14 Histology Small Bowel and Colon Flashcards
1
Q
Small intestine (p.2)
- length
- major functions
- Structural Specializations
- The small intestine wall is extensively folded at multiple levels
- The epithelium is characterized by
A
- approximately 20 feet or 6 meters long.
- major functions: further digestion, absorption, the production of GI hormones and immunological defense.
- Structural Specializations: structure dictates function.
- The small intestine wall is extensively folded at multiple levels: plicae circulares, villi & crypts, and microvilli.
- The epithelium is characterized by villi & crypts.
2
Q
Small intestine: Plicae circulares (p.3)
A
- folds of the entire mucosa into the lumen with a core of submucosal tissue.
- circularly arranged around the lumen of the small intestine.
- increase the surface area of the organ about three-fold,
- most important as mechanical mixers.
- the most extensive in the jejunum, where they can protrude as far as 8mm into the lumen.
3
Q
Small intestine:
Villi (p.4-6)
- Villi
- The lamina propria core contains diverse connective tissue structures:
- The muscularis mucosa
A
-
Villi
- folds of the epithelium into the lumen with a core of lamina propria.
- extend approximately 1mm out from the mucosa
- increase the small intestinal surface area another 10-fold.
- can be leaf or finger shaped (tallest in jejunum, shortest in ileum).
-
The lamina propria core contains diverse connective tissue structures:
- white blood cells (lymphocytes, plasma cells, eosinophils);
- lacteals (blind ending lymphatic capillaries);
- an arteriole from the submucosal artery;
- a rich fenestrated capillary network (permeable to macromolecules);
- unmyelinated nerves;
- scattered smooth muscle cells from the muscularis mucosa out to the tip of the villus (contraction forces lymph from lacteals into larger lymphatic vessels).
- The muscularis mucosa passes under the base of the crypts, separating the mucosa from the submucosa.
4
Q
Small intestine:
Villi (p.7-11)
- The villus epithelium is composed of two cell types,
- Goblet cells
- secrete /
- as more food is absorbed from the intestinal lumen/
- goblet cells increase in number from/
- Enterocytes,
- Goblet cells
- Neighboring cells are connected by/
- Occluding or tight junctions/
- The claudins/
- Adhering junctions and desmosomes/
A
-
The villus epithelium is composed of two cell types,
-
Goblet cells
- secrete mucins to provide a protective coating for the digestive enzymes of the microvillar glycocalyx.
- as more food is absorbed from the intestinal lumen, the need for lubrication increases.
- goblet cells increase in number from only a few in the epithelium of the upper small intestine, to the predominant cell type in the large intestinal surface epithelium.
-
Enterocytes,
- columnar absorptive cells,
- display an apical striated border of microvilli.
- major cell in coordination of the gut immune system & bacterial homeostasis.
-
contain pattern recognition receptors called Toll-like receptors that respond to bacteria components: peptidoglycans, lipopolysaccharides and flagellin.
- These surface epithelial cells respond by secreting beta-defensin, cytokines and chemokines.
-
Goblet cells
- Neighboring cells are connected by junctional complexes.
-
Occluding or tight junctions
- prevent intercellular passage of nutrients from the intestinal lumen to the interstitium,
- prevent mixing of apical and basolateral plasma membrane proteins.
- The claudins that form the intestinal tight junctions are permeable to water.
- Adhering junctions and desmosomes physically link one cell to its neighbors.
5
Q
Small intestine:
Crypts (p.4-5+12-15)
- Crypts
- Crypt enterocytes secrete/
- The secretion of these anions causes/
- Paneth cells
- ?
- secrete/
- involved in/
A
-
Crypts
- folds of the epithelium that invaginate down into the lamina propria.
- serve as precursors for enterocytes or goblet cells of the villi.
-
Crypt enterocytes secrete chloride ions and bicarbonate ions into the intestinal lumen.
