Digestive tractDigestive tract

Li Zhong Jie（李仲杰） , Ph. D

School of Medicine,Zhejiang University

---Digestive system: ---Digestive system: • Digestive tract Digestive tract • Digestive glandDigestive gland

---Digestive system function:---Digestive system function: This system is responsible for the This system is responsible for the

mechanical and chemical breakdown of mechanical and chemical breakdown of food material, and for absorbing these food material, and for absorbing these digestive products into the blood for use as digestive products into the blood for use as nutrients by the individual cells and nutrients by the individual cells and tissues of the bodytissues of the body

The Digestive The Digestive ProcessProcess• IngestionIngestion

– Taking in food through the mouthTaking in food through the mouth• PropulsionPropulsion (movement of food) (movement of food)

– SwallowingSwallowing– Peristalsis – propulsion by alternate Peristalsis – propulsion by alternate

contraction &relaxationcontraction &relaxation• Mechanical digestionMechanical digestion

– ChewingChewing– Churning in stomachChurning in stomach– Mixing by segmentationMixing by segmentation

• Chemical digestionChemical digestion– Carbohydrates, Fat, and Proteins are Carbohydrates, Fat, and Proteins are

broken down by enzymes.broken down by enzymes.• AbsorptionAbsorption

– Transport of digested end products Transport of digested end products into blood and lymph in wall of canal into blood and lymph in wall of canal

• DefecationDefecation– Elimination of indigestible substances Elimination of indigestible substances

from body as feces from body as feces

• Chemical digestionChemical digestion– Complex food molecules Complex food molecules

(carbohydrates, proteins and lipids) (carbohydrates, proteins and lipids) broken down into chemical building broken down into chemical building blocks (simple sugars, amino acids, and blocks (simple sugars, amino acids, and fatty acids and glycerol)fatty acids and glycerol)

– Carried out by enzymes secreted by Carried out by enzymes secreted by digestive glands into lumen of the digestive glands into lumen of the alimentary canalalimentary canal

Components of digestive tractComponents of digestive tract

---oral cavity ---oral cavity

---pharynx---pharynx

---esophagus---esophagus

---stomach---stomach

---small intestine---small intestine

---large intestine---large intestine

---rectum and anus---rectum and anus

General plan of digestive tractGeneral plan of digestive tract---Except for oral cavity and pharynx, all other organs share ---Except for oral cavity and pharynx, all other organs share

a similar histological plana similar histological plan

1.1. MucosaMucosa

2.2. SubmucoSubmucosasa

3.3. MusculariMuscularis externas externa

4.4. AdventitiaAdventitia

from lumen (inside) from lumen (inside) outout

1.1. MucosaMucosa

2.2. SubmucoSubmucosasa

3.3. MusculariMuscularis externas externa

4.4. AdventitiaAdventitia

from lumen (inside) from lumen (inside) outout

General plan of digestive tract

Inner layer: Inner layer: the mucosathe mucosa** (mucous membrane)(mucous membrane)

Three sub-layersThree sub-layers– Lining epitheliumLining epithelium

– Lamina propria (may Lamina propria (may contain glands)contain glands)

– Muscularis mucosae Muscularis mucosae (Smooth muscle)(Smooth muscle)

*

More about the mucosaMore about the mucosa

• Epithelium: absorbs nutrients, secretes mucusEpithelium: absorbs nutrients, secretes mucus– Continuous with ducts and secretory cells of Continuous with ducts and secretory cells of

intrinsic digestive glands (those within the wall)intrinsic digestive glands (those within the wall)– Extrinsic (accessory) glands: the larger ones such Extrinsic (accessory) glands: the larger ones such

as liver and pancreasas liver and pancreas

• Lamina propriaLamina propria– Loose connective tissue with nourishing and Loose connective tissue with nourishing and

absorbing capillariesabsorbing capillaries– Contains most of mucosa-associated lymphoid Contains most of mucosa-associated lymphoid

tissue (MALT)tissue (MALT)

• Muscularis mucosaeMuscularis mucosae– Thin layer of muscle producing only local Thin layer of muscle producing only local

movementsmovements

the the submucosasubmucosa**

*

– Loose C.T. Loose C.T. containing containing major blood and major blood and lymphatic vessels , glands lymphatic vessels , glands and nerves ,and nerves , many elastic many elastic fibers so gut can regain fibers so gut can regain shape after food passesshape after food passes

