Lecture 22 - The Digestive Tract (Histology)

7/27/2019 Lecture 22 - The Digestive Tract (Histology)

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The Digestive Tract


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The GI tract(gastrointestinal tract)

The muscular alimentary canal

Mouth

Pharynx

Esophagus

Stomach

Small intestine

Large intestine Anus

The accessorydigestive organs

Supply secretions contributingto the breakdown of food Teeth & tongue

Salivary glands

Gallbladder

Liver

Pancreas


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The Digestive Process

Ingestion Taking in food through the mouth

Propulsion (movement of food) Swallowing

Peristalsis propulsion by alternatecontraction &relaxation

Mechanical digestion Chewing

Churning in stomach

Mixing by segmentation

Chemical digestion By secreted enzymes: see later

Absorption Transport of digested end products into

blood and lymph in wall of canal

Defecation Elimination of indigestible substances from

body as feces


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Chemical digestion

Complex food molecules (carbohydrates,

proteins and lipids) broken down into

chemical building blocks (simple sugars,amino acids, and fatty acids and glycerol)

Carried out by enzymes secreted by digestive

glands into lumen of the alimentary canal


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Ways to divide.

The more common

Plus:

epigastric

periumbilical

suprapubic

flank


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Histology of alimentary canal wall

Same four layers from esophagus to anal canal

1. Mucosa

2. Submucosa

3. Muscularis

externa

4. Serosa

from lumen (inside) out


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Inner layer: the mucosa*(mucous membrane)

Three sub-layers

1. Lining epithelium

2. Lamina propria

3. Muscularis

mucosae

*


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More about the mucosa

Epithelium: absorbs nutrients, secretes mucus Continuous with ducts and secretory cells of intrinsic

digestive glands (those within the wall)

Extrinsic (accessory) glands: the larger ones such as

liver and pancreas

Lamina propria

Loose connective tissue with nourishing and

absorbing capillaries

Contains most of mucosa-associated lymphoid tissue

(MALT)

Muscularis mucosae

Thin layer of muscle producing only local movements


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Second layer: the submucosa*

Connective tissuecontaining majorblood and

lymphatic vesselsand nerves

Many elastic fibersso gut can regainshape after foodpasses

*


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Next in, the muscularis externa*(AKA just muscularis)

Two layers of smoothmuscle responsiblefor peristalsis andsegmentation

Inner circular layer(circumferential) Squeezes

In some places formssphincters (act asvalves)

Outer longitudinallayer: shortens gut

*


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Last (outer), the serosa*(the visceral peritoneum)

Simple squamousepithelium(mesothelium) Thin layer of areolar

connective tissueunderneath

Exceptions: Parts not in peritoneal

cavity have adventitia,

lack serosa Some have both, e.g.

retroperitoneal organs

*


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Smooth muscleSmooth muscle

Muscles are spindle-shaped cellsOne 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 hormones6 major locations:1. inside the eye 2. walls of vessels 3. respiratory tubes

4. digestive tubes 5. urinary organs 6. reproductive organs


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Nerves

Enteric nervous system: the guts own Visceral plexuses within gut wall controlling the

muscles, glands and having sensory info

Myenteric: in muscularis

Submucosal

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

Autonomic input: speeds or slows the system

Parasympathetic

Stimulates digestive functions

Sympathetic

Inhibits digestion

Largely automatic


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Review of some definitions.

Peritoneum: serous membranes of the

abdominopelvic cavity

Visceral peritoneum: covers external

surfaces of most digestive organs

Parietal peritoneum: lines body wall

Peritoneal cavity: slit-like potential spacebetween visceral and parietal peritoneum

Serous fluid lubricating


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New definitions

Mesentery Double layer of peritoneum

Extends to digestive organs from body wall

Hold organs in place

Sites of fat storage Route by which circulatory vessels and nerves reach

organs

Most are dorsal

Extend dorsally from gut to posterior abdominal wall Ventral mesentery from stomach and liver to

anterior abdominal wall

Some mesenteries are called ligaments though nottechnically such


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Mesenteries

Note dorsal, ventral and formation ofretroperitoneal position


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Mesenteries

Two ventral mesenteries

Falciform ligament

Binds anterior aspect of liver

to anterior abdominal walland diaphragm

Lesser omentum (=fatty

skin) see diagram*

All other mesenteries are

dorsal (posterior)

*


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Mesenteries continued (all these are dorsal)

