GIT

Gastrointestinal Tract: Generic Wall Plan

  • Dr Eleanor Peirce, Anatomy & Pathology, Adelaide Medical School

  • Room N240, Helen Mayo North

  • Phone: 831 35191

  • Email: eleanor.peirce@adelaide.edu.au

Learning Objectives

  • Identify the histological components (layers, tissues & cells) that contribute to the generic wall plan of the gastrointestinal tract (GIT).

    • 4 “standard” layers

    • Tissue & cellular composition of each wall layer

  • Correlate individual layers & structural components of the generic wall plan with their functions.

    • Structure-function relationships of generic wall plan components

The GIT as a Flexible Tube

  • Structural Components

    • Lumen: enables passage of contents through tube.

    • Flexible Wall: enables movement in response to variations in environmental conditions.

  • Functional Considerations

    • Modification & extraction of, or addition to, contents from the lumen must occur during passage through the tube.

    • Multilayered wall structure with specific roles for each layer, tissue & cell type.

Generic Wall Plan of the Gastrointestinal Tract

  • Extends from the oesophagus to the rectum.

  • 4 “standard” layers.

  • Specific tissues &/or cell types within each layer.

  • Generic plan based on jejunum (middle part of small intestine).

Wall Layers of the GI Tract

  • From lumen:

    1. Mucosa

    2. Submucosa

    3. Muscularis externa

    4. Serosa (or adventitia
      *CT only, no mesothelium)

Tissues within GIT Wall Layers

  • Mucosa (M)

    • Epithelium (varies with location along GIT)

    • lamina propria = loose connective tissue

    • muscularis mucosae = smooth muscle

  • Submucosa (SM)

    • dense (more fibrous) connective tissue than lamina propria

Tissues within GIT Wall Layers

  • Muscularis externa (ME)

    • smooth muscle

      • inner circular sub-layer (relative to lumen)

      • outer longitudinal sub-layer

  • Serosa (S)

    • Connective tissue covered by mesothelium

    • If no mesothelium = adventitia

Mucosa: Definitions of Mucosa & Lamina Propria

  • Innermost epithelial layer & underlying loose CT layer that line tubular systems that open to the outside of the body, i.e. GIT, respiratory, renal, & reproductive systems.

  • In GIT only, mucosa = epithelial lining + lamina propria + a thin band of smooth muscle, muscularis mucosae

Lamina Propria

  • loose CT layer of a mucosa

  • Often contains glands

Mucosa - Structural Features

  • lining epithelium + lamina propria +
    muscularis mucosae

  • crypt

  • lumen

  • villus

  • MALT

Mucosa - Composition

  • Epithelium

    • stratified squamous, or simple columnar

    • various cell types (enterocytes, goblet cells)

  • Lamina propria

    • loose connective tissue

    • high in amorphous ground substance(AGS) & cells

    • abundant capillaries & lymphatic tissue

    • Glands/crypts/pits

  • Muscularis mucosae

    • Smooth muscle

Mucosa

  • General Functions Cumulative functions of individual cells & tissues

    • Epithelium

      • Absorption (brush border on enterocytes)

      • Secretion e.g. enzymes, hormones, mucus

      • Protection (barrier b/w lumen & deeper tissues)

    • Lamina propria

      • Transport (through AGS to vessels)

      • Defense (various cells)

    • Muscularis mucosae

      • Localised movement (muscle contraction)

Mucosa - Lining Epithelium

  • Epithelial lining from stomach to rectum = simple columnar epithelium; (oesophagus = stratified squamous)

  • Small & large intestines lined by absorptive cells (= enterocytes) & goblet cells (secrete mucus)

  • General functions of mucus: lubrication & protection (small amount of adhesion)

Mucosa - Lamina Propria

  • AGS provides diffusion pathway for transfer of nutrients from epithelial cells where they are absorbed to blood capillaries.

  • Absorbed lipids, tissue fluid, & larger particulate substances (pathogens) enter lymph capillaries (lacteals).

  • CT cells target & destroy absorbed pathogens & antigens

Mucosa - Muscularis Mucosae

  • Thin layer of smooth muscle.

  • Located between CTs of mucosa & submucosa.

  • Allows local movement of mucosa, independent of the external muscle layers.

  • Muscle fibres extend from the muscularis mucosae into the core of the villi – contraction of muscle maximises the contact of contents in lumen with the epithelial lining.