– Meissner’s autonomic Meissner’s autonomic nerve plexusnerve plexus

the muscularis externa*the muscularis externa*Two layers of smooth muscle Two layers of smooth muscle

responsible for peristalsis responsible for peristalsis and segmentationand segmentation

• Inner circular layer Inner circular layer (circumferential)(circumferential)

– SqueezesSqueezes– In some places forms In some places forms

sphincterssphincters (act as valves) (act as valves)

• Myenteric (Auerbach’s) Myenteric (Auerbach’s) autonomic nerve plexusautonomic nerve plexus

• Outer longitudinal layer: Outer longitudinal layer: shortens gutshortens gut

*

Smooth muscle

•Muscles are spindle-shaped cells•One central nucleus•Grouped into sheets: often running perpendicular to each other•Peristalsis•No striations (no sarcomeres)•Contractions are slow, sustained and resistant to fatigue•Does not always require a nervous signal: can be stimulated by stretching or hormones

• Tunica adventitiaTunica adventitia– Fibrosa or serosa (covered Fibrosa or serosa (covered

by mesothelium)by mesothelium)

NervesNerves• Enteric nervous system: the gut’s ownEnteric nervous system: the gut’s own

– Visceral plexuses within gut wall controlling the Visceral plexuses within gut wall controlling the muscles, glands and having sensory infomuscles, glands and having sensory info• Muscularis external: Muscularis external: Myenteric (Auerbach’s) plexusMyenteric (Auerbach’s) plexus

• Submucosal: Submucosal: Meissner’s autonomic nerve plexusMeissner’s autonomic nerve plexus

– 100 million neurons! (as many as the spinal cord)100 million neurons! (as many as the spinal cord)

• Autonomic input: speeds or slows the systemAutonomic input: speeds or slows the system– ParasympatheticParasympathetic

• Stimulates digestive functionsStimulates digestive functions

– SympatheticSympathetic• Inhibits digestionInhibits digestion

• Largely automaticLargely automatic

outer

longitudin

al

Nerve plexus

inner circular

* Myenteric (Auerbach’s) plexus : : regulate the movement of SMregulate the movement of SM

EsophagusEsophagus

mucosa submucosa

muscularis

adventitia

EsophagusEsophagus Passage way for food from the pharynx to the stomachPassage way for food from the pharynx to the stomachmucosa:mucosa: • epithelium: stratified squamous epitheliumepithelium: stratified squamous epithelium• lamina propria: compact CT contain simple tubular glandslamina propria: compact CT contain simple tubular glands• muscularis mucosa: longitudinal muscularis mucosa: longitudinal arranged smooth musclearranged smooth muscle• At GE junction – thin simple columnarAt GE junction – thin simple columnar epitheliumepithelium

submucosa:submucosa: • LCTLCT• esophageal gland: mucous glandesophageal gland: mucous gland ---acidic mucin---acidic mucin

Muscularis externa:Muscularis externa: • inner circular and outer longitudinalinner circular and outer longitudinal• upper 1/3: skeletal muscleupper 1/3: skeletal muscle• middle 1/3: mixed of skeletal muscle and smooth middle 1/3: mixed of skeletal muscle and smooth

musclemuscle• lower 1/3: smooth musclelower 1/3: smooth muscleTunica adventitia:Tunica adventitia: a fibrous coat of loose connective tissuea fibrous coat of loose connective tissue

When empty, mucosa and submucosa lie in longitudinal folds

StomachStomach

---widest part of alimentary canal---widest part of alimentary canal---Temporary storage and mixing – 4 hours---Temporary storage and mixing – 4 hours---digest food partially to form a semi-fluid ---digest food partially to form a semi-fluid

mass, termed chymemass, termed chyme---absorb part of water and ions---absorb part of water and ions

– Water, electrolytes, some drugs like aspirin and Water, electrolytes, some drugs like aspirin and alcohol (absorbed through stomach)alcohol (absorbed through stomach)

Stomach (regions)Stomach (regions)

• Cardia (Cardiac Cardia (Cardiac junction)junction)– Surrounds esophageal Surrounds esophageal

entranceentrance

• Fundic stomach defined Fundic stomach defined histologically includeshistologically includes– FundusFundus

– BodyBody

• Pylorus (Pyloric junction)Pylorus (Pyloric junction)– Pylorus is continuous with Pylorus is continuous with

the duodenumthe duodenum

• RugaeRugae: : longitudinal longitudinal folds on internal surface folds on internal surface (helps distensibility)(helps distensibility)

