stem cell niche Flashcards
what does the embryo start off as
-clump of totipotent stem cells (embryonic stem cells
what are the distinct populations of stem cells 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
- others
-Adult stem cells are found in special microenvironments called “niches”
-It is thought that there are special mechanisms in the developing embryo that set aside these cells for use in the adult
Why do we need adult stem cells?
-a lot of cells are always dying so we need new ones
-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 - harsh environment in digestive system
-Sperm cells about three days
-Many brain cells can last an entire lifetime - not every cell dies off
why does the Epidermis needs to be continually renewed
-The skin has three main layers - the epidermis, dermis and subcutis.
-There are also hair follicles, sebaceous glands and sweat glands
-The epidermis is made up of keratinocytes
-The dermis contains fibroblasts and blood vessels
-Signals from the dermis and basement membrane stimulate keratinocyte proliferation.
-BL = basal layer; SL = spinous layer; GL = granular layer; SC= stratum corneum
what are the different types of Stem cell maintenance
-classiscal stem- cell division:
=maintains same stem cell population
=always divides to make 2 daughter cells
=one replaces one of the daughter cells so one daughter cell differentiates in transit-amplifying cells
-populational asymmetry
=division rates must be balanced so as to keep stem cell population constant
=2 daughter cells
=one daughter cells differentiates
=then both daughter cells differentiate both from progenitor cells
whats The basal layer of the epidermis
-stem cell niche
-The basal cell layer contains stem cells that proliferate (basal keratinocytes). The daughter cells are pushed progressively to the skin surface where they die.
-Cells produce high levels of the fibrous protein keratin that provides strength to the skin
-Cells produce different keratins as they move towards the surface.
-ec matrix is very thick
what do signals regulate
-basal stem ell niche
-Wnt signals from the dermis inhibits differentiation (maintains stem cell population). this inhibits stem cell maintenance
-Integrins (adhesion molecules) hold cells to the basement membrane (maintains stem cell population). - so it doesnt move therefore held in stem cell niche
-Notch signals in maturing keratinocytes inhibit integrins
-EGF inhibits Notch activity which inhibits integrals which activate adhesion which activates stem cell maintenance
what do Bulge cells provide
-an additional source of stem cells
-There is another stem cell niche in the hair follicle called the bulge.
-These cells provide the cells that form the follicle.
-When skin is badly damaged, there is a greater need for replacement cells. In this situation, bulge cells can also contribute to epidermis and sebaceous glands.
-More generally, this suggests that after injury when larger areas of tissue need restoration, different stem cell niches may be able to provide cells.
-A third stem cell niche is in the area of the dermal papilla.
whats Junctional epidermolysis bullosa (JEB) and regenerative medicine
-skin never properly attaches
-Adhesion between the dermis and epidermis is impaired.
-Patients have mutations in adhesion genes – for example LAMB3
-Skin epidermis (basal keratinocytes) can be cultured to replace skin in burn patients
-Scientists are now culturing cells from JEB patients, replacing the damaged gene in cell culture, then growing it to replace the skin in the patient.
-Inactivated viruses (viral vectors) can be used to introduce DNA into cells
why is Our gut lining is continuously replenished.
-The contents of the gut create a very harsh environment where cells need to be continuously replenished.
-The small intestine is well-studied model.
-Villi increase the surface area of the intestine to aid in absorption. Cells at the ends of the villi are continuously shed.
what are the Cells of the small intestine
-Enterocyte – absorption
-Goblet cells – secrete mucus
-Enteroendocrine cells – secrete peptide hormones
-Paneth cells – secrete anti-microbial peptides
Stem cells
-submucosa cells – help to maintain the stem cells
stem cells in SI of mouse
-Stem cells at the base of the crypt proliferate, in mouse each crypt produces 12 cells per hour. Each crypt is made up of about 250 cells.
-This produces a conveyor belt of cells
-Slow dividing stem cells express Bim1
Fast dividing stem cells express Lgr5
-These genes are markers for stem cells
How do we know that Lgr5 positive cells are stem cells?
-Lineage tracing allows us to determine the fate of a cell.
-Transgenes can be made that cause single cells to express a marker. In this case the transgene is controlled by the promoter for Lgr5.
-And the marker is a bacterial gene called beta galactosidase which can be used to turn cells blue.
-The marker is expressed in the cell and in all of its offspring permanently.
Culturing mouse embryonic stem cells
– replicating the embryonic stem cell niche
-Embryonic stem cells can be isolated from the epiblast and grown in cell culture.
-Specially defined conditions maintain them in the pluripotent state (BMP and LIF). This replaces the stem cell niche found in vivo.
-Removal of BMP/LIF causes the cells to differentiate into different cell types. Each cell type needs a specific set of conditions.
-None of these steps is very efficient – sometimes less than 1% of the cells form the desired cell type
-BMP and LIF are ligands
-look at slide 14
what do Human and mouse ES cells require
-Human ES cells require different treatment than mouse ES cells (FGF + Activin)
-Assays for pluripotency:
=Expression of epiblast markers: e.g. Nanog, Oct4, Sox2, Klf4
=Chimeras are made when you mix ES cells in with a normal embryo and the ES cells contribute to different tissues in the adult.
=Teratomas are benign tumours that contain differentiated tissues.
=Direct the ESCs to adopt cell fates in vitro
How do maintenance conditions block differentiation
-In mouse ES cells:
=LIF, BMP4 -> epiblast markers (Nanog, Oct3/4, Sox2, Esrrb, Klf4) -| differentiation
-cell dedifferentiate : Yamanaka factors
(Oct3/4, Sox2, Klf4 +c-Myc) -> differentiated cells -> induced pluripotent stem cells