Adaptations for Absorption

  • Folds consist of a core of submucosa & a covering of mucosa = plicae circulares

  • Macroscopically visible folds increase surface area (SA)

Mucosa – Adaptations for Absorption

  • Upward (finger-like) extensions of epithelial covered lamina propria = villi (singular = villus); villus height approx. 1mm1 mm

  • Downward extensions of epithelial lined lumen between bases of villi = crypts (glands) specialised cells with secretory activity

  • Villi & crypts of mucosa increase surface area

Mucosa – Adaptations for Absorption

  • Apical surfaces of enterocytes have microfilament supported extensions of plasma membrane = microvilli

  • High density of microvilli = brush border

  • Microvillous height = approx. 1μm1 μm

  • Microvilli increase surface area of cells

Adaptations for Absorption Summary

  • Maximum surface area for absorption is achieved by long length of tube (>5m>5m), folds, villi & microvilli.

  • Surface area of small intestine = ~200m2200m^2

  • Total SA = 600x600x > smooth cylinder of same luminal diameter

Submucosa – Components & General Functions

  • Layer of dense (fibrous) connective tissue containing:

    • large blood & lymph vessels,

    • glands & lymphatic tissue (“MALT”] in some regions (“overflow” area)

  • Connects mucosa to outer muscle layers, conduit for vessels & nerves

Submucosa Characteristics

  • Irregular dense CT

  • Usually very few defence cells

  • Connects mucosa to layers in outer wall

  • Houses large vessels & nerves (= services route)

  • CT layer forms core of submucosal folds (plicae circulares; * area on image)

Muscularis Externa

  • Two sub-layers of muscle some exceptions

  • Inner sub-layer: circular orientation of fibres

  • Outer sub-layer: longitudinal orientation of fibres

  • Smooth muscle from oesophagus to rectum

    • Mixes, crushes & propels food along by peristalsis

    • Myenteric (Auerbach’s) plexus (MP) innervates both circular & longitudinal sub-layers of smooth muscle

  • Limited skeletal muscle in mouth, pharynx, upper esophagus & anus

    • Voluntary control over swallowing & defecation

Sub-layers of Muscularis Externa

  • Muscularis externa consists of a sub-layer of muscle cells with an inner circular orientation, i.e. a concentric arrangement around the lumen & an outer longitudinally oriented sub-layer, i.e. muscle cells run parallel to the length of the intestine

Functions of Muscularis Externa

  • Peristalsis: Waves of contraction of circular & longitudinal muscle layers that propel gut contents forward through tube

  • Segmentation: coordinated contraction of localised regions of circular & longitudinal muscle layers mixes luminal contents

Serosa – Structure & Function

  • CT covered by simple squamous epithelium (mesothelium)

  • Secretes slippery fluid movement of organ independent of body wall

  • Conduit for entry/exit of vasculature & nerves into/out of organ

Learning Summary

  • Identified the histological components (layers, tissues & cells) that contribute to the generic wall plan of the gastrointestinal tract (GIT)

    • 4 “standard” layers: mucosa, submucosa, muscularis externa, serosa

    • Tissue & cellular composition of each wall layer

  • Correlated individual layers & structural components of the generic wall plan with their functions

    • Structure-function relationships of generic wall plan components à layer/regional function = cumulative function of constituent parts

Regional Specialisations - Duodenum & Ileum

  • Dr Eleanor Peirce Anatomy & Pathology, Adelaide Medical School

  • Room N240, Helen Mayo North

  • Phone: 831 35191

  • Email: eleanor.peirce@adelaide.edu.au

Gastrointestinal Tract 1

  • Key Concept Structural adaptations/variations from the generic wall plan of the GIT (i.e. jejunum) reflect regional differences in digestive function

    • Most structural variations are associated with the mucosa & muscularis externa wall layers

Learning Objectives

  • Appreciate that structural adaptations of the GIT wall plan reflect differences in function between regions

    • Correlate shared structural characteristics (common with jejunum) of the small intestine (SI) with their functions

    • Investigate how regional variations of luminal environment & function influence wall structure of the duodenum & ileum

    • Identify & link the distinguishing features of the duodenum & ileum with their functions

  • Distinguish between regions of the GIT according to their structural and functional characteristics

Recap: Generic Wall Plan of GIT

  • Based on jejunum (J)

  • Features reflect function, i.e. completion of digestion, nutrient absorption & endocrine secretion

  • However:

    • Mucosal & submucosal adaptations in

      • Duodenum (D)

      • Ileum (I)