• MuscularisMuscularis: : additional additional innermost oblique layer innermost oblique layer (along with circular and (along with circular and longitudinal layers)longitudinal layers)

• MucosaMucosa– Epithelium (simple columnar mucus-secreting)Epithelium (simple columnar mucus-secreting)

– Lamina propria (gastric glands of different types)Lamina propria (gastric glands of different types)

– Muscularis mucosae (Smooth muscle)Muscularis mucosae (Smooth muscle)

• SubmucosaSubmucosa– Loose C.T. no glandsLoose C.T. no glands

• Muscularis externaMuscularis externa inner oblique, middle circular, outer longitudinalinner oblique, middle circular, outer longitudinal

• Tunica adventitiaTunica adventitia– Mostly serosaMostly serosa

Stomach Histology OverviewStomach Histology Overview

mucosamucosa• RugaeRugae

– Longitudinal folds of Longitudinal folds of mucosamucosa

• A mucosal fold contains A mucosal fold contains submucosasubmucosa

• Gastric pits: small Gastric pits: small depressions, 3-5 gastric gland depressions, 3-5 gastric gland open into the bottomopen into the bottom

• Diffuse lymphoid tissue and Diffuse lymphoid tissue and nodules may be presentnodules may be present

Gastric pitGastric pit

Rugae in the stomachRugae in the stomach

Mucosa

Muscularis mucosa

Submucosa

Muscularis externa

Rugae

Cross section of gastric pitsCross section of gastric pits

Simple columnar epitheliumGastric pit

Laminia propria between pits

①①epitheliumepithelium: : simple simple columnar epitheliumcolumnar epithelium

• surface mucous cell: surface mucous cell: -tall columnar -tall columnar -ovoid, basally-located nuclei-ovoid, basally-located nuclei-apical mucin granule-apical mucin granule-tight junction-tight junction

The mucus(The mucus( bicarbonate-bicarbonate-buffered mucus) is secreted on buffered mucus) is secreted on to the epithelial surface to to the epithelial surface to form a form a barrier layerbarrier layer which which protects it from injury by protects it from injury by ingested substance and the ingested substance and the stomach’s own secretion of stomach’s own secretion of acid and enzymes.acid and enzymes.

②②lamina proprialamina propria: :

CT contains fibroblast, LC, CT contains fibroblast, LC, plasma cell, mast cell and plasma cell, mast cell and eosinophil, smooth muscleeosinophil, smooth muscle

• gastric gland (fundic gland)-gastric gland (fundic gland)-oxyntic glandoxyntic gland

• cardiac gland: mucous glandcardiac gland: mucous gland

• pyloric gland: mucous glandpyloric gland: mucous gland

* * Fundic glandFundic gland --long, straight, branched or long, straight, branched or

unbranched glandunbranched gland

Three part of gland:Three part of gland:

The neckThe neck

The bodyThe body

The baseThe base

Five type cells are Five type cells are foundfound::

Chief cellsChief cells

Parietal cellsParietal cells

Mucous neck cellsMucous neck cells

Stem cellsStem cells

Enterendocrine cellsEnterendocrine cells

Fundic gland

chief cellchief cell or zymogenic cell or zymogenic cell

---structure: ---structure: LM: LM:

• columnar columnar • Round, basally-located NucleiRound, basally-located Nuclei• cytoplasm: cytoplasm: /basal-basophilic/basal-basophilic /apical-zymogen granules/apical-zymogen granules

EM:

---RER, Golgi apparatus

---function: secret pepsinogen(the precursor of pepsin)

parietal cellparietal cell or oxyntic cell or oxyntic cell

---structure:---structure:LM: LM: • large, pyramidal or sphericallarge, pyramidal or spherical

• round centrally-located nucleiround centrally-located nuclei

• eosinophilic cytoplasmeosinophilic cytoplasm

EM:

• intracellular secretory canaliculus -invaginations

• tubulovesicular system

• mitochondria

Active secreting cell

Resting cell

---function: ---function:

i.i.. secret hydrochloric acid (HCl) . secret hydrochloric acid (HCl) synthesis processes of HCl: synthesis processes of HCl:

in intracellular secretory canaliculusin intracellular secretory canaliculus

• HH++ K K++ -ATP pump: get H -ATP pump: get H++ from cell from cell

• ClCl-- channel: get Cl channel: get Cl-- from blood from blood

• HH++ +Cl +Cl--→HCl→HCl

function of HCl:function of HCl:

