L7- Stem cells

Question 1

Requirements of a stem cell

Answer 1

• A stem cell can make other stem cells (self renew)
• Stem cells are competent to respond to differentiation cues
• Stem cells differentiate to form other cells and tissues

Question 2

Definition of the stem cells potency hierarchy

Answer 2

TOTIPOTENT (entire potential)

• a cell that can give rise to an entire organism, including the tissues needed to support development of an embryo
• fertilised egg, early embryo

PLURIPOTENT (many potential)

• a cell that can give rise to the tissue of the embryo
• embryonic stem cell, induced pluripotent stem cell

MULTIPOTENT (several, more than one potential)
- a cell that has restricted differentiation potential, generally limited to some or all of the cells of the tissue where it resides

UNIPOTENT/ terminally differentiated

Question 3

How definitive are molecular markers?

Answer 3

CORRELATIVE EVIDENCE

• Proteins expressed on stem cells may coincide with stem cell phenotypes
• Not useful if expressed by other cell types
• Sometimes absence of a differentiated cell marker is used to isolate the stem cell

FUNCTIONAL EVIDENCE
- Proteins expressed on stem cells may determine stem cell behaviour

Question 4

Tools to study stem cells and their challenges

Answer 4

• Share common molecular markers (Antibody staining)
  > Not suited for microscopy (few stem cells sit on isolation not visible)
  > Many stem cells types still don’t have molecular markers. Difficult to purify
• Share common molecular markers (Transcriptomics)
  FANTOM molecular atlas
  > Tissue stem cells are rare and may not be represented in the atlases
• Capacity to differentiate:
  • ex vivo (cell based assays)
  • in vitro (transplantation)
    > Transgenic approach not suited for humans
    > Differentiation in a dish doesn’t mean function in vivo

Question 5

Evidence for tissue stem cells

Answer 5

Adult stem cells coexist in a tissue niches

• hematopoietic stem cells (HSC) gives rise to all cell types
• mesenchymal stromal cell (MSC) gives rise to adipose, cartilage, bone, muscle, stroma
• endothelial stem/ progenitor cell gives rise to vasculature

Evidence of residency?

• animal studies
• organ transplantation

Evidence of multipotency?

Evidence of function?
- long lived organ donation:
bone marrow, skin, cornea (eye)

Question 6

Issue with Mesenchymal stromal cells

Answer 6

• the cells are poorly defined
• the evidence from clinical trial is scarce
• the mode of action is not known
• the claims do not match facts

Question 7

What is an organoid?

Answer 7

Pluripotent cells make organoids:

• Pluripotent stem cells respond to physical and biochemical signals
• Differentiation proceeds developmental steps
• Differentiated cells form into structures with recognisable features of tissue

ORGANOID:

• In vitro tissue derived from a cultured stem or progenitor cell
• 3D structure
• May be derived from one cell linage e.g. gut organoids are epithelial
• May include multiple cell linages
• