Intestines Flashcards
What role does the small intestine play in digestion?
Chemical digestion of food and absorption of products of digestion
What role does the small intestine play in digestion?
Chemical digestion of food and absorption of products of digestion
The small intestine is divided into:
- Duodenum - shortest segment, 25 cm long
- Jejunum - 2.5 cm long
- Ileum - 3.5 cm long
What are the four layers of the digestive tract?
- Mucosa
- Submucosa
- Muscularis externa
- Serosa
The mucosa of the small intestine has special features that serve to:
Increase the surface area available for digestion and absorption
The surface area of the small intestine is increased by: (2)
- Plicae circulares
2. Villi
What is the plicae circulares?
Valves of Kerckring - permanent submucosal folds. Since the submucosa folds, the overlying mucosa also folds and increases the epithelial surface area
What are villi?
Finger-like mucosal folds that project into the lumen. This is characteristic of the small intestine only to increase the surface area.
How do you distinguish between villi and microvilli?
Microvilli are cytoplasmic projections of individual cells
Describe the mucosa of the small intestine:
- Characterized by villi - which are covered by simple columnar epithelium containing primarily ENTEROCYTES (absorptive cells). Underlying the epithelium is the lamina propria
- Also has intestinal glands - CRYPTS OF LIEBERKUHN - simple tubular glands that open at the base of the villi and empty into the lumen.
The Crypts of Lieberkuhn extend thru the lamina propria to the:
Muscularis mucosae
Describe the lamina propria of the small intestine:
- Delicate CT that forms the core of the villus and surrounds the glands
- Contains blood and lymphatic vessels
- Capillary network right under the epithelium
- Has a central lacteal
- Lymphatic infiltration is common
What is the central lacteal?
A lymphatic vessel: blind-ending vessel that begins near the tip of the villus, runs thru the central core of the villus, and drains to a plexus in the lamina propria and submucosa
Describe the lymphatic infiltration of the lamina propria:
It can be diffuse or nodular. This is part of the gult-associated lymphatic tissue (GALT) which is found throughout the entire GI tract
Where is the muscularis mucosae?
Thin layer deep to the glands
Describe the submocosa:
- Coarse collagenous CT (dense irregular)
The small intestine is divided into:
- Duodenum - shortest segment, 25 cm long
- Jejunum - 2.5 cm long
- Ileum - 3.5 cm long
What are the four layers of the digestive tract?
- Mucosa
- Submucosa
- Muscularis externa
- Serosa
The mucosa of the small intestine has special features that serve to:
Increase the surface area available for digestion and absorption
The surface area of the small intestine is increased by: (2)
- Plicae circulares
2. Villi
What is the plicae circulares?
Valves of Kerckring - permanent submucosal folds. Since the submucosa folds, the overlying mucosa also folds and increases the epithelial surface area
What are villi?
Finger-like mucosal folds that project into the lumen. This is characteristic of the small intestine only to increase the surface area.
How do you distinguish between villi and microvilli?
Microvilli are cytoplasmic projections of individual cells
Describe the mucosa of the small intestine:
- Characterized by villi - which are covered by simple columnar epithelium containing primarily ENTEROCYTES (absorptive cells). Underlying the epithelium is the lamina propria
- Also has intestinal glands - CRYPTS OF LIEBERKUHN - simple tubular glands that open at the base of the villi and empty into the lumen.
The Crypts of Lieberkuhn extend thru the lamina propria to the:
Muscularis mucosae
Describe the lamina propria of the small intestine:
- Delicate CT that forms the core of the villus and surrounds the glands
- Contains blood and lymphatic vessels
- Capillary network right under the epithelium
- Has a central lacteal
- Lymphatic infiltration is common
What is the central lacteal?
A lymphatic vessel: blind-ending vessel that begins near the tip of the villus, runs thru the central core of the villus, and drains to a plexus in the lamina propria and submucosa
Describe the lymphatic infiltration of the lamina propria:
It can be diffuse or nodular. This is part of the gult-associated lymphatic tissue (GALT) which is found throughout the entire GI tract
Where is the muscularis mucosae?
