Histology of the GI Tract - Part 2 Flashcards
Macromolecules partially digested in the stomach are further digested in the small intestine largely by the action of…
pancreatic enzymes
Where does the terminal digestion of proteins and carbohydrates occur?
At the mucosal surface of the small intestine by enzymes of intestinal origin
Amino acids, monosaccharides, fatty acids and monoglycerides (resulting from SI breakdown) will be absorbed…
along the vast absorptive surface of the small intestine
Where is the opening of the main pancreatic duct
Duodenum (first part of the SI)
Name the layers of the wall of the small intestine (duodenum, jejunum and ileum)
- Mucosa
- Submucosa
- Tunica muscularis (inner circular, outer longitudinal)
- Serosa (visceral peritoneum)
The small intestine consists of 3 segments called…
duodenum
jejunum
ileum
histologically distinct at the level of the submucosa
What is the characteristic of the submucosa of the duodenum?
It is filled by a plethora of Brunner’s glands within the dense CT
What is the role of Brunner’s glands?
Secrete alkaline fluid (pH9) into the lumen of the duodenum
What is the characteristic of the submucosa of the jejunum
It penetrate deep into the lumen and pushes the submucosa, forming bulbs called plicae ciruclaris.
What is the name of the “bulbs” formed by the submucosa of the jejunum?
Plicae circularis
What happens to the submucosa as we move into the ileum?
The plicae circularis disappear and the submucosa is just a thin, plain layer of dense CT
Describe the mucosa of all 3 regions of the SI
It forms projections called villi which increase the surface area.
What is found at the bottom of the villi in the SI?
Intestinal crypts
What epithelium lines the Si
Simple columnar epithelium
What tissue is found beneath the villi and in between the crypts?
Lamina propria (loose CT) filled with plasma cells, macrophages, lymphatics, blood vessels, fibrocytes.
What is the purpose of the alkaline fluid (pH9) secreted by Brunner’s glands in the duodenum?
Neutralizes the highly acidic chyme exiting the stomach
Name of the layer immediately beneath the lamina propria of the SI, but still contained within the mucosa
Muscularis mucosa (may have 2 layers - inner circular and outer longitudinal)
The lamina propria of the mucosa (SI) contains many…
lymphoid cells
True or false:
Glands are present in the submucosa of the duodenum and jejunum
False! Glands are only in the submucosa of the duodenum
Serosa of the SI can be infiltrated by..
adipocytes
Nerve plexus in between the 2 muscle layers of the tunica muscularis (SI)
Auerbach plexus
Cell type at the base of the crypts (intestinal villi)
Undifferentiated stem cells
What are Paneth’s cells?
Cells that contain granules with enzymes called lysozyme (destroy bacteria) - these cells regulate the intestinal flora
Where are Paneth’s cells located?
At the base of the crypts in the small intestine ONLY (not colon)
Describe columnar cells with striated border
Cells of the small intestine (epithelium). Contain a brush border on the surface.
Describe enterodocrine cell sin the small intestine
Line the crypts, secrete hormones
Name all the cells you can see in the SI crypts
Undifferentiated stem cells
Paneth’s cells
Columnar cells with striated border
Enteroendocrine cells
Goblet cells
Note that you can see a lot of mitoses in the crypts
Two cell types that are most abundant at the tips of the villi
- Columnar cells with striated border
- Goblet cells
Goblet cells contain and secrete…
mucin, made of glycoproteins (to lubricate the surface of the villi)
Microfold cells are another type of cell that make up the villi in the SI. Describe them.
They contain fewer microvilli than enterocytes. They remove antigens from the surface of the intestine by endocytosis and introduce them to lymphocytes, to mount an immune response in peyer’s patches via IgA.
What are Peyer’s patches?
Lymphoid tissue in the lamina propria of the small intestine
In the crypts of the SI enteroendocrine cells secrete… (2)
Cholecystokinin (CCK)
Secretin
What is the action of cholecystokinin (CCK)
Stimulates contraction of the gallbladder and pancreatic secretion
What are enterocytes (SI)?
They are absorptive cells of the SI. They are columnar cells with a striated brush border.
What is at the core of microvilli (SI) and were does it originate from?
Actin filaments are at the core of the microvilli. They originate from the terminal web.
Microvilli (SI) are coated in…
a glycoprotein coat (glycocalyx)
Lipase and bile acids break down lipids into… (3)
glycerol
fatty acids
monoglycerides
These are then absorbed by the microvilli of the SI
Lipids coming from… and proteins coming from… will be conjugated into…
Lipids coming from the smooth ER and proteins coming from the rough ER will be conjugated into chylomicrons (this happens in the Golgi of enterocytes).
How do chylomicrons enter the circulation?
They are secreted from enterocytes to the interstitial space and enter the circulation via lymphatic capillaries in the intestine.
After entering the bloodstream, where will chylomicrons be stored?
In the liver
The actin filaments in the microvilli extend from the terminal web (also made of actin) and anchor to a protein called..
villin
Name the components of the glycoprotein coat of the microvilli (4)
Disaccharides
Dipeptidases
Alkaline phosphatase
Enterokinase (duodenum only)
What is enterokinase?
Enzyme only found in the microvilli of the duodenum. Its function is to activate pancreatic enzymes
How is the mucosa of the large intestine different from that of the large intestine?
There are ONLY intestinal crypts, but no villi
Name the different regions of the large intestine (5)
Cecum
Appendix
Ascending, transverse & descending colon
Sigmoid colon
Rectum
All histologically similar
What is the function of the large intestine
- Reabsorbs water and maintain fluid balance
- Absorbs some vitamins
- Processes undigested food material (eg. fiber)
- Stores waste before elimination
The vitamins absorbed in the colon are produced by…
bacteria of the gut via fermentation
Two main cell types of the epithelium of the large intestine
- Goblet cells
- Columnar cells with striated brush border (absorptive cells, i.e. enterocytes)
Two cell types in the epithelium of the LI (other than goblet and absorptive)
- Entero-endocrine
- Stem cells
How are lymphatic nodules of the small and large intestine different?
SI: located in lamina propria (mucosa)
LI: located in the submucosa
What is the teniae coli?
Longitudinal muscle cells of the tunica muscularis in the large intestine will sometimes form a thick band called teniae coli
The serosa of the large intestine is often…
infiltrated by adipocytes
Describe the wall of the appendix
very thin
Describe the wall of the appendix
Similar to large intestine, but fewer crypts
MANY lymphatic nodules in submucosa (sometimes penetrate into the lamina propria)
Why are infections common in the appendix?
Because the wall is so much thinner, so infection can quickly spread to the peritoneum and cause peritonitis
How does chemotherapy affect the intestinal epithelium?
It damages it a lot, because it destroys the stem cells which are necessary for the high renewal rate normally seen in the intestine.
Inflammation of the appendix
Appendicitis
Irritation of the colon can cause… (3)
Colitis
Diarrhea
Dehydration
Second most common cause of cancer death in North America
Malignant tumours of the large bowel (mostly adenocarcinomas)
