Stem cell niche Flashcards
Stem cells
Embryo starts of clump totipotent stem cells
In adult are distinct populations stem cells often with restricted potency:
-Mesenchymal stem cells: Bone cells (osteoblasts), cartilage cells (chondrocytes) and fat cells (adipocytes)
- Blood stem cells: red blood cells, platelets and white blood cells
- Satellite stem cells: muscle
- Germ cells: oocytes and sperm, because they then form zygotes
Adult stem cells found in special microenvironments called “niches”
Is thought that there are special mechanisms in developing embryo that set aside these cells for use in adult
Why do we need adult stem cells?
Red blood cells live for about four months
White blood cells more than a year
Skin cells two or three weeks
Colon cells about four days
Sperm cells about three days
Many brain cells can last an entire lifetime
Stem cell maintenance
Classical stem cell division:
- Maintains same stem cell population
- Stem cell divides into 1 stem cell and 1 transit-amplifying cell
- TC then divides into 2 TC’s and then those TC’s divide into 2 TC’s
- Then the TC divides into 2 differentiated cells
Population asymmetry:
- Three sorts of division
- Division rates must be balanced so as to keep stem cell population constant
- Progenitor cells divide into 2 progenitor cells
- Then those new progenitor cells will divide into 1 progenitor cell and 1 differentiated cell
- That 16% will then divide into 2 differentiated cells
Often see mix of the two modes of division
Epidermis needs to be continually renewed
Skin has 3 main layers - epidermis, dermis and subcutis
BL = basal layer; SL = spinous layer; GL = granular layer; SC = stratum corneum
The dermis contains fibroblasts and blood vessels
Epidermis made up of keratinocytes
Signals from dermis and basement membrane stimulate keratinocyte proliferationi
Basal layers of epidermis is a stem cell niche
BCL contains stem cells that proliferate
Most daughter cells pushed to skin surface and die but some stay stuck to basement membrane to continue creation SC
Cells produce high levels keratin, provide strength to skin, produce different keratins as they move towards the surface
If it produces keratin 5 and 14 then its a stem cell
If it produces keratin 1 and 10 then its specialised (spinous cells)
Layers of the skin from outside to inside:
- Outer layer of dead cells
- Dividing and differentiating keratinocytes
- Spinous cells expressing 1 and 10
- Basal-layer cells expressing 5 and 14
- Basement membrane made of laminin and collagen (niche)
Signals regulate basal stem cell niche
Wnt signals from dermis inhibit differentiation (maintains stem cell population)
Integrins hold cell to basement membrane (maintains stem cell population)
Notch signals in maturing keratinocytes inhibits integrins
EGF inhibits Notch activity
(Look at diagram for this)
EFG activity in basal-layer inhibits Notch and basal cells express integrin ɑ6β4, Wnt signals from dermis inhibit basal-layer cell differentiation
But then in the next two layers there is high Notch activity which inhibits integrin gene expression
Bulge cells provide an additional source of stem cells
Another stem cell niche in hair follicle called the bulge
These cells provide the cells that from the follicle
After wounding is greater need for replacement cells, then bulge cells can also contribute to epidermis and sebaceous glands
More generally suggest that after injury when larger areas tissue need restoration, different stem cell niches may be able to provide cells
Third stem cell niche is in area of dermal papilla at the very end of the hair follicle
Junctional epidermolysis bullosa (JEB)
Illness where adhesion between dermis and epidermis is impaired, integrins affected
Patients have mutations in adhesion genes e.g. LAMB3
Skin epidermis can be cultured to replace skin in burn patients
Scientists now culturing epidermal cells from JEB patients, replacing damaged gene in cell culture (infection of isolated epidermal cells with retroviral vector containing unmutated LAMB3) then growing it to replace skin in the patient (forms stem cells, differentiated cells and partly differentiated cells)
Our gut lining also continuously replenished
Contents of the gut create very harsh environment where cells need continuously replenished
Small intestine well-studied model of this
Villi increase surface area of intestine to aid in absorption
Cells at end villi continuously shed
Cells of small intestine
Enterocyte - absorption
Goblet cells - secrete mucus
Enteroendocrine cells - secrete peptide hormones
Paneth cells - secrete antimicrobial peptides
Stem cells
Submucosa cells - help to maintain the stem cells
SC at base of crypt proliferate
In mice each crypt produces 12 cells per hour
Each crypt made of about 250 cells
Produces conveyor belt of cells
Slow dividing stem cells express Bim1
Fast diving stem cells express Lgr5
These genes are markers for stem cells
How do we know the Lgr5 positive cells are stem cells?
Lineage tracing allows determination of cell fate
transgenes can be made that cause single cells express marker
In this case transgene is controlled by promoter for Lgr5
Marker is expressed in cell and in all of its offspring permanently
In this example marker is bacterial gene beta galactosidase, used to turn cells blue
Once marker turned on can never be turned off even if cell turns of Lgr5, is a permanent marker
Mouse embryonic stem cells - replicating the embryonic stem cell niche
ESC can be isolated from epiblast and grown in cell culture
Specially defined conditions maintain them in pluripotent state (BMP and LIF), this replaces stem cell niche found in vivo
Removal BMP/LIF causes cells to differentiate into different cell types, each types needs specific set conditions
None these steps very efficient, sometimes less than 1% cells form desired cell types
Human and mouse ESC
Human ESC require different treatment (FGF and Activin/Nodal) then mouse ESC
Mice ESC also appear to be pluripotent
Assays for pluripotency:
- Expression of epiblast markers
- Chimaeras made when mic ESC in with normal embryo and ESC contribute to all mouse tissues
- Teratomas are benign tumours that contain differentiated tissues
- Direct the ESCs to adopt cell fate
Similarity between mouse and human ESC:
- Express transcription factors Nanog, Oct4, Sox2, Klf4
- Can form teratomas
Differences:
- Can contribute to chimeras (M)
- Can differentiate into all cell types in vitro (M)
- Can differentiate into a wide range of cell types in vitro (H)
How do maintenance conditions block differentiation?
In mouse ESC:
LIF and BMP4 -> Nanog, Oct3/4, Sox2, Esrrb and Klf4 which block differentiation but just return to themselves
Using Yamanaka factors (Oct3/4, Sox2, Klf4 and c-Myc) you can get differentiated cells to form induced pluripotent stem cells (iPS cells)