Thin layer deep to the glands
Describe the submocosa:
- Coarse collagenous CT (dense irregular)
- Ganglion cells of submucosal (Meissner’s) plexus may be present
- Submucosal glands in DUODENUM ONLY - Brunner’s glands
What are Brunner’s glands?
They are present in the duodenum, branched tubular glands that produce an alkaline mucus, buffering the acidic chyme entering from the stomach.
What do Brunner’s glands empty into?
Crypts of Lieberkuhn
Describe the muscularis externa of the small intestine?
- Two layers of smooth muscle - inner circular and outer longitudinal
- Ganglion cells of the myenteric plexus between the smooth muscle layers
Contraction of the INNER layer of smooth muscle results in:
Segmentation
Contraction of the OUTER layer of smooth muscle results in:
Peristalsis
Describe the serosa:
When covered by peritoneal epithelium
What are the different types of cells in the epithelium? (5)
- Enterocytes - absorptive cells
- Goblet cells - mucus
- Enteroendocrine cell - hormones: gastrin, secretin, serotonin
- Immature enterocyte - secretory IgA
- Undifferentiated stem cell
What are enterocytes?
- Columnar absorptive cells
- Primary cell type covering the villi
- Replaced every 5-6 days
- Tall, regualr microvilli at apical surface of cells greatly increased the surface area for absorption
What are enterocytes replaced by?
Undifferentiated cells in the crypt of Lieberkuhn
In LM sections, they are visible as:
The striated border
In TEM section, they can be identified as:
Absorptive microvilli
What are actin microfilaments within these microvilli bundled by?
Fimbrin and Villin
How are the actin filaments linked to the plasma membrane?
By myosin I and calmodulin
How do microfilaments interact within the terminal web?
With a spectrin isoform that connects them to intermediate filaments (cytokeratins)
Microvilli are covered with a glycocalyx surface coat that contains enzymes including:
Oligosaccharidases, disaccharidases, and pepitdases
Where are the junctional complexes?
They form a tight seal near the apical margin
What is the function of enterocytes?
- Uptake of ions
- Uptake of sugars
- Uptake of peptides and amino acids
- Uptake of lipids
- Uptake of vitamin B12
- Recycling of unconjugated bile salts
- Absorption of water
- Release of secretory IgA at the luminal surface
Describe the uptake of sugar by the enterocytes:
The plasma membrane of microvilli is covered with a glycocalyx surface coat, which includes enzymes such as oligosaccharidases and dissaccharidases (lactase, maltase, sucrase) for terminal digestion of carbohydrates
What happens to the monosaccharides?
They are then absorbed by active transport and move to the basal membrane where they are exported by other transporters
What is the cause of lactose intolerance?
A genetic defect in lactase
Describe the uptake of peptides and amino acids by the enterocytes:
ENTEROKINASE which is an enzyme present in the glycocalyx of the apicl (microvilli) membrane - this activates trypsinogen (from the pancreas) to form trypsin
What does trypsin do?
Activates other pancreatic enzymes –> these enzymes cleave proteins to amino acids or very small peptides that can be transported into and then out of enterocytes
Describe the uptake of lipids:
In the intestinal lumen, lipids are broken down by pancreatic lipase in the presence of bile salts to free fatty acids and monoglycerides, which then diffuse into the enterocytes
What is the role of sER in the uptake of lipids?
It resynthesizes triglycerides and then transfers them to Golgi complexes for further processing and incorporation into vesicles with protein componenets - CHYLOMICRONS
What happens to the chylomicra?
They migrate to the lateral margin where they are released by exocytosis into the intercellular space –> they then move toward lymphatic vessels (lacteals) in the center of the villus
Describe the uptake of vitamin B12 by the enterocytes:
Gastric intrinsic factor (IF) is produced by PARIETAL CELLS in gastric glands in the body of the stomach –> IF binds to B12 in intestinal lumen and then this complex can be taken up by the enterocyte
What happens to vitamin B12 within the enterocyte?