Shared Structures & Features of SI Regions

  • Plicae Circulares

    • Permanent folds that have a core of submucosa

    • Not found in lower ileum

    • Can not stretch out like rugae in stomach

    • Functional implication: Increase surface area for absorption

  • Villi

    • Finger-like projections of mucosa

    • Core of lamina propria covered by epithelium

  • Microvilli

    • Extensions of enterocyte apical plasma membrane

  • Large Surface Area of Small Intestine (SI)

    • Plicae circulares (circular folds) up to 10mm10 mm tall involve only mucosa & submucosa chyme flows in spiral path causing more contact

    • Villi are fingerlike projections 1mm1 mm tall contain blood vessels & lymphatics (lacteals) for nutrient absorption

    • Microvilli are 1μm(0.001mm)1 μm (0.001mm) tall form brush border on cells brush border enzymes for final stages of digestion

    • 3 levels of folding provide 600 times greater surface area than that of a smooth cylinder

Shared Structures in SI: Epithelial Lining Cells

  • 2 most common cell types

    • Enterocytes (absorptive cells)

    • Goblet cells (secrete mucus)

  • These cells:

    • Cover villi & line crypts

    • Have a 4-8 day turnover time

    • Arise from stem cells at base of crypts

    • Differentiate as they move up crypts to tip of villi where they undergo apoptosis & enter the lumen

Goblet Cells & Enterocytes

  • Goblet cells secrete mucus that:

    • coats all lining cells & protects them from digestive enzymes (viscous)

    • lubricates lining (slimy)

    • Adheres small molecules (sticky), facilitating absorption

    • Goblet cell density increases from duodenum to ileum

  • Function of enterocytes

    • Tight junctions form strong barrier between gut lumen & tissues of body

    • Besides absorption, have role in digestion

Tight Junctions (TJs) between Enterocytes

  • Seal intercellular gap via fusion of adjacent cell membranes

  • Block intercellular passage barrier between two environments

  • Functional significance: Prevent:

    • Digestive secretions in lumen destroying gut lining through seepage between lining cells

    • Pathogen penetration through the epithelial layer Enable:

    • Selective absorption - must occur through microvilli & cytoplasm of cells (not via intercellular space)

    • Absorption against concentration gradient epithelial layer lumen of gut intercellular environment

Enterocytes – Role in Digestion in SI

  • Cells release enterokinase that activates/converts inactive proenzymes from pancreas into active enzymes

  • Membrane of microvilli houses digestive enzymes that catalyse final conversion of partially digested carbohydrates & proteins into monosaccharides & small peptides prior to absorption by cells

Enterocytes – Lipid Absorption

  • Bile (from liver) & lipases convert lipids to monoglycerides & fatty acids that pass into enterocytes

  • Lipids resynthesised in SER as triglycerides & small droplets coated with a thin lipoprotein cover to form chylomicrons

  • Chylomicrons released from cells & enter lymph capillaries

Enterocytes: Transport of Antibody

  • Plasma cells in lamina propria produce antibodies (IgA) that are transferred through enterocytes via endosomes

  • In lumen, antibodies bind with pathogens, toxins etc.

Other Small Intestinal Epithelial Cells

  • Paneth cells secretes lysozyme

  • Enteroendocrine cells

    • Secretin

    • Cholecystokinin

    • Gastric inhibitory peptide

  • M (microfold) cells

    • Endocytose & present antigen

    • Mainly in ileum

Intestinal Lining: Paneth Cells

  • Located at base of crypts

  • Contain prominent acidophilic granules

  • Secretion of lysozyme into lumen à regulation of natural gut flora

Intestinal Lining: Enteroendocrine Cells

  • Secrete regulatory peptides, e.g. gastrin, CCK, secretin, into lamina propria

  • Paracrine & endocrine regulation of gastrointestinal physiology, including control of gall bladder & exocrine pancreatic activity

  • Hormone-containing granules accumulate in basal region of cell & will be released from basal surface into adjacent lamina propria & then capillaries

Shared Structures of SI: Role of Muscularis Externa in Mechanical Digestion & Absorption

  • Muscle facilitates:

    • Weak peristalsis cf. strength of contractions in stomach; chyme remains for 3-5 hours ample time for absorption during transit

    • Segmentation = local mixing of chyme with intestinal juices; contents sloshed back & forth to facilitate maximal contact with mucosa

SI Luminal Environment Changes with Increasing Distance from Stomach

  • Proximal

    • Acidic chyme from stomach

    • Alkaline pancreatic juice

    • High enzyme load

    • Nutrient rich (à high absorption)

    • Less intestinal flora

  • Distal

    • Less hostile pH

    • Reduced nutrient load (due to prior absorption)

