Intestines Histology

Question 1

Answer 1

Question 2

Answer 2

Question 3

Answer 3

Question 4

Answer 4

Question 5

Answer 5

Question 6

Answer 6

Question 7

Answer 7

Question 8

Answer 8

Question 9

Answer 9

Question 10

Answer 10

Question 11

Answer 11

Question 12

Large Intestine

Answer 12

Question 13

Answer 13

Question 14

Answer 14

Question 15

Answer 15

Question 16

Answer 16

Question 17

Answer 17

Question 18

Answer 18

Question 19

Answer 19

Appendix

Question 20

Mucosal surfaces are mainly protected by […] antibody functioning to cross-link pathogens in the lumen.

Answer 20

Mucosal surfaces are mainly protected by IgA antibody functioning to cross-link pathogens in the lumen.

Question 21

Answer 21

• Langerhans Cells are dendritic cells.
• Not found in intestines, M cells instead.

Question 22

Answer 22

Question 23

Answer 23

Question 24

Answer 24

Question 25

The down side of M Cells!

Answer 25

Question 26

Outer Layer of Small Intestine

Answer 26

The outer layer of the small intestine is a serosa, except where the duodenum and terminal ileum attach to the posterior abdominal wall. In these regions the outer layer is an adventitia.

Question 27

Muscularis Propria of Small Intestine

Answer 27

The muscularis propria is made up of outer longitudinal and inner circular layers. The inner circular layer forms a sphincter at the ileocecal valve.

Question 28

Submucosa of Duodenum

Answer 28

The submucosa of the duodenum (but not the jejunum or ileum) contains glands (Brunner’s glands) which secrete a bicarbonate rich mucus to protect the small intestine from the acidic stomach contents.

Question 29

Mucosa of Small Intestine

Answer 29

The mucosa has a simple columnar epithelium which is invaginated to form the crypts of Lieberkühn, and evaginated to form villi; isolated lymphoid follicles are common, and in addition, aggregates of lymphoid follicles, the Peyer’s patches, are present in the ileum.

Question 30

Epithelial Cell Types in Small Intestine

Answer 30

• Enterocytes
• Goblet Cells
• Paneth Cells
• Enteroendocrine Cells
• Intraepithelial Lymphocytes
• M Cells
• Stem Cells

Question 31

Enterocytes

Answer 31

Enterocytes (aka absorptive cells) are tall columnar cells which make up most of the epithelium of the small intestine. They have a prominent brush border (dense and regular microvilli) at the apical surface, a prominent Golgi apparatus, lots of mitochondria, a well-developed smooth endoplasmic reticulum compartment in the apical cytoplasm, and an extensively folded basolateral membrane. They are multifunctional cells, involved in

• digestion
• Absorptionof monosaccharides and amino acids
• absorption of fats
• absorption of water and electrolytes
• transepithelial transport of IgA and IgM

Question 32

Enterocytes - Digestion

Answer 32

• Enterocytes produce membrane-bound digestive enzymes, called brush border enzymes, which remain attached to the plasma membrane of the microvilli and function at the cell surface. One of these, enteropeptidase, cleaves the pancreatic enzyme precursor, trypsinogen, to its active form, trypsin. Trypsin then goes on to activate other digestive enzyme precursors secreted from the pancreas.
• The activated pancreatic enzymes digest fats and convert proteins to oligopeptides and carbohydrates to oligosaccharides. Brush border enzymes digest oligopeptides to amino acids and oligosaccharides to monosaccharides. Some examples of brush border enzymes are lactase, sucrase, maltase, aminopeptidase, carboxypeptidase - the idea being that there is a variety of enzymes that provide for complete digestion of oligosaccharides and oligopeptides. Note that it is insufficient production of the brush border enzyme lactase that is the basis for lactose intolerance.

Question 33

Enterocytes - Absorption of monosaccharides and Amino Acids

Answer 33

These nutrients are brought into the enterocyte by active transport and/or facilitated diffusion; they then diffuse down a concentration gradient out of the cell at the basolateral surface, where they are picked up by fenestrated capillaries.

Question 34

EmEnterocytes - Absorption of Fats

Answer 34

Monoglycerides and fatty acids diffuse across the apical membrane of enterocytes into a smooth endoplasmic reticulum compartment where they are resynthesized into triglycerides. A protein coat is added to the triglyceride and the resulting particles, called chylomicrons are transported in vesicles to the basolateral membrane and exocytosed. The chylomicrons are then absorbed into a lacteal - the large lymphatic capillary present in each villus.

Question 35

Enterocytes - Absorption of Water and Electrolytes

Answer 35

Enterocytes express high levels of the enzyme Na+K + /ATPase in their basolateral membranes. Na+ ions enter the enterocytes by facilitated diffusion through the apical membrane. They are actively pumped out of the cell at the basolateral membrane, through the action of Na+K + /ATPase. The basal and lateral membranes of enterocytes are highly folded to increase surface area for Na+K + /ATPase, and there are many mitochondria present near the membrane to supply ATP for active transport. The pumping out of sodium maintains a high salt concentration in the intercellular space near the base of the cell, and water is drawn out of the lumen down an osmotic gradient and enters the lymphatic and blood capillaries.

