gut stem Flashcards
(12 cards)
gut lining
-made of loose connective tissue (lamina propia) with continious line of epithelium with crypts (invaginations which house stem cells) and vili protuding to increase SA for absorption
- epithelial cells migrate from base to tips of vili where lost - 3-5 days
crypts have stem cells at bottom with cell cycle of 24 hours and above are rapidly dividing cells with cell cycle of 12 hours, non dividing differentiated cells lie on top
villi composition
-absorptive brush border cells
-mucus secreting goblet cells
how to investigate stem cells division
mice - give mice dosage of radioactive thymidine which gets incorporated into dna during cell cycle, those in s phase will incorborate marker into dna and followed over hours and days, when cells divide then label becomes diluted so can be quantified
describe composition of crypt
-dividing cells exclusive in crypt
-differentiated cells do not divide
- middle layer of cells divide most rapidly
- just above base of crypt, interspersed amongst paneth cells (cells that have role in gut immunity and stem cell support) , lie cells that divide slowly
- rapidly dividing cells above these stem cells are called commited precursors (definitly going to differentiate) or transit amplyfying cells (transition from stem cells to differeniated cell)
absorptive cells
brush boarder cells, densely packed microvilli
- job is to take up nutrients from gut lumen
- produce hydrolytic enzymes that perform some of final steps of extracellular digestion, majority cell type in epithelium
goblet cells
secret mucus into gut lumen that covers the epithelium with protective coat
paneth cells
form part of the gut lumen that covers epethelium with protective coat
enteroendocrine cells
- of more than 15 different subtypes, secrete serotonin and peptide hormones that act on neurons and other cell types in the gut wall and regulates growth, proliferation and digestive activities of cells of the gut and other tissues
wnt signalling and stem cell niche
Some people have a hereditary predisposition to colorectal cancer and develop large numbers of small precancerous tumors (adenomas) in the lining of this part of the gut.
The appearance of these tumors suggests that they have arisen from intestinal crypt cells - lots of crypts
- mutations causing loss of Apc gene (adenomatous polyposisi coli) - loss of Apc therfore mimics effect of continiual exposure to Wnt signal
-wnt signalling normally keeps crypt cells in proliferative state and cessation of exposure of Wnt signalling normally makes them stp dividing as leave crypt
- Lgr5 gene strongly upregulated in repsonse to Wnt signalling
- 15 Lgr5 stem cells in each crypt amongst paneth cells, intenstinal stem cell niche = paneth cells generate signals including stromg wnt signal to act over short range to mainatain stem cell state, signal proteins from connective tissue surround crypt to reinforce localizing signal from paneth cells- Lgr5 itself is a receptor for protein R-spondin
- The niche created by the Paneth cells has space for only a limited number of stem cells
When the stem cells divide, it is a random matter which of the daughter cells are pushed out of the niche and committed to differentiation and which stay in place as stem cells for the future for self renewal
In most other stem-cell systems where the question has been examined, it appears that the fates of the daughters of a stem cell are assigned in a similar way.
how can a single lgr5 expressing cell in culture generate an entire organised crypt villus
-Paneth cells are progeny of the stem cells, suggesting that the intestinal stem-cell system is in some way self-maintaining and self-organising.
- Single dissociated Lgr5-expressing cells can proliferate in culture, embedded in a cell-free matrix rich in the basal-lamina component laminin
-cells first form round epithelial vesicles.
- Within a few days, one or another of the cells in the vesicle, at random, begins to differentiate as a Paneth cell.
- induces its neighbors to behave as stem cells and initiates transformation of the simple vesicle into an organized structure, or organoid.
- Protrusions resembling crypts grow out into the surrounding matrix and contain Paneth cells, Lgr5-expressing stem cells, and the transit amplifying cells derived from them.
- These cell types are confined to the cryptlike structures.
- Terminally differentiated, nondividing absorptive cells line the other parts of the organoid epithelium, with their microvilli facing the lumen.
- Goblet and enteroendocrine cells are also present, scattered through the epithelium.
- The whole “minigut” structure, with all its cell types, grows and renews itself in much the same way as the lining of the normal intestine.
Ephrin–Eph Signaling Drives Segregation of the Different Gut Cell Types
The self-organising behaviour of organoids suggests that
interactions among the different epithelial cells drives
them to segregate from one another.
* Cells that live in the crypts express the EphrinB receptor
* In contrast, non-proliferative differentiated cells switch off
expression of this receptor and instead switch on
expression of its ligands (cell-surface proteins of the
EphrinB family ).
* Cells expressing Eph-receptor proteins are repelled by
contacts with cells expressing ephrins on their surface
* This mechanism serves to keep the cells segregated and in
their proper places.
Notch Signaling Controls Gut Cell Diversification and Helps Maintain the Stem-Cell State
Wnt signaling leads to expression of Notch and Delta
in the cells of the crypt
* Delta-Notch signaling in the crypt then mediates
lateral inhibition between adjacent cells.
* Cells expressing higher levels of Delta eventually
activate Notch in their neighbours, whilst they go on
to adopt a secretory fate, and stop dividing
* Their neighbors, with activated Notch, are prevented
from differentiating and keep on dividing.
* This creates the greater number of aborptive cells
required
Essentially the same process operates at the crypt
base
* Paneth cells express higher levels of Delta to prevent
stem cells from differentiating
* in the transit amplifying population, where nascent
secretory cells express higher levels of Delta.
* Division continues in the Notch-activated cells as they
move up the crypt, until they escape from the
influence of Wnt and emerge onto the villi to become
absorptive cells
