lecture 12: adult stem cells and regeneration Flashcards Preview

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What is an Adult Stem Cell?

  • 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 


Where are stem cells found?

  • 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 
  • 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 (?)


How are adult tissues maintained?

  • 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 


What are some parameters by which rate of production of blood cells may be regulated?

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 


How can a stem cell divide to produce daughters with different fates and maintain homeostasis?

  • environmental asymmetry
    • haemopoietic stem cells 
  • divisional asymmetry 


What is an example of stem cells dividing by environmental asymmetry?

  • 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 


What is an example of divisional asymmetry?

  • 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 


What is population asymmetry?

  • 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 


How often do stem cells divide?

  • 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 


What are epidermal stem cells 

  • 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 


What are intestinal epithelial stem cells?

  • 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


What about non-homeostatic regeneration? Does it occur from a stem cell population?

  • some tissues have quiescent stem cell populations 
  • e.g. satellite cells in muscle 
  • only act when injury occurs


What are examples of regenerating tissues in animals?

  • limb regeneration in amphibians 
  • heart regeneration in zebrafish 


What are two types on non-homeostatic regeneration in adult animals?

  • 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 


What happens following limb amputation in an animal that is able to regenerate tissues?

  • 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  


How do we know that cells in the blastema do not revert to pluripotent stem cells?

  • 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 


Why are nerves important in limb regeneration?

  • 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 


Can humans regenerate limbs?

  • no
  • but finger tip regeneration is efficient in young children 
  • crucial on the stem cells in the nail bed


What is a major cause of morbidity and mortality in australia? Can it be treated using regeneration?

  • heart failure 
  • myocardial infarction results in scarring and heart tissue that cannot contract properly 
  • the tissue does not regenerate 
  • theoretical approaches to regenerative heart therapy 
  • the zebrafish heart does regenerate efficiently after wounding 
  • why? can this tell us about the capacity of the human heart to regenerate? 
  • firstly, zebrafish heart injuries do not scar like human hearts 
  • scarring prevents regeneration 
  • if we could understand why the scarring doesn't occur this could be a great leap forward 


Where do the regenerating cells arise from?

  • 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 


What causes proliferation of cardiomyocytes?

  • 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 


What are the review points?

  • where are adult stem cells found and what parameters can regulate them?
  • what different cell division strategies are used in homeostatic regeneration?
  • what are transit-amplifying cells?
  • which tissues only regenerate after injury?
  • what regeneration strategies are illustrated by morphollaxis and epimorphosis?
  • what is the process of salamander limb regeneration?
  • what is meant by cells keeping a memory of their origin in limb regeneration?
  • why are nerves required for limb regeneration?
  • describe how we know where regenerating cells arise in the zebrafish heart 
  • what is the role of retinoic acid in the regenerating zebrafish heart?