    • Relatively more toxins & pathogens

    • Higher levels of intestinal flora

  • Variations in Mucosal & Sub-mucosal Characteristics Linked to Differences in Luminal Environment

Duodenum – Luminal Environment

  • Luminal environment contains:

    • Acidic chyme entering through pyloric sphincter

    • Additional digestive secretions via hepatopancreatic ampulla

      • Bile from liver (stored in gall bladder)

      • Pancreatic juice = zymogens (inactive enzymes, activated by duodenal mucosa) + alkaline fluid rich in bicarbonate ions

  • Chemically hostile luminal environment Additional mechanisms required to protect epithelial lining à submucosal Brunner’s glands

Histology of Duodenum: Variation from Generic Plan

  • Definitive feature = submucosal (Brunner’s) glands that secrete alkaline mucus

Duodenum

  • Brunner’s glands provide additional alkaline mucus to counteract erosion of lining by gastric juices & pancreatic digestive enzymes

  • Muscularis mucosae is discontinuous (crossed by numerous ducts)

Ileum – Luminal Environment

  • Luminal environment:

    • Is less hostile chemically

    • Contains fewer absorbable nutrients

    • Contains higher relative concentration of toxins & pathogens

    • Has greater quantity of intestinal flora

  • But wall still has features that maximise absorptive capacity, therefore à Region prone to pathogenic invasion & additional defense mechanisms required to counteract threat à M cells, MALT/lymph nodules/Peyer’s patches

Histology of Ileum

  • Variation from Generic Plan

Ileum – Extensive Peyer’s Patches

  • MALT provides important defence mechanisms that prevent proliferation of micro-organisms & invasion

  • Microfold (M) Cells

    • Located in epithelium overlying lymphoid nodules, particularly ileum

    • Endocytose antigens from lumen & transport them to basal invagination where lymphocytes, macrophages etc., transfer material to adjacent lymph nodules for an immune response

Learning Summary

  • Appreciated that structural adaptations of the GIT wall plan reflect differences in function between regions

  • Correlated shared structural characteristics (common with jejunum) of the small intestine with their functions

  • Investigated how regional variations of luminal environment & function influence wall structure of the duodenum & ileum

  • Identified & linked the distinguishing features of the duodenum & ileum with their functions

  • Distinguished between regions of the GIT according to their structural and functional characteristics

Regional Specialisations - Appendix & Colon

  • Dr Eleanor Peirce Anatomy & Pathology, Adelaide Medical School

  • Room N240, Helen Mayo North

  • Phone: 831 35191

  • Email: eleanor.peirce@adelaide.edu.au

Gastrointestinal Tract 1

  • Key Concept Structural adaptations/variations from the generic wall plan of the GIT (i.e. jejunum) reflect regional differences in digestive function

    • Most structural variations are associated with the mucosa & muscularis externa wall layers

Learning Objectives

  • Appreciate that structural adaptations of the GIT wall plan reflect differences in function between regions

    • Investigate how variations in wall structure from the generic plan influence the functions of the large intestine

    • Identify the common features shared by the appendix & colon & link them with their functions

    • Identify & link the distinguishing features of the appendix & colon with their functions

  • Distinguish between all major regions of the GIT according to their structural and functional characteristics

Large Intestine: Anatomical Regions

  • Caecum

  • Appendix

  • Colon

  • Rectum

  • Anal canal

Large Intestine: Main Functions

  • Absorption (large surface area, enterocytes)

  • Lubrication of lining (abundant goblet cells)

  • Movement & Elimination (modifications to muscularis externa)

  • Modifies fluid content of faeces, i.e. absorbs water & salts

Large Intestine: Key Histological Features

  • Mucosa lined by simple columnar epithelium

    • Abundant Goblet cells in mucosal lining (> enterocytes)

  • Fewer submucosal folds and no villi à less surface area than small intestine

  • Intestinal crypts (glands sunken into lamina propria) produce mucus only

  • Modification of muscularis externa

    • Longitudinal muscle fibres of colon form taeniae coli producing haustra (pouches)

  • Transverse & sigmoid colon have a serosa with epiploic appendages (suspended fatty sacs);

    • Remainder of LI is retroperitoneal

  • Anal canal lined by stratified squamous epithelium

Appendix Histology

  • Variation from Generic Plan

    • Lymphoid tissue of appendix is part of MALT; contributes to overall defence & immune functions of intestinal mucosa

  • Epithelial cells lining crypts produce mucus & continue to do so even if opening of appendix into caecum is obstructed