Greater omentum Connects stomach to posterior abdominal wall very roundabout

Wraps around spleen: gastrosplenic ligament

Continues dorsally as splenorenal ligament

A lot of fat

Limits spread of infection by wrapping around inflamed e.g. appendix

Mesentery or mesentery proper Supports long coils of jejunum and ileum (parts of small intestine)

Transverse mesocolon Transverse colon held to posterior abdominal wall

Nearly horizontal sheet fused to underside of greater omentum

Sigmoid mesocolon Connects sigmoid colon to posterior abdominal wall

see next slides for pics


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Note mesenteries: falciform ligament, lesser

omentum, greater omentum


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Note: greater omentum, lesser omentum, falciform ligament,

transverse mesocolon, mesentery, sigmoid mesocolon


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Some organs are retroperitoneal

Are behind the peritoneum

Fused to posterior (dorsal) abdominal wall

Lack a mesentery

Include: Most of duodenum (1st part of small intestine)

Ascending colon

Descending colon

Rectum

Pancreas

Tend to cause back pain, instead of abdominal pain

(This is as opposed to the organs which are intraperitoneal,

or just peritoneal)


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Mouth = oral cavity

Lining: thick

stratified squamous

epithelium

Lips- orbicularis

oris muscle

Cheeksbuccinator muscle

The Mouth


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Vermillion border or redborder Between highly

keratinized skin of face

and mucosa of mouth Needs moisture

Note frenulums (folds ofmucosa)

Palate roof of mouth Hard plate anteriorly

Soft palate posterioly

Uvula


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Tongue Mostly muscles

Grip and reposition food

Forms bolus of food (lump)

Help in swallowing Speech help form some consonants

Note frenulum on previous slide: can be too tight

Taste buds contained by circumvallate and fungiform papillae

Lingual tonsil back of tongue


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Teeth

Called dentition (like dentist)

Teeth live in sockets (alveoli) in the gum-

covered margins of the mandible andmaxilla

Chewing: raising and lowering themandible and moving it from side to sidewhile tongue positions food between teeth


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Teeth

Two sets

Primary or deciduous Baby teeth

Start at 6 months

20 are out by about 2 years

Fall out between 2-6 years

Permanent: 32 totalAll but 3rd set of molars by

end of adolescence

3rdset = wisdom teeth Variable

Some can be impacted(imbedded in bone)


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Teeth are classified according to

shape and function

Incisors: chisel-shaped forchopping off pieces

Canines: cone shaped totear and pierce

Premolars (bicuspids) and

Molars - broad crownswith 4-5 rounded cusps forgrinding

incisor

canine

premolar

molar

Cusps are surface bumps


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Tooth structure Two main regions

A. Crown (exposed)

B. Root (in socket)

C. Meet at neck

Enamel

99% calcium crystals

Hardest substance inbody

Dentin bulk of thetooth (bone-like but

harder than bone, withcollagen and mineral)

Pulp cavity with vesselsand nerves Root canal: the part of

the pulp in the root

A

B

C


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Tooth structure

Cementum bonelayer of tooth root Attaches tooth to

periodontal ligament

Periodontal l igament

Anchors tooth in boneysocket of the jaw

Continuous with ging iva(gums)

Cavities orcaries- rot

Plaque film of sugar,bacteria and debris

A

B

C


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Extrinsic salivary glands

Parotids* - largest (think mumps)

Facial nerve branch at risk during surgery here

Submandibular # - medial surface mandible

Sublingual + - under tongue; floor of mouth

Compound = duct branches

Tubo = tubes

Alveolar = sacs

*

#

+


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Pharynx

Oropharynx andlaryngopharynx Stratified squamous

epithelium

Three constrictormuscles* Sequentially squeeze

bolus of food intoesophagus

Are skeletalmuscles

Voluntary action

Vagus nerve (X)

___oropharynx

___laryngopharynx

*

*

*


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Esophagus

Continuation of pharynx in

mid neck Muscular tube collapsed

when lumen empty

Descends through thorax On anterior surface of

vertebral column

Behind(posterior to) trachea

Esophagus___________

*


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Esophagus continued

Passes through esophageal hiatus in the diaphragm toenter the abdomen

Abdominal part only 2 cm long Joins stomach at cardiac orifice*

Cardiac sphincter at cardiac orifice to prevent regurgitation (foodcoming back up into esophagus)

Gastroesophageal junction and GERD

___________________esophageal hiatus

(hiatus means opening)

*

Mi i t f h


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Contains all 4

layers (see right)

Epithelium: nonkeratinized stratified squamous epithelium

At GE junction thin simple columnar epithelium Mucus glands in wall

Muscle (muscularis externa) changes as it goes down Superior 1/3 of esophagus: skeletal muscle (like pharynx)