- The secretion of these anions, especially Cl-, causes the electrical drag of Na+ out of the cells as well.
- the exit of all these ions together results in the osmotic flow of water.
- The Na+ is reabsorbed while facilitating villar cotransport of nutrients.
- The excess water secretion is reabsorbed by the small intestinal villi and large intestinal crypts.
- At the base of the crypts are Paneth cells.
- large, acidophilic cells that have abundant basal RER, and large apical storage granules.
-
secrete
- lysozyme (degrades peptidoglycan coat of bacterial cell walls),
- alpha-defensins (ion channels that insert into parsites & bacteria),
- TNFα (proinflammatory cytokine).
-
involved in population control of gut flora.
- the lumen of the small intestine is never completely sterile;
- however, there exists a large resident bacterial population in the large intestine.
- Paneth cells of the small intestine thwart its backward expansion.
6
Q
Small intestine: Crypts: Enteroendocrine cells (p.17)
- Enteroendocrine cells
- APUD cells found in the small intestine include:
- Vasoactive Intestinal Peptide (VIP)
- Somatostatin (D1 cell)
- Enteroglucagon (L cell)
- Neuropeptide YY
A
-
Enteroendocrine cells
- also scattered through the small intestinal crypts.
-
APUD cells found in the small intestine include:
-
Vasoactive Intestinal Peptide (VIP)
- increases HCO3- and water secretion from the pancreas and the small intestine
- relaxes the esophageal sphincter, fundic stomach and the gall bladder
- decreases stomach acid
-
Somatostatin (D1 cell)
- feeds back to the stomach to shut down histamine and gastrin release
- stimlulates MMC that initiate in the duodenum.
-
Enteroglucagon (L cell)
- stimulates hepatic glycogenolysis
-
Neuropeptide YY
- a satiety signal secreted by the cephalic phase of food intake, as well as by food in the small intestine.
-
Vasoactive Intestinal Peptide (VIP)
7
Q
Small intestine:
Crypts:
Enteroendocrine cells:
APUD cells found in the small intestine include: (p.18-20)
- Cholecystokinin (CCK) (I cell)
- Secretin (S cell)
- Glucose dependent Insulinotropic Peptide (GIP)
- Motilin (Mo cell)
A
-
Cholecystokinin (CCK) (I cell)
- mainly secreted from the duodenum by fats or proteins in the lumen.
- indirectly stimulates the secretion of digestive enzymes from the pancreas and stimulates gall bladder contraction
-
Secretin (S cell)
- released by the presence of acid in the duodenum
- results in secretion of bicarbonate and water from the pancreas and Brunner’s glands of the duodenum
-
Glucose dependent Insulinotropic Peptide (GIP)
- induces insulin secretion;
- release is stimulated by glucose in lumen
- increases lipid deposits via stimulation of lipoprotein lipase
- Initially this hormone was called Gastric Inhibitory Peptide because of its inhibition of gastric acid production.
-
Motilin (Mo cell)
- stimulates upper GI motility
- secreted during the fasting state at intervals of approximately 100 minutes
- thought to influence the migrating motor complex (MMC) or “housekeeping” waves.
8
Q
Small intestine:
Microvilli (p.21-23)
- Microvilli
- glycocalyx
- The digestive process that began in the stomach is continued in the small intestinal lumen via/
- enterokinase
- The pancreatic enzymes will reduce/
- Enzymes of the glycocalyx/
A
-
Microvilli
- folds of the apical plasma membrane on each individual enterocyte with a core of actin cytoskeleton.
- increase the surface area for nutrient digestion and absorption by at least another 20-fold.
- Extending from the microvilli is a thick glycoprotein coat called the glycocalyx.
- contains hydrolytic enzymes: enterokinase (enteropeptidase), dipeptidases and disaccharidases.
- These are intrinsic membrane proteins that span the bilayer and expose their enzymatic active sites toward the small intestinal lumen.