• pepsinogen→pepsinpepsinogen→pepsin

• kill the bacteriakill the bacteria

ii.ii.    secret intrinsic factor:secret intrinsic factor:

glycoprotein + Vitamin-Bglycoprotein + Vitamin-B1212→absorption →absorption of of

VBVB12 12 in ileumin ileum

result in Addison’s anaemia

an deficiency of VB12

mucous neck cellmucous neck cell• less, neck partless, neck part• pale stain in HE stainpale stain in HE stain• secrete mucussecrete mucus

stem cellstem cell undifferentiated cellundifferentiated cell

enterendocrine cellenterendocrine cell • L cell: secreting histamine, L cell: secreting histamine,

promote secretion of parietal cellpromote secretion of parietal cell• D cell: secreting somatostatin, D cell: secreting somatostatin,

inhibit the secretion of parietal inhibit the secretion of parietal cellcell

* * Mucous-HCOMucous-HCO33 barrier barrier

structure :structure :

• mostly of glycoproteins rich in mostly of glycoproteins rich in carohydrates and bicarbonate ionscarohydrates and bicarbonate ions

• 0.25-0.5mm thick mucous0.25-0.5mm thick mucous

• The pH of the surface is usually 0.9-The pH of the surface is usually 0.9-1.51.5

* * Mucous-HCOMucous-HCO33 barrier function :barrier function :

• The mucus( bicarbonate-buffered mucus) is secreted on to The mucus( bicarbonate-buffered mucus) is secreted on to the epithelial surface to form a barrier layer which protects it the epithelial surface to form a barrier layer which protects it from injury by ingested substance and the stomach’s own from injury by ingested substance and the stomach’s own secretion of acid and enzymes. secretion of acid and enzymes.

• HCl HCl PPH: 0.9-1.5H: 0.9-1.5

HH++ + HCO + HCO33-- → H → H22COCO33 →H →H22O + COO + CO22

↑ ↑

bicarbonatasebicarbonatase

• acid-base neutralizationacid-base neutralization

• Epithelial transitionEpithelial transition

---Stratified Squamous ---Stratified Squamous nonkeratinized to simple nonkeratinized to simple columnarcolumnar

• It is clinically important, It is clinically important, as it is the most site of as it is the most site of esophageal carcinoma. esophageal carcinoma.

Cardiac JunctionCardiac Junction

Small intestineSmall intestine

Duodenum – first region, only about 25cm long, Duodenum – first region, only about 25cm long,

Jejunum – second region is roughly 2.5m longJejunum – second region is roughly 2.5m long

Ileum – last region is roughly 3.5m longIleum – last region is roughly 3.5m long

Primary functionsPrimary functions• Transport food from stomach to Large intestineTransport food from stomach to Large intestine• Secretion of digestive enzymes to facilitate digestion of food Secretion of digestive enzymes to facilitate digestion of food

substancessubstances• Absorption of food substances into blood and lymph vesselsAbsorption of food substances into blood and lymph vessels• Secretion of certain hormonesSecretion of certain hormones

Small Intestine OverviewSmall Intestine Overview

• MucosaMucosa– Epithelium (simple columnar)Epithelium (simple columnar)

– Lamina propria (intestinal glands)Lamina propria (intestinal glands)

– Muscularis mucosae (Smooth muscle)Muscularis mucosae (Smooth muscle)

• SubmucosaSubmucosa– loose C.T. (contain duodenal glands in the duodenum)loose C.T. (contain duodenal glands in the duodenum)

• Muscularis externaMuscularis externa inner circular, outer longitudinalinner circular, outer longitudinal

• Tunica adventitiaTunica adventitia– serosa (except for the duodenum)serosa (except for the duodenum)

4444

Lymph NodulesLymph Nodules

Special structure of mucosaSpecial structure of mucosa

• Plicae circulares Plicae circulares – Mucosa and submucosa are arranged in permanent, Mucosa and submucosa are arranged in permanent,

circular mucosal foldscircular mucosal folds

• Intestinal villiIntestinal villi– Mucosal projections covered by epithelium and Mucosal projections covered by epithelium and

containing only lamina propriacontaining only lamina propria

• Crypt or intestinal glandsCrypt or intestinal glands– Surrounded by lamina propriaSurrounded by lamina propria