Question 36

Enterocytes - Transepithelial Transport of IgA and IgM

Answer 36

The poly-Ig receptor, expressed at the basolateral membrane of enterocytes, mediates transport of polymeric antibodies (IgA and IgM are polymeric antibodies) from the lamina propria into the lumen of the intestine.

Question 37

Goblet Cells

Answer 37

Secrete mucin. They are characterized by a prominent goblet-shaped aggregation of mucous granules.

Question 38

Paneth Cells

Answer 38

Secrete lysozyme, which is an enzyme that dissolves bacterial cell walls. They are tall columnar cells with large acidophilic granules in their apical cytoplasm. They are located at the base of the crypts.

Question 39

Enetroendocrine Cells

Answer 39

Morphology as in stomach. Cholecystokinin (CCK), secretin and Glucose dependent Insulinotrophic Peptide (GIP) are secreted by enteroendocrine cells in the small intestine and are the main regulators of exocrine pancreatic function.

Question 40

Intraepithelial Lymphocytes

Answer 40

Present among epithelial cells (above the basal lamina). Some of these may be suppressor T cells involved in blocking the immune response against food antigens.

Question 41

M Cells

Answer 41

Present in the GI tract (and the respiratory tract) in the epithelium overlying lymphoid follicles (Follicle Associated Epithelium): They function in transport of antigens from the lumen into the follicle

Question 42

Stem Cells

Answer 42

• Undifferentiated epithelial stem cells are located in the crypts, just above the Paneth cells. These cells provide new cells to replace those that are sloughed off regularly as a part of normal epithelial cell turnover.
• Various chemotherapeutic agents used in cancer therapy, function by disrupting the mitotic spindle and killing dividing cells. In the GI tract, because of the very high rate of epithelial cell turnover, loss of dividing stem cells causes severe damage to the epithelium and can result in nausea, dehydration and malnutrition.

Question 43

Outer Layer of Large Intestine

Answer 43

The outer layer is a serosa over most of the colon, and an adventitia over the rectum, which lies outside the abdominal cavity.

Question 44

Muscularis Propria of Large Intestine

Answer 44

The muscularis propria in the colon has the longitudinal muscle layer arranged into three longitudinal bands, the taeniae coli. At the recto-anal junction, the circular layer of the muscularis forms an internal anal sphincter (smooth muscle). An external anal sphincter (skeletal muscle) is present outside of this.

Question 45

Submucosa of Large Intestine

Answer 45

The submucosa contains the internal rectal plexus (aka internal hemorrhoidal plexus) of veins at the recto-anal junction.

Question 46

Mucosa of Large Intestine

Answer 46

The mucosa has a simple columnar epithelium invaginated to form crypts of Lieberkühn. There are no villi. At the recto-anal junction, there is an abrupt transition from simple columnar to stratified squamous epithelium. Lymphoid follicles are common, especially in the distal colon and the rectum.

Question 47

Epithelial Cell Types in the Large Intestine

Answer 47

• The epithelium of the large intestine is composed of the same cell types as are found in the small intestine, except that Paneth cells are present only in the proximal part of the large intestine, goblet cells are more numerous, and enterocytes are correspondingly fewer (though still in the majority).
• At the distal end of the rectum, just before it narrows to form the anal canal, there is a squamocolumnar junction at which the simple columnar, crypt epithelium of the rectum changes to stratified squamous epithelium. At a variable distance along the anal canal, the stratified squamous epithelium changes from nonkeratinized to keratinized.

Question 48

Immune Protection of the GI Tract

Answer 48

• M cells are specialized epithelial cells that transport particulates, including potential pathogens, from the lumen of the GI tract into lymphoid follicles where they can interact with antigen presenting cells and immune responses cab be initiated where appropriate. Stimulated B cells differentiate into pre-plasma cells in lymphoid follicle germinal centers. Pre-plasma cells leave the follicles via lymphatic capillaries and travel (via the same route as other lymphocytes) to connective tissue underlying mucosal surfaces of the GI tract where they differentiate into plasma cells that secrete IgM or IgA antibodies. Poly-Ig receptors expressed in enterocytes and localized to basal plasma membranes bind to IgA or IgM in underlying lamina propria and transport the antibodies (by receptor-mediated endocytosis) across the epithelial cell to the apical surface.
• At the apical surface, the poly-Ig receptor-carrying antibody is cleaved, releasing the antibody, and a portion of the receptor called secretory component, onto the mucosal surface where it can bind to specific pathogens. (Secretory component is a significant part of the secreted antibody complex, as it makes the antibody resistant to proteolytic cleavage – this is particularly important in the GI tract.)
• IgA antibodies function on mucosal surfaces by cross-linking pathogens, thereby preventing their binding to surface epithelia. In addition, IgA bound to antigen itself binds readily to mucus and hence promotes pathogen clearance in secreted mucus.