Appendix – has Lymphoid Tissue

  • lymphoid nodules Extend into submucosa

  • Numerous, diffuse lymphocytes in lamina propria

  • Appendix is a narrow, blind-ending tube àincreased risk of lodgement of luminal contents & infection àabundant MALT in lamina propria & submucosa for defence

Colon Histology

  • Variation from Generic Plan

    • No villi, only crypts

    • Modified muscularis externa à taeniae coli

Histology of Colonic Mucosa

  • Features align with function of colon: reabsorption of electrolytes & water; elimination of undigested food & waste

    • No villi, just crypts (“flat” luminal surface)

    • Fewer enterocytes & EE cells but abundant goblet cells

Colonic Mucosa

  • Note:

    • Only crypts (no villi)

      • Reduced SA cf. small intestine

    • Many goblet cells

      • Less liquid luminal environment, more lubrication needed

    • MALT

      • Threat of invasion (but < MALT ileum as less SA)

Features

  • Mucus from abundant goblet cells in colon provides lubrication & protects the mucosa from abrasion caused by hardening faecal mass

Colon

  • Modifications of Muscularis Externa

    • Outer longitudinal muscle sub-layer of colon arranged in 3 thickened bands (taeniae coli), producing sac-like bulges (haustra coli) along length

    • Intervening muscle between taeniae of longitudinal layer is very thin

    • Haustra due to muscle tone of taeniae coli à ME provides main propulsive force for movement of faeces through lumen

  • Adaptations of Muscularis Externa = Taeniae coli

    • Involve the outer longitudinally running sub-layer of muscularis externa = area between dotted lines

    • Are 3 “thickened” bands of the longitudinal smooth muscle

    • Intervening muscle of outer sub-layer is very thin

Distinguishing Characteristics of GIT Regions

  • Each GIT region has a histological wall feature or set of features that together distinguishes it from other regions (see table)

  • Features relate predominantly to the mucosa and/or muscularis externa

  • Application to histological images of your knowledge & understanding of the components of the GIT wall & their organisation & function is examined in the Slide Analysis Assessment Task

  • Practice your skills using the Slide Analysis Formative Learning – Digestive Tract Quiz in MyUni.

GIT Region Distinguishing Histological Characteristic/s

  • Oesophagus Stratified squamous nonkeratinized epithelium

  • Body & fundus of stomach

    • 3 indistinct sub-layers to muscularis externa + mucosa with short, wide pits & long glands containing parietal & chief cells

  • Pylorus of stomach

    • Very thick circularly oriented sub-layer to muscularis externa (sphincter) + mucosa with long mucus secreting pits & short glands (few parietal cells)

  • Duodenum

    • Villi & crypts + submucosal mucous secreting (Brunner’s) glands

  • Jejunum

    • Villi & crypts

  • Ileum

    • Villi & crypts + Peyer’s patches (strings of lymph nodules)

  • Appendix

    • Crypts with abundant goblet cells, no villi + abundant MALT (diffuse lymphocytes, nodules) extending into submucosa

  • Colon

    • Crypts with abundant goblet cell but no villi; taeniae coli

Learning Summary

  • Appreciated that structural adaptations of the GIT wall plan reflect differences in function between regions

  • Investigated how variations in wall structure from the generic plan influence the functions of the large intestine

  • Identified the common features shared by the appendix & colon & linked them with their functions

  • Identified & linked the distinguishing features of the appendix & colon with their functions

  • Distinguished between all major regions of the GIT according to their structural and functional characteristics

Regional Specialisations - Appendix & Colon

  • Features

Key concept

  • Structural adaptations/variations from the generic wall plan of the GIT reflect regional differences in digestive functions

  • Most structural variations are associated with the mucosa & muscularis externa wall layers

Learning Objectives

  • Appreciate that structural adaptations of the GIT wall plan reflect differences in function between regions

  • Investigate how variations in wall structure from the generic plan influence the functions of the large intestine

  • Identify the common features shared by the appendix & colon & link them with their functions

  • Identify & link the distinguishing features of the appendix & colon with their functions

  • Distinguish between all major regions of the GIT according to their structural and functional characteristics

Large Intestine: Anatomical Regions

  • Caecum

  • Appendix

  • Colon

  • Rectum

  • Anal canal

Large Intestine: Main Functions

  • Absorption (large surface area, enterocytes)

  • Lubrication of lining (abundant goblet cells)

  • Movement & Elimination (modifications to muscularis externa)

  • Modifies fluid content of faeces, i.e. absorbs water & salts

  • Small intestine fluid suspension

  • Large intestine faeces (solid, indigestible wastes)

