Histology Of Gi Tract

Front 1

Layers - 3

Back 1

1. First layer - mucosa, which has three layers.
a. the innermost layer is the lining epithelium. Depending on its location in the GI tract, the lining epithelium may function as protective, absorptive, or secretory. It may fold inward to increase surface areas, such as the case with plicae in the jejunum.

b. Next is the lamina propria, which is simply a term for connective tissue located under a lining epithelium.

c. Outside of that is a layer of muscle, the muscularis mucosa.

2. submucosa. The submucosa is typically thicker than the mucosa. Note that glands may be present in the mucosa or submucosa. This will again vary upon location. On the outside of the submucosa we have the muscularis externa. This consists of circular muscle on the inside, and longitudinal muscle on the outside.

Front 2

layers continued

Back 2

3.Most externally we have either the adventitia (connective tissue continuous with the surrounding organs) or a serosa (slippery thin membrane made
up by a single layer of mesothelial cells, ie- visceral pleur, ie- visceral pleura, visceral layer of serous pericardium, visceral peritoneum).

Serosas will produce an exudate to allow organs to slide against each other.

There are also
two important nerve plexus: Meissner’s Plexus, located in the submucosa, and Auerbach’s Plexus,
found between the circular and longitudinal muscles which will control peristaltic contractions.
(“Aurbach’s is on the autside”)

Front 3

Cross sectional

Back 3

Study the cross section

Front 4

Esophagus

Back 4

The esophagus’ lumen is lined by non-keratinized stratified squamous epithelium. Because
it is non-keratinized, that means the cells in this lining epithelium are still alive and have their organelles (unlike the cells in our skin).

The esophagus is surrounded by an adventitia.

The top 1/3 of the esophagus’ muscularis externa layer is made up of skeletal muscle, the middle 1/3 is made up of a mixture of skeletal and smooth, and the bottom 1/3 is completely smooth muscle. Important: ALL of the
muscularis externa in the esophagus is involuntary- even the skeletal muscle in the upper parts. It is
all innervated by the parasympathetic fibers from the vagus nerve.

Front 5

Esophagus - glands and arrows

Back 5

There are two types of glands to be familiar with in the esophagus: (1) esophageal glands proper/submucosal glands, within the submucosa, located throughout the length of the esophagus to provide a protective mucin

(2) cardiac glands. within the lamina propria, located only at the cricoid cartilage near the 5th tracheal ring and at the junction of the esophagus and stomach. The cardiac glands also function to secrete a protective mucin

The dark layer at the base of the lining epithelium is due to basophilic, mitotic cells. These cells are dividing so that the GI lining can be replaced every couple of days. why chemotherapy has negative affects on the GI tract- it targets active mitotic cells in the body

The dark blob to the left is just a lymph nodule.

The red arrow is pointing to mucosal glands.

On the outside, we can see the longitudinal and circular muscle layers. Also note how you can
tell that the inner is running as a circle and the outer fibers go into the pg

Front 6

High magnification view of esophagus’ lining epithelium

Back 6

Surrounding the epithelial lining we can see DARK basophilic cells (mitotic!)

Front 7

High magnification view of esophagus’ lining epithelium.

Back 7

Note all the little black dots within
the lining epithelium are just each cell’s nucleus.

The yellow arrows are pointing to the basophilic,
actively mitotic cells.

The dark vertical arrow (pointing toward the text that says lamina propria) is actually pointing to the muscularis mucosa. This layer was not easily seen on the lower magnification
slides.

Front 8

SEM of esophagus surface

Back 8

The ridges on the surface are microplicae, which are just tiny folds.

The role of these in the esophagus is unknown. The black arrows are pointing to bacteria.

Front 9

TEM of esophagus surface

Back 9

Here we can actually see some of the epithelial lining cells getting ready to slough off into the lumen. These cells still have nuclei and other organelles, making the epithelial lining a non-keratinized stratified squamous epithelium.

what looks like villae is actually plicae

Front 10

Basal Cells in the Lining Epithelium

Back 10

These are just the mitotic cells. The daughter cells of these will be displaced toward the surface. They are euchromatic cells due to their high activity.

Front 11

Meissner’s Plexus.

Back 11

The arrow on the slide is pointing to a ganglionic cell body of Meissner’s plexus within the submucosa.

Front 12

Auerbach’s Plexus.

