Digestive tract Digestive tract Li Zhong Jie 李李李李 () , Ph. D School of Medicine,Zhejiang University
Jan 03, 2016
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!