Middle 1/3 mixture of skeletal and smooth muscle

Inferior 1/3 smooth muscle (as in stomach and intestines)

When empty, mucosa and submucosa lie in longitudinal folds

Microscopic anatomy of esophagus


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Esophagus histology


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Stomach J-shaped; widest part of alimentary canal

Temporary storage and mixing 4 hours Into chyme

Starts food breakdown Pepsin (protein-digesting enzyme needing acid

environment) HCl (hydrochloric acid) helps kill bacteria

Stomach tolerates high acid content but esophagusdoesnt why it hurts so much when stomachcontents refluxes into esophagus (heartburn; GERD)

Most nutrients wait until get to small intestine tobe absorbed; exceptions are: Water, electrolytes, some drugs like aspirin and

alcohol (absorbed through stomach)


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Stomach

Lies mostly in LUQ But pain can be epigastric or

lower

Just inferior to (below)diaphragm

Anterior (in front of) spleenand pancreas

Tucked under left lowermargin of liver

Anchored at both ends butmobile in between

Main regions in drawing toright--------------------------------

Capacity: 1.5 L food; maxcapacity 4L (1 gallon)

epigastrium

junction with

esophagus

funnel shaped

contains pyloric

sphincter

dome


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Stomach Regions

Cardiac region Fundus (dome shaped)

Body

Greater curvature

Lesser curvature

Pyloric region

Antrum

Canal

Sphincter

junction with

esophagus

funnel shaped

contains pyloric

sphincter

dome

Rugae: longitudinal folds


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Rugae: longitudinal foldson internal surface (helpsdistensibility)

Muscular is: additionalinnermost oblique layer(along with circular andlongitudinal layers)

Histology of


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Histology of

stomach

Simple columnar

epithelium: secretebicarbonate-bufferedmucus

Gastric pits openinginto gastric glands

Mucus neck cells Parietal cells

HCL

Intr ins ic factor(forB12 absorption)

Chief cells Pepsinogen

(activated to pepsinwith HCL)

Stimulated by gastr in:a stomach hormone


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Small intestine

Longest part of alimentary canal (2.7-5 m) Most enzymatic digestion occurs here

Most enzymes secreted bypancreas, not

small intestine

Almost all absorption of nutrients

3-6 hour process

Runs from pyloric sphincterto RLQ

Small intestine___________

S ll i t ti h 3 bdi i i


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Small intestine has 3 subdivisions Duodenum 5% of length

Jejunum almost 40%

Ileum almost 60%

Blood supp ly : super io r

mesenter ic artery;

Veins drain in to hepat ic

po rtal vein

Duodenum is retroperitoneal (stuck down under peritoneum); others are loose

Duodenum receives

bile from liver and gallbladder via bile duct*

enzymes from pancreas via main pancreatic duct*

*

*

Small intestine designed for absorption


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Small intestine designed for absorption Huge surface area because of great length

Structural modifications also increase absorptive area Circular folds (plicae circulares)

Villi (fingerlike projections) 1 mm high simple columnar epithelium: velvety

Microvilli

Lacteal*: network of bloodand lymph capillaries-Carbs and prote ins into b lood to

l iver via hepat ic por ta l vein

-Fat into lymp h:fat-solub le toxins

e.g. pest ic ides circu late

sys temical ly before going to l iverfor detoxi f icat ion

*

Absorpt iv ie cel l

with microv i l l i to

increase surface

area & m any

mitochondr ia:

nutr ient up take is

energy-

demanding

Intest inal cryp ts* (of Lieberkuhn) inbetween villi


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Intest inal cryp ts(of Lieberkuhn) inbetween villi Cells here divide every 3-6 days to renew epithelium (most rapidly dividing cells of the

body)

Secrete watery intestinal juice which mixes with chyme (the paste that food becomesafter stomach churns it)

Intest inal f lo ra the permanent normal bacteria Manufacture some vitamins, e.g. K, which get absorbed

Mucus to counteract acidity

from stomach

Hormones:

Cholecystokin in(stimulates GB

to release stored bile, also pancreas)

Secretin(stimulates pancreaticducts to release acid neutralizer)

*

-have many

mitochondr ia:

nutr ient uptakeis energy-

demanding

-produce

mucus

Duodenal glands* *


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General histology of digestive tract


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Large intestine

Subdivisions

CecumAppendix

Colon

RectumAnal canal

Digested residue reaches it

Main function: to absorb waterand electrolytes

1. Teniae coli (3 longitudinal


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Three special

features

1. Teniae coli (3 longitudinalmuscle strips)

2. Haustra (puckering into sacs)

3. Epiploic appendages (omental

or fat pouches)

1.