Back 12

In between the two muscle layers we see ganglionic cell bodies of Aurbach’s Plexus. Given their location, it is unsurprising that these nerves stimulate the contraction of the surrounding muscle layers.

All smooth muscle - so this is lower layer

Front 13

Junction of esophagus and stomach

Back 13

Note how abrupt this transition is. Remember the first portion of the stomach is called the cardiac region of the stomach.

Front 14

SEM of stomach’s inside surface

Back 14

The dark arrows are pointing to gastric pits. Can also see the outline of each cells. White material is probably mucin.

Front 15

Lining epithelium of stomach

Back 15

Here, can see the simple columnar epithelium making up the lining epithelium. Below is the submucosa- recall that there are NO glands in this layer.

Front 16

Neck and base of gland.

Back 16

The major cell type making up the base will be the chief cell.

Front 17

High magnification view.

Back 17

See the dark chief cells and also the pinker, round parietal cells (remember these are filled with membrane so they are very eosinophilic)

Front 18

TEM of gastric pit (cross section).

Back 18

Can see invaginations (white areas) on parietal cell

Front 19

Higher mag of parietal cell.

Back 19

Front 20

Cardiac region vs. pyloric region of stomach

Back 20

See that it is possible to differentiate between the
two because gastric pits are shallow in the cardiac region and glands are straight. In the pyloric region
glands are convoluted so we don’t get a full cross section.

Front 21

Moving on from the stomach to the intestines

Back 21

Chyme spends 3-4 hours in the stomach and then it is dumped into the duodenum where pancreatic duct and bile duct is also dumping …

Small intestines, mainly continue digestions and reabsorption here. Small intestine is 1. really long (20 feet) 2. increases surface area with really large folds (you can see some here) – first 10 inches = duodenum, then next 2/5ths is jejunum, then the last 3/5s are ileum
In the duodenum:

Plicae circularis begin in duodenum, then again in jejunum, then disappear in ileum
-also intestinal VILI !
1.Tall columnar absorptive cells with microvillus brush border (back-to-back) – enzymes are stored here!
2. Goblet cells (simple columnar) filled with mucin granules to make slippery/slimy surface!
3. There are also glands in the small intestine that are BETWEEN the intestinal villi – “intestinal absorptive glands” “crypts of librica” - -- at surface they are either sloughed off or apoptosis – 3-5 day turnover! (very active/mitotic)
4.Paneth in the bottom – lots of zinc and lysozyme, but otherwise we don’t really know what they are doing!

Front 22

Distinguishing Features of the Small Intestine:

Back 22

1.) From Duodenum to Ileum the intestinal villi decrease in #, become thinner, and more finger-like.
2.) From Duodenum to Ileum the goblet cells increase in number.
3.) Contain Brunner’s Glands (secrete alkali Mucin that neutralizes acidic chime from stomach) are present in the first part of the duodenum.
4.) Contain Peyer’s Patches (prominent lymphatic nodules) in the ileum and provide a means of discerning histological specimens from this region from any other part of the GI tract.
5.) Contain Plicae Circulares, which are large folds in the small intestine that increase surface area to
facilitate further absorption. They involve the lining mucosa and submucosa running in circular pattern
around lumen of intestine most prominent in jejunum, less visible in duodenum and ileum

Front 23

Really great picture of the duodenum

Back 23

"Can't get a better picture than this!" - Dr. Andrews.

Front 24

Looking down the surface of the lumen of the duodenum

Back 24

These are intestinal villi! broad, leaf-shaped, packed together

the are moving up and down! and there is a central lacteal in the middle that helps it move. Called a "lacteal" because after a fatty meal these become REALLLL fat and looks WHITE! ew?

Front 25

Intestinal lumen

Back 25

Check out the villi and the large fold that is the plicae

Front 26

Surface of intestinal villus

Back 26

Cells at the pointer that are leached out are goblet cells

Other wise, simple columnar epithelial!

Front 27

Great example of the microvilli brush border

Back 27

back-to-back microvilli for brush border!

We will also see back-to-back in proximal tubules of kidney

Goblet cells at the black pointer cells
Dark circles are lymphocytes...

Front 28

TEM of Microvillus Brush Border

Back 28

Nuclei of adjacent intestinal absorptive cells. The borders are interdigitating.

Front 29

Surface of the Microvilli here....