Large Intestine: Key Histological Features

  • Mucosa lined by simple columnar epithelium*

  • Abundant Goblet cells in mucosal lining (> enterocytes)

  • Fewer submucosal folds and no villi → less surface area than small intestine

  • Intestinal crypts (glands sunken into lamina propria) produce mucus only

  • Modification of muscularis externa – longitudinal muscle fibres of colon form taeniae coli producing haustra (pouches)

  • Transverse & sigmoid colon have a serosa with epiploic appendages** (suspended fatty sacs); remainder of colon is retroperitoneal

    • Anal canal lined by stratified squamous epithelium

Variation from Generic Plan

  • Abundant lymphoid tissue is part of MALT; contributes to overall defence & immune functions of intestinal mucosa (high bacterial load)

  • Epithelial cells lining crypts produce mucus & continue to do so even if opening of appendix into caecum is obstructed

Appendix

  • Has Lymphoid Tissue lymphoid nodules extend into submucosa

  • Numerous, diffuse lymphocytes in lamina propria, deeper nodules

  • Appendix is a narrow, blind-ending tube →increased risk of lodgement of luminal contents & infection from bacteria →abundant MALT in lamina propria & submucosa for defence

Colon Histology

  • Variation from Generic Plan

  • No villi, only crypts

  • Modified muscularis externa

  • Section not cut through taeniae coli; - longitudinal muscle layer

Histology of Colonic Mucosa

  • Features align with function of colon: reabsorption of electrolytes & water; elimination of undigested food & waste

  • No villi, just crypts (“flat” luminal surface)

  • Fewer enterocytes & EE cells but abundant goblet cells

Colonic Mucosa Note

  • Only crypts (no villi) → reduced SA cf. small intestine

  • Many goblet cells → less liquid luminal environment, more lubrication needed

  • MALT → threat of invasion (but < MALT ileum as less SA)

Mucus

  • From abundant goblet cells in colon provides lubrication & protects the mucosa from abrasion caused by hardening faecal mass

Colon – Modifications of Muscularis Externa

  • Outer longitudinal muscle sub-layer of colon arranged in 3 thickened bands (taeniae coli), producing sac-like bulges (haustra coli) along length

  • Intervening muscle between taeniae of longitudinal layer is very thin

  • Haustra created due to high muscle tone of taeniae coli → ME provides main propulsive force for movement of faeces through lumen

Taeniae Coli

  • Involve the outer longitudinally running sub-layer of muscularis externa

  • Are 3 “thickened” bands of the longitudinal smooth muscle

  • Intervening muscle of outer sub- layer is very thin

  • Do the images show TS or LS views of the colon?

Distinguishing Characteristics of GIT Regions

  • Each GIT region has a histological wall feature or set of features that together distinguishes it from other regions (see table)

  • Features relate predominantly to the mucosa and/or muscularis externa

  • Application to histological images of your knowledge & understanding of the components of the GIT wall & their organisation & function is examined in the Slide Analysis Assessment Task

  • Practice your skills using the Slide Analysis Formative Learning – Digestive Tract Quiz in MyUni.

GIT Region Distinguishing Histological Characteristic/s

  • Oesophagus Stratified squamous nonkeratinized epithelium

  • Body & fundus of stomach 3 indistinct sub-layers to muscularis externa + mucosa with short, wide pits & long glands containing parietal & chief cells

  • Pylorus of stomach Very thick circularly oriented sub-layer to muscularis externa (sphincter) + mucosa with long mucus secreting pits & short glands (few parietal cells)

  • Duodenum Villi & crypts + submucosal mucous secreting (Brunner’s) glands

  • Jejunum Villi & crypts (no submucosal glands or Peyer’s patches)

  • Ileum Villi & crypts + Peyer’s patches (strings of lymph nodules)

  • Appendix Crypts with abundant goblet cells, no villi + abundant MALT (diffuse lymphocytes, nodules) extending into submucosa

  • Colon Crypts with abundant goblet cell but no villi; taeniae coli

Learning Summary – Take Home Message

  • The mucosa of both the colon & appendix is characterised by pits/crypts, but no villi & is adapted for water absorption.

  • Abundant MALT that extends into the submucosa is present in the appendix and reflects a high risk of infection due to a high bacterial load.

  • Abundant goblet cells provide lubricating mucus to assist with movement of increasing solid faeces through the colon.

  • Taenia coli are confined to the muscularis externa of the colon.

  • Each major region of the GIT has distinguishing histological characteristics of its wall that reflect its specific functions & are useful for identifying the region.

