes)
• Life cycle
• 5 days

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Paneth cells in base of crypt
Stem cells
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Nutrition and the Gut
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Feed additives and gut health

• Probiotics
• Yeast cell well components
• Spray-dried animal plasmas
• Enzymes

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Probiotics

• Categories
• Viable (active and culturable)
• Dormant (inactive but culturable)
• Active but not culturable
• Dead but not culturable
• Positive influence on growth performance
• Modifies gut microbiota
• Blocks pathogenic colonization of intestine
• Inhibits immune system activation
• Inconsistent performance and health

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Common organisms

• Lactobacilli
• Non pathogenic E. coli
• Enterococci
• Streptococci
• Yeasts

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Yeasts

• Baker yeast (Saccharomyces cerevisiae)
• Positive influence on growth performance
• Modifies gut microbiota
• Blocks pathogenic colonization of intestine
• Inhibits immune system activation
• Increases
• villus height
• Epithelial proliferation
• Intestinal immune cell number

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Sray-dried Plasma

• Improves growth rates and potentially feed
  conversion
• Contributed to the immunoglobulin fraction
  Increases voluntary feed intake
• Highly palatable
• Rich in fibrinogen immunoglobulin and albumin
• Alters mucosal lamina propria in pigs
• Lowers immune cell density
• Decreases proinflammatory gene expression in many
  organs

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Enzyme Additives
Adopted from Odetallah (2005) Pierce and
Stevenson (2008)