2.

3.


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Between ileum

and cecum

1stpart

Bl ind tube

Colon has segments: ascending , transverse and descending c olon; then sigm oid colon

Right angle turns:hepatic f lexure* in RUQan dsplenic f lexure* in LUQ

*

*

S-shaped

Movement sluggish and weak except for a few mass peristaltic

movements per day to force feces toward rectum powerfully

Rectum


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In pelvis

No teniae

Strong longitudinal musclelayer

Has valves

Anal canal Pectinate line*

Inferior to it: sensitive topain

Hemorrhoids (enlargedveins) Superior to pectinate

line: internal

Inferior to pectinate line:external

Sphincters (close opening) Internal*

smooth muscle

involuntary

External* skeletal muscle

voluntary

*

*

*


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Defecation

1. Triggered by stretching ofwall, mediated by spinalcord parasympathetic reflex

2. Stimulates contraction ofsmooth muscle in wall and

relaxation of internal analsphincter

3. If convenient to defecatevoluntary motor neuronsstimulate relaxation ofexternal anal sphincter

(aided by diaphragm andabdominal wall muscles -called Valsalva maneuver)


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Histology large intestine

No villi Fewer nutrients

absorbed

Columnar cells in pic

= absorptive cells Take in water and

electrolytes

A lot of goblet cells formucus Lubricates stool

More lymphoid tissue A lot of bacteria in stool

Th Li


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The Liver

Largest gland in the body

(about 3 pounds) Over 500 functions

Inferior to diaphragm inRUQ and epigastric areaprotected by ribs

R and L lobes Plus 2 smaller lobes

Falciform ligament Mesentery binding liver to

anterior abdominal wall

2 surfaces Diaphragmatic

Visceral

Covered by peritoneum Except bare area fused to

diaphragm


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anterior

poster ior

Fissureon visceral surface

Porta hepat is: major vessels and nervesenter and leave - see pics

Ligamentum teres: remnant of

umbilical vein in fetus, attaches to navel

see next slide


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Just some of the livers repertoire

Produces bile

Picks up glucose from blood

Stores glucose as glycogen

Processes fats and amino acids

Stores some vitamins

Detoxifies poisons and drugs

Makes the blood proteins

Li hi t l


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Liver histology Liver lobules (about one million of them)

Hexagonal solid made of sheets ofhepatocytes(liver

cells) around a central vein

Corners of lobules have portal triads

(see next pic)


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Portal tr iad

Portal arteriole

Portal venule Branch of hepatic

portal vein Delivers substances

from intestines forprocessing byhepatocytes

Bile duct Carries bile away

Liver sinusoids Large capillaries

between plates ofhepatocytes

Contribute to centralvein and ultimately to

hepatic veins and IVC Kupffercells

Liver macrophages

Old blood cells andmicroorganismsremoved


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Hepatoc tes (li er cells)


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Hepatocytes (liver cells)

Many organelles

Rough ER manufactures blood proteins Smooth ER help produce bile salts and detoxifies

blood-borne poisons

Peroxisomes detoxify other poisons, includingalcohol

Golgi apparatus packages Mitochondria a lot of energy needed for all this

Glycosomes - role in storing sugar and regulation ofblood glucose (sugar) levels

Produce 500-1000 ml bile each day Secrete into bile canaliculi (little channels) then ducts

Regeneration capacity through liver stem cells

Gallbladder*


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Gallbladder Bile is produced in the liver

Bile is stored in the gallbladder

Bile is excreted into theduodenum when needed (fattymeal)

Bile helps dissolve fat andcholesterol

If bile salts crystallize, gallstones are formed

Intermittent pain: ball valveeffect causing intermittentobstruction

Or infection and a lot of pain,fever, vomiting, etc.

*

PancreasLies in LUQ kind of behind stomach

Is retroperitoneal


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Pancreas

(exocrine and

endocrine)

Is retroperitoneal

Has a head, body and tail

Head is in C-shaped curve of duodenum

Tail extends left to touch spleen

Main pancreatic duct runs thelength of thepancreas, joins bile duct
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Pancreaticone acinus


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Pancreatic

exocrine function

Compound acinar(sac-

like) glands opening into

large ducts (therefore

exocrine) Acinarcells make 22

kinds of enzymes

Stored in zymogen

granules Grape-like arrangement

Enzymes to duodenum,

where activated


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Endocrine cells:

Documents

Lecture 22 - The Digestive Tract (Histology)