lecture 12: adult stem cells and regeneration Flashcards
1
Q
What is an Adult Stem Cell?
A
- an undifferentiated cell, found amongst differentiated cells in a tissue or organ that can renew itself and differentiate to yield some or all of the major specialised cell types of the tissue or organ (stemcells.nih.gov)
- they must be able to self renew
- they must be long lived
- they are generally multipotent
2
Q
Where are stem cells found?
A
- in niches in tissues capable of regeneration
- tissues with constant turnover
- haematopoietic system
- Location: bone marrow
- niche components: macrophages, T reg cells, osteoblasts, adipocytes, nestin, MSCs, CAR cells, glia
- intestine
- fast-cycling: base of crypt, slow-cycling: +4 position
- paneth cells, mesenchymal cells
- interfollicular epidermis
- basal layer of epidermis
- dermal fibroblasts
- hair follicle
- bulge
- K6 bulge, dermal pilla, adipocyte precursor cells, subcutaneous fat, dermal fibroblasts
- haematopoietic system
- Tissues with low or no turnover
- brain
- subventricular zone, subgranular zone
- ependymal cells, vasculature
- skeletal muscle
- between the basement membrane and the muscle fibres
- myofibres (?)
- brain
3
Q
How are adult tissues maintained?
A
- by a balance between cell division and cell growth
- the balance is not rigid
- e.g. wound healing, blood cell replacement
- skin is replaced at a regular rate until the wounding of the epidermis induces epithelial cells (perhaps due to loss of contact inhibition) to increase their rate of proliferation
- tissues have different capabilities for renewal: bone marrow >> epidermis >> liver, muscle >> nervous tissue
- renewal is constant in some tissues and only occurs in after wounding in others - cf. bone marrow and liver
4
Q
What are some parameters by which rate of production of blood cells may be regulated?
A
controllable parameter
- frequency of stem-cell division
- probability of stem-cell death
- probability that stem-cell daughter will become a committed progenitor cell of the given type
- divison cycle time of committed progenitor cell
- probability of progenitor-cell death
- number of committed progenitor-cell divisions before a terminal differentiation
- lifetime of differentiated cells
5
Q
How can a stem cell divide to produce daughters with different fates and maintain homeostasis?
A
- environmental asymmetry
- haemopoietic stem cells
- divisional asymmetry
6
Q
What is an example of stem cells dividing by environmental asymmetry?
A
- haemopoietic stem cells
- stem cell ‘sitting on’ stromal cell
- signal to this cell through a variety of receptors - Kit and Kit ligand on stromal cell
- divides along a plane so one daughter no longer sees this ligand from the stromal cell and therefore commits to differentiation or dies
7
Q
What is an example of divisional asymmetry?
A
- neuroblasts
- neuroblasts have asymmetrically localised protein components
- division in the one plane results in symmetric division
- division in the perpendicular plane results in asymmetric division
8
Q
What is population asymmetry?
A
- a third option
- is this more common?
- balance between proliferation and differentiation is achieved at a cell population level
- to achieve homeostasis both outcomes must occur with similar frequencies – hence the stem cell number will remain constant
9
Q
How often do stem cells divide?
A
- stem cells divide rarely, but produce transit amplifying cells which are committed to differentiation and reproduce rapidly
- slow cycling populations but different populations divide at different rates
- committed transit amplifying cell
10
Q
What are epidermal stem cells
A
- epidermal stem cells are located in the basal layer
- descendants of stem cells, which will become karatinocytes, become detached from the basal lamina, divide several times, and leave the basal layer before beginning to differentiate
- in the intermediate layers, the cells are still large and metabolically active
- whereas in the outer epidermal layers, the cells lose their nuclei, become filled with keratin filamets, and their membranes become insoluble due to deposition of the protein involucrin
- the dead cells are eventually shed from the skin surface
11
Q
What are intestinal epithelial stem cells?
A
- cancer is a clonal disease of regenerating tissues
- it is a pertubation of normal growth controls - cell division, differentiation, growth and death
- cancer cells proceed along a path of uncontrolled growth and migration that can kill the organism
- there is a progression from benign localised growth to malignancy in which the cells metastasize – migrate to many parts of the body where they continue to grow
- the life of a cell in an intestinal crypt is 2-3 days - except stem cells
- so colon cancer is a disease of stem cells
- stem cells found in the base of crypts
12
Q
What about non-homeostatic regeneration? Does it occur from a stem cell population?
A
- some tissues have quiescent stem cell populations
- e.g. satellite cells in muscle
- only act when injury occurs
13
Q
What are examples of regenerating tissues in animals?
A
- limb regeneration in amphibians
- heart regeneration in zebrafish
14
Q
What are two types on non-homeostatic regeneration in adult animals?
A
- morphallaxis:
- little new growth, regeneration occurs by re-patterning of existing tissues and the re-establishment of boundaries e.g. regeneration in Hydra
- new boundary regions are established first and then new positional values are specified in relation to them
- epimorphosis
- growth of new, correctly patterned structures e.g. Newt (urodele amphibian) limb regeneration
- new positional values are linked to growth from the cut surface
- after amputation, limb cells reconstruct missing parts but no more
- reconstruction occurs by cell de-differentiation, proliferation and re-specification
- these can be illustrated by considering a gradient in positional value in the French Flag model
15
Q
What happens following limb amputation in an animal that is able to regenerate tissues?
A
- following limb amputation, there is a rapid migration of epidermal cells over the wound surface to heal the wound and form the apical ectodermal cap
- cells beneath the cap de-differentiate and proliferate to form a blastema
- the blastema consists of a heterogenous collection of restricted progenitor cells
- don’t de-differentiate completely
- as the limb regenerates these cells re-differentiate to form the missing parts of the limb
- proliferation of cells in the blastema is dependent on the presence of nerves
16
Q
How do we know that cells in the blastema do not revert to pluripotent stem cells?
A
- blastema cells retain their specification, even though they dedifferentiate
- have restricted developmental potential
- took some cartilage from a GFP-expressing limb and transplanted on a wild-type
- they then amputed and watched to see what happened
- the green cells only ended up in the cartilage in the bone i.e. the cells that were originally cartilage, after de-differentiation and formation of blastema, only formed cartilage
- cells keep a memory of their tissue of origin during limb regeneration
- blastema cells are a heterogenous population of progenitor cells with restricted differentiation potential
17
Q
Why are nerves important in limb regeneration?
A
- nAG supplied by limb nerves (secreted from Schwann cells) is required for regeneration
- nAG binds to a cell surface protein, Prod1 and can substitute for the presence of nerves
20
Q
Where do the regenerating cells arise from?
A
- lineage tracing has shown that regenerated cardiomyocytes are derived from de-differentiated cardiomyocytes
- a tamoxifen-inducible Cre recombinase enzyme was driven by a cardiomyocyte specific promoter which acted on a reporter transgene to cause red cardiomyocytes to become green (express GFP)
- after injury, regenerated tissue all derived from green cells (i.e. it came from existing, differentiated cardiomyocytes)
- cre/lox
- inducible cre - only causes recombination when in the presence of a drug
21
Q
What causes proliferation of cardiomyocytes?
A
- retinoic acid produced by the endocardium acts as a paracrine signal to stimulate localised proliferation of cardiomyocytes
- perhaps by manipulating scarring, retinoic acid production and cardiomyocyte de-differentiation in mammals we may be able to assist human heart regeneration after myocardial infarction
