Small intestine Flashcards
What is the function of the small intestine?
To absorb water, nutrients and salts
What are the three main regions of the small intestine and their lengths?
duodenum (25cm)
jejunum (2.5m)
ileum (3.75m)
What is the small intestine lined in to increase surface area?
The small intestine is lined on the inside by fan-shaped mesentery which increases the surface area of the intestine with folds and supports the blood supply. The mesentery then has villi covering it.
What kind of muscles are in the external wall of the small intestine?
Longitudinal and circular muscles which are important for motility.
Is there a transition between the duodenum, jejunum and ileum?
No, all have the same basic histological organisation
What are the invaginations into the intestine known as?
Crypts of Lieberkuhn
What is the villi made of (cells) and what does it have to help improve its function?
- Have a good blood and lymph supply
- Have a good innervation from the submucosal plexus
- Has simple columnar epithelium: primarily enterocytes (absorptive cells), scattered goblet cells and enteroendocrine cells
What do the Crypts of Lieberkühn comprise of?
Stem cells - replace high turnover
Paneth cells - base of crypt, secrete acidophilic granules, lysozyme
Enterocytes
Their features, abundance, their lifespan and cell shape
- most abundant in small intestine
- tall columnar cells with microvilli
- have a basal nucleus
- specialised for absorption and transport
- 1-6 days lifespan
What are the microvilli on enterocytes covered in and why?
Glycocalyx - rich carbohydrate layer on apical membrane that serves as a protection from the digestional lumen yet allows absorption
It traps a layer of water and mucus (unstirred layer) to regulate rate of absorption and to protect surface of microvilli from pH of lumen
How is the surface area of the small intestine increased?
Cylindrical internal surface area of the small intestine is 0.4m2 and the folds, villi and microvilli increase it to 200m2
Goblet cells
Abundance, what do they do, how and distribution?
- 2nd most abundant
- produces mucus which facilitate passage of material through the bowel
- mucus containing granules accumulate at the apical end causing the goblet shape
- the abundances increases as you move through the bowel as more lubrication is required
Enteroendocrine cells
Shape, distribution and function?
- columnar epithelial cells
- scattered among absorptive cells
- found in lower parts of the crypts
- secrete hormones e.g. histamine to influence gut motility
- aka chromaffin cells
Paneth cells
Where are they found, what do they contain and their functions?
- Found only in the base of the crypts
- Contain large, acidophilic granules that contain:
lysozyme (antibacterial to protect stem cells), glycoproteins (to protect the enzymes from themselves) and zinc (to act as a co-factor to many of the enzymes) - Can have phagocytic roles
- May regulate intestinal flora.
Epithelial cell turnover
- Cell proliferation, differentiation and death are continuous
- Enterocytes and goblet cells have lifespan of around 36 hrs
- Replaced by dividing stem cells in the crypt
- Stem cells are pluripotent
How do stem cells move from the crypt to the tip?
- Dividing stem cells in the crypt are shunted up and at the tip the cells become senescent (old)
- Apaptosed cells are sloughed into lumen and digested/rebabsorbed by intestine
Why does the intestine have such fast turnover?
Enterocytes are the first line of defence so they may be directly affected by toxic substances in the diet -> interference in cell function is not dangerous as any lesions are short lived
What may cause intestinal dysfunction?
If the escalator transit of enterocytes is interrupted through impaired production of new cells e.g. radiation