– Extend to the muscularis mucosaeExtend to the muscularis mucosae

Plicae circularesPlicae circulares villivilli

Plicae circularesPlicae circulares

VilliVilli

• EpitheliumEpithelium (Simple columnar) (Simple columnar)– Absorptive cellsAbsorptive cells

• Numerous, regular microvilli form striated-borderNumerous, regular microvilli form striated-border

• Well formed junctional complexWell formed junctional complex

Plicae circulares, villi and microvilli are serve to increase the surface Plicae circulares, villi and microvilli are serve to increase the surface area of the small intestine by as much as 600-foldarea of the small intestine by as much as 600-fold

surface coat: a layer of glycoprotein film, protect the underlying cells surface coat: a layer of glycoprotein film, protect the underlying cells from mucolytic and proteolytic agentfrom mucolytic and proteolytic agent

– Goblet cellsGoblet cells :secrete mucus to lubricate and protect the :secrete mucus to lubricate and protect the epitheliumepithelium

– Enteroendocrine cells: Enteroendocrine cells: produce hormonesproduce hormones

striated-striated-borderborder

microvillimicrovilli

lamina propria:lamina propria:

LCT contains network of LCT contains network of blood and lymph capillariesblood and lymph capillaries

-Carbs and proteins into -Carbs and proteins into blood to liver via hepatic blood to liver via hepatic portal veinportal vein

-Fat into lymph-Fat into lymphcentral lacteal:central lacteal: lymphatic vessel, lymphatic vessel, absorb fatabsorb fat

Crypt or small intestinal glandCrypt or small intestinal gland:: the invagination of epithelium into lamina propria the invagination of epithelium into lamina propria

• absorptive cellabsorptive cell

• goblet cellgoblet cell

• endocrine cellendocrine cell

• stem cellstem cell

• Peneth cellPeneth cell

Peneth cellPeneth cell::LM: LM:

---pyramidal in shape, locate in ---pyramidal in shape, locate in basal portion of the glands, in basal portion of the glands, in groupsgroups

---apical: acidophilic granules- ---apical: acidophilic granules- contain defensin (cryptdin), contain defensin (cryptdin), LysozymeLysozyme

EM: EM:

-protein-secreting cell feature-protein-secreting cell feature

Function: related to immune Function: related to immune function, anti-bacterial activityfunction, anti-bacterial activity

Large intestineLarge intestine

---Consists of:---Consists of:

-cecum-cecum

-ascending colon-ascending colon

-transverse colon-transverse colon

-descending colon-descending colon

-sigmoid colon-sigmoid colon

-rectum-rectum

-anal canal-anal canal

--- function:--- function:

absorb water and ions absorb water and ions

• MucosaMucosa– No villi or plicae circularesNo villi or plicae circulares

– Glands are longer than in small Glands are longer than in small intestineintestine

– Single columnar epithelium Single columnar epithelium contain numerous goblet cells contain numerous goblet cells but absorptive cells are still but absorptive cells are still presentpresent

– Occasional solitary lymph Occasional solitary lymph nodulesnodules

– At anal junction there is an At anal junction there is an abrupt transition to stratified abrupt transition to stratified squamous non-keratinized squamous non-keratinized epithelium. epithelium.

• SubmucosaSubmucosa– Similar to small intestine Similar to small intestine

except nerve plexus are except nerve plexus are more easily found heremore easily found here

• Muscularis externaMuscularis externa– Inner circular layer is Inner circular layer is

evidentevident

– Outer longitudinal layer Outer longitudinal layer forms three bands, the forms three bands, the taenia colitaenia coli

• Adventitia Adventitia – Both serosa and fibrosa Both serosa and fibrosa

are foundare found

• Mucosa is like the colon exceptMucosa is like the colon except– Numerous lymph nodules are present in the youngNumerous lymph nodules are present in the young

– These decrease with ageThese decrease with age

– They break up the muscularis mucosae so that it is They break up the muscularis mucosae so that it is difficult to finddifficult to find

– The glands are also often not very evident.The glands are also often not very evident.

• Submucosa and muscularis externa are like the Submucosa and muscularis externa are like the rest of the colonrest of the colon

Appendix Appendix

AppendixAppendix

Review and exerciseReview and exercise

1. Compare the mucosal structure of stomach with that of esophagus, small intestine and large intestine.2. Describe the fine structure and ultra-structure of the parietal cell and chief cell in the gastric gland.3. Describe the different formation of villi and plicae in small intestine.4. Describe the structure of mucosa of the digestive tract.

Thanks for your Thanks for your attention!attention!