-
The digestive process that began in the stomach is continued in the small intestinal lumen via the pancreatic enzymes.
- Note that the glycocalyx enzyme, enterokinase, converts the pancreatic enzyme trypsinogen to its active form trypsin, which in turn activates the remaining pancreatic enzymes.
- The pancreatic enzymes will reduce most luminal proteins and carbohydrates to di/tripeptides and disaccharide forms.
- Enzymes of the glycocalyx complete the protein and carbohydrate digestion and provide single amino acids and monosaccharides for the enterocyte transporters.
9
Q
Small intestine:
Microvilli (p.24-25)
- nutrient symporters
- secondary active transporters
- The majority of the lipids
- Together, the microvilli and glycocalyx are responsible for/
A
- The second major class of intrinsic proteins of the microvillus plasma membrane is the nutrient symporters.
- These carriers cotransport amino acids or monosaccharides with Na+.
- These are passive processes which utilize the existing Na+ electrochemical gradient within the enterocyte.
- Since the apical nutrient symporters are driven by the Na+ gradient created by basolateral 3Na+/2K+ ATPase active transport pumps, the symporters are often referred to as secondary active transporters.
-
The majority of the lipids diffuse across the plasma membrane as 2-monoglycerides, cholesterol, lysolecithin and free fatty acids.
- Fatty acids greater than 24 C bind an enterocyte carrier protein for absorption.
- Together, the microvilli and glycocalyx are responsible for the absorption of several hundred grams of carbohydrate, 100 grams of fat, 50-100 grams of amino acids, 50-100 grams of ions, and 7-8 liters of water per day.
10
Q
Small intestine:
GALT (p.26)
- GALT
- lymphoid cells
- seen in normal GI mucosa
- The notable exception
- Diffuse lymphocytes and solitary nodules/
- Peyer’s Patches
A
-
GALT
- an acronym for “gut associated lymphoid tissue.”
- one component of the larger Mucosal Immune System.
-
lymphoid cells
- transient,
- always found throughout the GI tract lamina propria, submucosa and even the epithelium itself.
- Lymphocytes, plasma cells, macrophages, eosinophils and mast cells can all be seen in normal GI mucosa.
- The notable exception is the neutrophil, whose presence indicates mucosal injury or inflammation.
- Diffuse lymphocytes and solitary nodules (clustered lymphocytes with or without a pale secondary center) are not unusual in any portion of the GI tract.
- Unique aggregated nodules called Peyer’s Patches are found only in the ileum.
11
Q
Small intestine:
GALT (p.26-28)
- At the site of Peyer’s patches, the overlying villi/
- follicle associating epithelium (FAE)
- “M” cells
- The sensitized T- cells/
- T and B cells/
A
- At the site of Peyer’s patches, the overlying villi are frequently absent.
- The epithelium covering a patch is called the follicle associating epithelium (FAE).
- The FAE is composed of specialized cells called “M” cells.
- do not display a border of microvilli, like the neighboring enterocytes, but rather irregular small folds called “microplicae.”
- pinocytose a representative sampling of intraluminal antigens and transcytose them across to intraepithelial antigen presenter cells (APC) on the basolateral surface.
- cup shaped with the flat “base” facing the intestinal lumen.
- The “cup” portion is filled with APCs and T-lymphocytes.
- The sensitized T- cells migrate into the underlying lymphatic nodule, and activate the maturation of B lymphocytes, which exit the Peyer’s patch.
-
T and B cells migrate to lymph nodes, where they proliferate into T-helper, T-cytotoxic and plasma cells.
- This sensitization and proliferation is considered the afferent (sensory) limb of the Mucosal Immune System.
12
Q
Small intestine: GALT: The afferent (sensory) limb of the Mucosal Immune System (p.29-30)
- The committed T-cells and plasma cells/
- Once out in the systemic circulation, these cells/
- alpha4beta7
- Once in the lamina propria, plasma cells produce/
- IgA
A
- The committed T-cells and plasma cells leave the lymph nodes and continue along lymphatic drainage, eventually reaching the right side of the heart.