It is liberated and transported across the basal membrane into a capillary
What is vitamin B12 necessary for?
RBC production
What happens to the recycled unconjugated bile salts?
They are returned to liver hepatocytes
Describe the absorption of water by the enterocytes:
Na+-K+ ATPase in the lateral plasma membranes moves Na+ ions into the intercellular spaces and water follow. During active absorption, lateral plications separate, enlarging the space
Describe the release of secretory IgA at the luminal surface:
IgA is synthesized by plasma cells in the lamina propria as a dimer –> taken up by receptor-mediated endocytosis at the basal surface of enterocytes –> IgA-receptor complex is transported up through the cell –> then released at the apical margin with the stabilizing portion of the receptor still attached
What is special about secretory IgA?
It is less easily degraded within the intestinal lumen
What are goblet cells and what is their function in the small intestine?
Unicellular exocrine glands, the apical cytoplasm packed with coalescing glycoprotein mucinogen granules that are released to form a coat of mucus.
What are enteroendocrine cells?
They contain basal granules that are released into the underlying CT. They make up 1% of the cell population. There are more than a dozen types of enteroendocrine cells throughout the digestive tract
What are the 4 types of enteroendocrine cells and what organs do they impact?
- Cholecystokinin (CCK)
- Gastric inhibitory peptide (GIP)
- Secretin
- Motilin
Impacts the stomach, intestines, pancreas, or gallbladder
What is CCK’s function?
Responds to lipid content, stimulates the gallbladder contraction and pancreatic enzyme secretion
What is the function of gastric inhibitory peptide?
It inhibits gastric secretion from gastric glands
What is the function of secretin?
It is released in duodenum in response to input of HCl from the stomach, inhibits gastric acid secretion and causes pancreatic ducts to release bicarbonate ion (buffer)
What is the function of motilin?
Stimulates gastrointestinal motility
What are the cells in the intestinal glands (Crypts of Lieberkuhn)? (5)
- Immature enterocytes
- Goblet cells
- Enteroendocrine cells
- Paneth cells
- Undifferentiated stem cells
What are paneth cells?
They are located in the base of glands and contain prominent acidophilic granules. The products are lysozyme, which degrades bacterial surface coats, and defensins which increase membrane permeability of parasites and bacteria
What are the undifferentiated stem cells?
They are precursors of other cells. The cells undergo mitosis within the glands, then migrate toward an extrusion zone at tips of villi.
How often do the undifferentiated stem cells replace epithelium?
5-6 days.
Which cells are replaced more slowly?
Paneth cells and Enteroendocrine cells
What is gut-associated lymphatic tissue (GALT)?
Lymphatic infiltration is common in small intestine and may be diffuse or nodular. This is part of GALT, which is found throughout the entire GI tract
Describe the lymphoid elements in CT of lamina propria and/or submucosa:
- Lymphatic nodules with B cells and macrophages
- Diffuse lymphatic tissue
- In cooperation with T cells, B lymphocytes differentiate into plasma cells which setlle down in the GI tract or elsewhere to produce an antibody response
What is nodular GALT and what makes up the epithelium?
It is follicular and covered by a specialized epithelium and has:
- M (microfold) cells: flattened cells with short apical folds of the plasma membrane. They take up antigens and transport them to underlying lymphocytes
- Dendritic cells: sample and transport antigen from the lumen
- Enterocytes: assist by degrading antigens
Where is the nodular form of GALT most prominent?
In the ileum where it can be seen macroscopically as Peyer’s patches
Defining characteristics of the small intestine:
- Presence of villi - not found elsewhere
- Duodenum is identified by the presence of Brunner’s glands in the submucosa
- The jejunum has NO submucosal glands
- The ileum has NO submucosal glands and shows an increase in the number of goblet cells and the amount of lymphatic infiltration and is the site of Peyer’s patches
What does the large intestine play a major role in?