Back 29

brush border, supported by actin filaments

Front 30

Demonstrates the shape of the cells

Back 30

Cells are hexagonal or pentagonal - we can see microvillus brush border (SEM!)

Front 31

Goblet Cell

Back 31

Full of mucin granules! adjacent to two absorptive brush border cells with their mitochondria inside

Front 32

Back 32

Front 33

PANETH CELLS (higher mag than previous flashcard)

Back 33

Paneth cells at base of microvilli - filled with esophillic granules

Front 34

Enteroendorcine cells are also called

Back 34

argentaffin cells

(just know this maybe? IDK but he said it)

Front 35

Peyer's Patches

Back 35

Huge lymphatic nodules that are in the lamina propria puckering into the lumen in the ileum

Front 36

Large Intestines

Back 36

A little digestion from left over enzymes
but MAINLY reabsorption of water and solidification of POOP

-about 5 feet long.

Front 37

Large Intestine Continued

Back 37

We need a lot of slippery mucin, so we have a LOT MORE GOBLET CELLS

- we do not find ANY intestinal villi here.
-we DO find the intestinal glands "crypts of librica"
-very few, if any, Paneth cells or enteroendocrine cells
-The muscularis externa is broken up into three different bands - tenai coli - which causes haustra or puckering of the large intestines... (looks like a scrunchy)

Front 38

Appendix!

Back 38

No Tenai Coli
-lots of debris
-lots of lymphatic material

Front 39

Lining of Large Intestine

Back 39

Deep intestinal glands
-goblet cells are purple
-tenai coli at bottom

Front 40

Lumen of the large intestine

Back 40

Lots of goblet cells

Front 41

Abrupt transition at the Anal Canal

Back 41

As we go down the anal canal, there is an abrupt transition from what we see throughout the length of the large intestine to NON-KERATINIZED, STRATIFIED, SQUAMOUS EPITHELIUM HERE

verrrry abrupt. Clinically important -- site where cancers form. Here. Cervix. anywhere there is an abrupt transition.

Front 42

Lining of the Anal Canal

Back 42

Non-kertinized, stratified, squamous epithelium

Front 43

Portal Circulation

Back 43

FACTS:
-Liver is the largest gland in the body
-The liver is surrounded by a capsule of connective tissue called Gleason's Capsule
-Portal Vein carries 75% of the blood and carries low levels of O2 and carries products of digestion (except the chylomicrons)
-The Hepatic Artery delivers 25% of the blood and carries O2 rich blood
-The Hepatic vein, bile duct, and lymphatic vessels all carry material away from the liver
-The hepatic vein delivers blood from the liver to the IVC and the blood is derived from both the hepatic artery and portal vein.
-Bile exits the liver in the common hepatic duct;
-the liver produces 600-1200 ml of bile per day. ------Lymphatics exit the liver and enter the thoracic duct, and the liver produces more lymph than any other organ. The lymph is uniquely rich in plasma protein, particularly albumin, which is produced in the liver.

Front 44

Lobules - The Functional Unit of the Liver

Back 44

-Each lobule is formed by connective tissue of the Gleason's Capsule and are hexagonal in shape
-The Portal Triad is at the periphery and the central vein is at the center
- The Portal Triad contains the portal venuole, a hepatic arteriole, a bile duct branch, and a lymphatic vessel branch
-Blood travels from the triad through the sinusoids ("the second capillary plexus of the portal system") before collecting in the central vein
-The sinusoids are TRUE SINUSOIDS (permeable endothelium without basal lamina supported by reticular fibers)
-Blood flows from the central vein into the soblobular vein, emptying in to the hepatic vein
-Blood from the portal vein and from the hepatic artery immediately mix in the sinusoids of the liver

Front 45

Lobules Continued

Back 45

-Most of the liver cells are hepatocytes (in 1 or 2 cells thick) and they line the sinusoids
-Some hepatocytes are binucleated because the liver is very regenerative
-Fat storying cells are aka "Ito cells" and they also store Vit A
-Kuppfer Cells are protective, mobile, phagocytic cells of the liver and they are phagocytized foreign material that are derived from monocytes
-Bile produced by hepatocytes travels in the opposite direction as blood flow in the bile canaliculi
-Before joining the bile duct, bile canaliculi aggregate as the "Hering's Canal"
-Lympathics travel in the opposite direction of blood flow, too
-Space in between sinusoid endothelial cells and hepatocytes is known as the space of Disse, and lymphatic material
collects here. It accumulates in the lymphatic vessels found in the portal triad. Ito cells are also found in the space of Disse.