- Once out in the systemic circulation, these cells extravasate into the organs of the Mucosal Immune System (GI, respiratory, reproductive).
- The immune cells display an integrin, alpha4beta7, which serves as the “addressin” for MadCAM-1 (Mucosal adhesion, Cell Adhesion Molecule-1) on the endothelial cells of these organs.
- Once in the lamina propria, plasma cells produce large quantities of IgA, a dimeric form of antibody.
- IgA is the main immunoglobulin found in mucous secretions: tears, saliva, colostrum, GI tract, genito-urinary tract, prostate and respiratory tract.
13
Q
Small intestine:
GALT (p.30-32)
- poly-IgA receptor (Pig-r)
- Once IgA binds to SC/
- Eventually the transport vesicle/
- Secretory IgA/
- return “homing” of immune cells
- M cells and IgA help to
A
- The basolateral plasma membranes of the enterocytes contain a glycoprotein receptor for IgA called the poly-IgA receptor (Pig-r) with a detachable segment called the secretory component (SC).
-
Once IgA binds to SC,
- the complex is endocytosed.
- The secretory component remains with the IgA and is transcytosed to the apical surface of the enterocyte.
- Eventually the transport vesicle fuses with the apical plasma membrane and releases secretory IgA into the intestinal lumen.
-
Secretory IgA binds antigens and enterotoxins, decreases the adherence of microbes to the epithelial surface, and neutralizes viruses and bacterial toxins.
- This return “homing” of immune cells and the secretion and transport of IgA are considered the efferent (effector) limb of the Mucosal Immune System.
- Clearly, M cells and IgA help to clear unwanted antigens before they even have the opportunity to breech the wall of the GI tract.
14
Q
Small Intestinal Sections:
Duodenum (p.33-36+38)
- duodenum
- Duodenal villi
- The most distinguishing feature of the duodenum
- The duodenum co-ordinates with other organs of the GI system by/
- The proximal small bowel is the site of absorption for:
A
-
duodenum
- approximately 25 cm long
- curves around the head of the pancreas.
-
Duodenal villi
- leaf-like in shape (broad, flat);
- this appearance varies in microscopic specimens depending on the plane of sectioning.
- very few goblet cells in the epithelium, as there is yet little need for lubrication.
- leaf-like in shape (broad, flat);
-
The most distinguishing feature of the duodenum is Brunner’s glands in the submucosa.
- The only other portion of the GI tract which has glands in the submucosa is the esophagus.
- Brunner’s glands are branched tubuloalveolar glands that produce a slightly alkaline mucus secretion to neutralize the acidic chyme arriving from the stomach.
- The duodenum co-ordinates with other organs of the GI system by sensing luminal content and secreting multiple enteroendocrine hormones.
-
The proximal small bowel is the site of absorption for:
- Folate
- Calcium
- Iron
- Vit D
- Vit Bs (not 12)
15
Q
Small Intestinal Sections:
Jejunum and Ileum (p.37-40)
- Jejunum
- length
- displays/
- goblet cells
- major area for/
- Ileum
- length
- characterized by
- There are only/
- goblet cells
- The terminal ileum is the site for absorption for:
A
-
Jejunum
- 2.5 meters long.
- displays the best developed plicae circulares and numerous, long, finger-like villi.
- There are a medium number of goblet cells, more than in the duodenum.
- major area for final digestion and absorption in the small intestine.
-
Ileum
- longest portion of the small intestine (3.5 meters).
- characterized by Peyer’s patches and short, club-like villi.
- There are only shallow crypts and low plicae circulares.
- The epithelium contains a large percentage of goblet cells.
-
The terminal ileum is the site for absorption for:
- Vitamin B12 /intrinsic factor complex
- bile acids
- secretory-IgA