Reabsorption of electrolytes and water and elimination of undigested foods
The colon is divided into:
- Cecum - with projecting vermiform appendix
- Colon - ascending, transverse, descending, and sigmoid
- Rectum
- Anal canal
Does the colon have villi?
No, only crypts of Lieberkuhn (it does have the same 4 layers - mucosa, submucosa, muscularies externa, and serosa/fibrosa)
Describe the mucosa of the colon:
- The epithelium is simple columnar with a smooth surface (no villi) - has enterocytes with the primary role of reabsorption of electrolytes and water and also has goblet cells: produce mucus
- Has Crypts of Lieberkuhn
- Lamina propria withGALT
- Muscularis mucosae
The crypts of Lieberkuhn (simple tubular glands) are located in the mucosa and contain:
- Immature enterocytes
- Goblet cells - increase in number as the rectum is approached
- Enteroendocrine cells
- Undifferentiated cells - surface renewal every 5-6 days
Are there paneth cells in the colon mucosa?
No, they are lost from the glands at the beginning of the large intestine
Describe the submucosa of the colon:
No glands, but lymphatic tissues is usually present
Describe the muscularis externa:
It has a continuous inner circular layer and an outer longitudinal layer that is organized into 3 distinct and separate bands - the taeniae coli
How it the appendix different than the colon?
- Smaller diameter
- Presence of a continuous outer longitudinal layer of muscle
- More extensive lymphatic infiltration than the colon
- Less regularly spaced crypts
Describe the rectum:
- Makeup is similar to the colon
- Crypts of Lieberkuhn are lined predominantly by goblet cells
What is the anal canal?
The most distal portion of the alimentary canal and about 4 cm long. Has mucosa, submucosa, muscularis externa, and fibrosa
Describe the mucosa of the anal canal:
- The upper anal canal resembles the large intestine with simple columnar epithelium with simple intestinal tubular glands
- Glands (crypts of Lieberkuhn) contain many goblet cells as well as undifferentiated cells and a few enteroendocrine cells
When the upper anal canal transitions into the lower anal canal, there are 3 changes in the mucosa:
- Transition from simple columnar to stratified columnar epithelium (sometimes stratified cuboidal)
- Crypts of Lieberkuhn are lost
- Muscularis mucosae disappears
Within the lower anal canal, the epithelium will further transition to:
Stratified squamous nonkeratinized epithelium
At the end of the anal canal, there is a transition of the stratified squamous nonkeratinized epithelium to:
Stratified squamous keratinized epithelium of thin skin
Describe the submucosa of the anal canal:
Very vascular with extensive plexus of hemorrhoidal veins
Describe the muscularis externa of the anal canal:
- Inner circular layer of smooth muscle forms the internal anal sphincter
- Outer longitudinal layer of smooth muscle terminates in adjacent CT
- An increasing layer of skeletal muscle forms the external anal sphincter
Review of the stomach wall:
- Mucosal glands are simple branches tubular glands that empty into gastric pits
- NO VILLI
- 3 poorly defined layers of smooth muscle in muscularis externa
Review of the small intestinal wall:
- Finger-like mucosal projections into the lumen are villi and are only present in the small intestine
- Mucosal glands are simple tubular glands that open at the base of the villi (crypts of Lieberkuhn)
- Submucosal glands are only present in duodenum - Brunner’s glands
- Villi + submucosal glands = duodenum
- Villi but no submucosal glands = jejunum or ileum
- 2 smooth muscle layers in muscularis externa (inner circular and outer longitudinal)
Review of the large intestinal wall:
- NO VILLI
- Mucosal glands are simple tubular glands (crypts of Lieberkuhn)
- 2 layers of smooth muscle in muscularis externa: inner circular and outer longitudinal
In the colon, outer longitudinal is organized into 3 distinct bands:
Taeniae coli