Shared SI Features & Regional Specialisations of Duodenum & Ileum

  • Dr Eleanor Peirce Anatomy & Pathology, School of Biomedicine Room N240, Helen Mayo North Phone: 831 35191 Email: eleanor.peirce@adelaide.edu.au

Gastrointestinal Tract

  • Key Concept Structural adaptations from the generic wall plan of the GIT reflect regional differences in digestive functions.
    Most variations in wall structure occur within the mucosa & muscularis externa.

Learning Objectives

  • Appreciate that structural adaptations of the GIT wall plan reflect differences in function between regions
    Indentify shared structural characteristics of all regions of the small intestine (SI) & correlate them with their functions
    Investigate how regional variations of luminal environment & function influence wall structure of the duodenum & ileum
    Identify & link the distinguishing features of the duodenum & ileum with their functions
    Distinguish between regions of the GIT according to their structural and functional characteristics

Recap: Generic Wall Plan of GIT

  • Based on the jejunum (J) = middle portion of SI

  • Structural features reflect SI functions, i.e. completion of digestion, nutrient absorption & exocrine & endocrine secretion
    However:

  • Structural adaptations of the mucosa and submucosa in the
    Duodenum (D) = initial 20cm of SI
    Ileum (I) = distal 2/5 of SI
    1 Mucosa (purple) 2 Submucosa (light pink) 3 Muscularis externa (brighter pink) 4 Serosa

Primary Function of SI is Absorption

  • How is absorption achieved?
    By having a large luminal surface area : volume ratio achieved via
    Submucosal circular folds ( = plicae circulares) overlaid by
    Villi = finger-like extensions of mucosa
    Microvilli = bristle-like extensions of apical plasma membranes of enterocytes
    Long length of the small intestine (>6 m) plicae circulares

Shared Features of SI Regions for Absorption

  • Plicae circulares Permanent folds that have a core of submucosa
    Cannot stretch out/flatten like rugae in the stomach Not found in lower ileum Up to 10mm tall
    Villi Finger-like projections of the mucosa Have a core of lamina propria containing blood capillaries & lacteals covered by simple columnar epithelium of enterocytes & goblet cells 1mm tall
    Microvilli Extensions of enterocyte apical plasma membrane Form a brush border that houses enzymes for final stages of digestion 1 um (0.001mm) tall 3 levels of folding create 600x more surface area than that of a smooth cylinder

Intestinal Villi Facilitate Nutrient Transport

  • Simple columnar epithelium
    Enterocytes enable absorption
    Goblet cells secrete mucus
    Lamina propria has abundant AGS for diffusion to peripheral capillaries
    Centrally located lacteal

Feature

  • All epithelial cells arise from stem cells located at the bases of crypts Enterocytes & goblet cells differentiate as they move up crypts to tip of villi where they undergo apoptosis & enter the lumen Crypts also contain Paneth cells (P) Enteroendocrine cells 4-8 day cell turnover from crypt → tip of villus Stem cells & enteroendocrine cells not distinguishable with H&E staining Cell Types in the Intestinal Epithelium P

Functions of Goblet Cells & Enterocytes

  • Goblet cells secrete mucus that: coats all lining cells & protects them from digestive enzymes
    lubricates lining (slimy) Adheres small molecules (sticky), to facilitate their absorption Goblet cell density increases from duodenum to ileum Other functions of enterocytes (besides absorption) Tight junctions form strong barrier between gut lumen & tissues of body Brush border enzymes have role in digestion

Tight Junctions (TJs) between Enterocytes

  • Seal intercellular gap via fusion of adjacent epithelial cell membranes
    Block intercellular passage → barrier between two environments
    Functional significance: Prevent: Digestive secretions in lumen destroying gut lining through seepage between lining cells Pathogen penetration through the epithelial layer Enable: Selective absorption - must occur through microvilli cytoplasm of cells (not via intercellular space) Absorption against concentration gradient epithelial layer lumen of gut intercellular environment

Enterocytes – Role in Digestion in SI

  • Enterocytes release enterokinase that activates


To provide a more in-depth look at the histology of the gastrointestinal tract (GIT) and link what you would observe on a micrograph to the structure and function of each component, consider the following expanded details:

1. Mucosa:

  • Epithelium:

    • Micrograph Observation: In the esophagus, you would see a stratified squamous epithelium characterized by multiple layers of cells. From the stomach to the rectum, a simple columnar epithelium is present, featuring a single layer of elongated cells. Within the small and large intestines, you'll notice specialized cells like enterocytes (absorptive cells) and goblet cells (mucus-secreting).