Front 46

Liver Lobules Conceptual View

Back 46

Can be viewed in three ways: classic, portal and acinus view
-Classic: triad on the periphery and central vein in the middle
-Portal: Viewing the triad in the CENTER of lobule and central vein at the periphery of the picture
-Aciunus View: hepatocellular heterogeneity: important for pathologists becasue it is always for the lobules to be broken up into different zones. Zone 1 = closest to triad (and most likely to be exposed to toxins). Zone 3= closest to central vein (most likely area to become necrotic)

Front 47

Lobule

Back 47

lobule surrounded by connective tissue with the triads on the periphery (black arrows) and the central
vein in the middle (yellow arrow).

Front 48

Portal Triad of a Liver Lobule

Back 48

Notice the portal venuole is much larger than the rest of the structures and it is surrounded by endothelial cells.

The hepatic arterial is much smaller and is surrounded by endothelial cells and smooth muscle
cells. Both the venuole and the arterial have RBCs
within them.

Lymphatic vessels can be seen and they do not contain RBCs.

Bile ducts are surrounded by cuboidal epithelium. You can see the connective tissue surrounding these structures, and the pink hepatocytes lining the sinusoids

Front 49

CENTRAL VEIN

Back 49

Both of the above pictures are of central veins. You can see the sinusoids funneling into them.

Front 50

HIGH YIELD PIC: Higher magnification SEM Image of the liver

Back 50

You can see the bile canaliculus (BC) in between two hepatocytes. The true sinusoids (S) can be
seen with multiple fenestrations. The space of
Disse (DS) can be seen between the
hepatocytes and sinusoid endothelium (DS is
referring to the larger arrow, not the small
arrow). A lot of exchange is occurring in the
space of Disse and this is where the lymph
forms.

Front 51

Hepatocyte Schematic

Back 51

Notice how many different types of organelles
hepatocytes contain. Also you can see the reticular
fibers in the space of Disse supporting the sinusoids (sinusoids would be found above and below the pic). Microvilli of the hepatocytes extend into the space of Disse to increase surface area for exchange. Notice the space between the endothelial cells (the fenestrations).

The bile canaliculus can also be seen on the R and L. The bile canaliculus is bound by tight junctions
(can’t see the tight junctions in the picture).

Desmosomes also hold hepatocytes together as we can see.

Front 52

Hepatocyte info

Back 52

This picture shows that 90% of bile produced by the liver is recycled. This means that the bile is retaken up in the intestines, transported to the liver, taken up by hepatocytes in the space of
Disse. And transported back into the bile canaliculus.

10% of bile is
made new in hepatocytes via synthesis of cholic acid and conjugation with glycine and taurine in the smooth ER.

Front 53

Bile Canaliculus

Back 53

This picture shows the bile canaliculus (yellow arrow). The red arrows are actually pointing to desmosomes. Although these desmosomes are adjacent to the bile canaliculus in the pic, tight junctions are even more adjacent to the canaliculus. The tight
junctions are just too small to see.

Front 54

Glycogen Storage

Back 54

Glycogen is stored in the hepatocytes. The purple on the left and the black on the right is glycogen.

Larger clumps are called alpha glycogen and smaller spots are called beta glycogen.

Front 55

Fenestrated Endothelium lining the sinusoids between layers of hepatocytes

Back 55

To the right we can see a sinusoid bounded on either side by hepatocytes. The holes in the true sinusoid can be clearly seen along with the space of Disse (red arrows). You can see the microvilli of the hepatocytes lateral to the space of Disse, and
the black arrows are pointing out the bile canaliculi.

Front 56

Bile Canaliculi

Back 56

The red arrow in this picture is displaying a Kupffer cell, which as you recall is phagocytic. The black arrows are again bile canaliculi.

Front 57

Kupffer Cells

Back 57

The two pictures below are showing more Kupffer cells.