    • Structure-Function Correlation:

      • Stratified Squamous Epithelium: Provides protection against abrasion in the esophagus.

      • Simple Columnar Epithelium: Facilitates absorption and secretion due to its single-layered structure and specialized cells.

      • Enterocytes: Columnar cells with microvilli (visible as a brush border on the apical surface) to increase surface area for absorption.

      • Goblet Cells: Clear, goblet-shaped cells interspersed among enterocytes, responsible for secreting mucus to lubricate and protect the epithelium.

  • Lamina Propria:

    • Micrograph Observation: This layer appears as loose connective tissue filled with blood vessels, lymphatic vessels (including lacteals in the small intestine), immune cells (lymphocytes, plasma cells, macrophages), and occasional glands (intestinal crypts).

    • Structure-Function Correlation:

      • Loose Connective Tissue: Supports the epithelium while allowing for nutrient and waste exchange.

      • Blood and Lymphatic Vessels: Supply nutrients and remove waste products; lacteals specifically absorb fats.

      • Immune Cells: Provide immune surveillance and defense against pathogens.

      • Intestinal Crypts: Invaginations of the epithelium that contain stem cells for cell renewal and specialized secretory cells.

  • Muscularis Mucosae:

    • Micrograph Observation: A thin layer of smooth muscle located beneath the lamina propria, often appearing as a dark band.

    • Structure-Function Correlation:

      • Smooth Muscle Contraction: Causes local movements of the mucosa, enhancing contact between the epithelium and the contents in the lumen, aiding absorption and secretion.

2. Submucosa:

  • Micrograph Observation: This layer is characterized by dense irregular connective tissue containing larger blood vessels, lymphatic vessels, nerve fibers, and potentially glands (e.g., Brunner's glands in the duodenum).

  • Structure-Function Correlation:

    • Dense Irregular Connective Tissue: Provides structural support and connects the mucosa to the muscularis externa.

    • Blood and Lymphatic Vessels: Supply the mucosa and muscularis externa with nutrients and remove waste products.

    • Nerve Fibers: Part of the submucosal (Meissner’s) plexus, which regulates glandular secretion and mucosal blood flow.

    • Brunner's Glands: Secrete alkaline mucus to neutralize acidic chyme entering from the stomach (duodenum specific).

3. Muscularis Externa:

  • Micrograph Observation: Typically composed of two sublayers of smooth muscle: an inner circular layer and an outer longitudinal layer. The myenteric (Auerbach's) plexus is often visible between these layers as clusters of nerve cell bodies.

  • Structure-Function Correlation:

    • Inner Circular Layer: Contraction decreases the diameter of the lumen.

    • Outer Longitudinal Layer: Contraction shortens the length of the GIT segment.

    • Myenteric Plexus: Coordinates peristalsis and segmentation movements.

4. Serosa/Adventitia:

  • Micrograph Observation:

    • Serosa: A thin layer of loose connective tissue covered by a mesothelium, which is a simple squamous epithelium.

    • Adventitia: Dense connective tissue that blends into surrounding tissues without a mesothelial covering.

  • Structure-Function Correlation:

    • Serosa: Reduces friction between the GIT and adjacent organs, allows for movement within the abdominal cavity.

    • Adventitia: Anchors the GIT to surrounding structures (e.g., esophagus).

5. Regional Specializations:

  • Duodenum:

    • Micrograph Observation: Identify Brunner’s glands in the submucosa, which appear as clusters of mucus-secreting cells with pale-staining cytoplasm.

    • Structure-Function Correlation: These glands secrete alkaline mucus to neutralize acidic chyme and protect the duodenal lining.

  • Ileum:

    • Micrograph Observation: Look for Peyer’s patches (lymphoid nodules) in the lamina propria and extending into the submucosa. They appear as dense aggregates of lymphocytes.

    • Structure-Function Correlation: Peyer’s patches are part of the gut-associated lymphoid tissue (GALT) and are critical for immune surveillance and response.

  • Appendix and Colon:

    • Micrograph Observation: Note the absence of villi and the presence of abundant goblet cells and crypts in the mucosa. In the colon, identify taeniae coli as thickened bands in the muscularis externa.

    • Structure-Function Correlation: The absence of villi indicates reduced absorptive capacity compared to the small intestine. Abundant goblet cells secrete mucus to lubricate the passage of feces. Taeniae coli create haustra (sacculations) that aid in the mixing and propulsion of colonic contents.

By carefully examining these histological features on a micrograph, you can correlate the structure of each component with its specific function within the GIT, enhancing your understanding of its physiological processes.