Kupffer cells can be seen in the left pic as black dots in the sinusoids since they phagocytized India ink. The right pic show Kupffer cells after they have phagocytized trypan blue (black arrows). The yellow arrows are just showing binucleated hepatocytes

Front 58

Bile Ducts

Back 58

This picture is showing how the bile dumps into the bile duct of the portal triad. Bile travels along bile canaliculi, before reaching Hering’s canals, then interlobular ducts, and finally a bile duct. The bile ducts eventually dump out into the common hepatic duct. Cuboidal cells line the interlobular ducts and bile ducts.

Front 59

Gallbladder - If bile does not travel directly into the duodenum, it gets stored in the gall bladder.

Back 59

The epithelium of the gall bladder is very folded as you can see to the right and the gall bladder is surrounded by smooth muscle. This smooth muscle can contract to squeeze out bile when triggered by
cholecystokinin.

Remember that the gall bladder releases bile into the cystic duct, which joins the common hepatic duct to form the common bile duct. The common bile duct joins the pancreatic duct to form the ampulla of Vater. The ampulla of Vater empties into the second part of the duodenum through the sphincter of Oddi. The sphincter of Oddi is closed in between meals.

Front 60

Gallbladder CONT.

Back 60

The picture shows the folds of the gall bladder epithelium, and the second
picture below shows that the gall bladder epithelium is simple columnar. These simple columnar epithelial cells contain microvilli, but it is NOT a microvilli brush border. The gall bladder epithelium functions to concentrate bile up to 5 times.

Front 61

Pancreas

Back 61

The pacreas is an exocrine and endocrine organ. 90% of the pancreas functions as an exocrine organ, releasing digestive enzymes via acinar cells.

10% of the pancreas has endocrine functions and releases hormones into the blood stream via Islets of Langerhans. Both the endocrine and exocrine pancreas can be seen above

Front 62

Exocrine Pancreas Schematic

Back 62

The picture shows acinar cells of the exocrine
pancreas. The digestive enzymes are produced in the RER and stored in membrane bound granules,
released by exocytosis into the lumen. This is a
typical serous gland. The unique feature of the
pancreas serous gland is that the intercalated duct
cells can be found within the center of the acinous.
These are called centroacinar cells.

Front 63

Centroacinar Cells

Back 63

Centroacinar cells (black arrows) produce a
bicarbonate solution that gets added to the digestive enzymes in the acinous. This helps reduce the acidity of the chyme, bringing the pH of the exocrine pancreas fluid up to 8.

Front 64

Pancreatic Acinar Cells

Back 64

you can see lots of RER and granules in
the acinar cells. The granules are filled with proenzymes (zymogens). Also you can notice there is a polarity to these acinar cells.

Front 65

Enzymes of the Acinar

Back 65

Amyolytic enzymes (e.g. amylase) break down carbohydrates. Lipolytic enzymes (e.g. lipase) break down fats. Proteolytic enzymes (e.g. trypsinogen) break down amino acids. Remember,
proteolytic enzymes are produced, stored, and released as proenzymes (zymogens).

Front 66

Islet of Langerhans

Back 66

Islet of Langerhans cells, as seen on the right, are
surrounded by connective tissue and have fenestrated capillaries inside of them. They have a variety of cells within them that produce a variety of hormones.

Front 67

Alpha Cells

Back 67

Alpha cells secrete glucagon. Glucagon functions to increase blood glucose levels. 20% of the Islet of Langerhans is composed of alpha cells, and the alpha cells are located on the periphery of the islet. Dense granules within the alpha cells are round with clear halos on the periphery,

Front 68

Beta Cells

Back 68

Beta cells secrete insulin. Insulin functions to decrease blood glucose levels. 70% of the
Islet of Langerhans is composed of beta cells, and the beta cells are centrally located in the islet.
Granules within the beta cells, as seen below, have irregular crystalline cores. Proinsulin, produced in
the beta cells, is cleaved to form insulin and C peptide; both are released into the blood stream.

Front 69

Delta Cells

Back 69

Delta cells in the Islet of Langerhans release somatostatin. Somatostatin inhibits other islet cells (paracrine effect) and it also slows motility of the intestinal tract, extending time for
digestion and nutrient uptake. 5% of islet cells are delta cells.

Front 70

PP Cells (no, not poopy poop cells)

Back 70

PP cells in the Islet of Langerhans, also known as F cells, release pancreatic polypeptides. These cells
are rare and may function to inhibit the exocrine pancreas

Front 71

Pancreatic Summary

Back 71

Just know this^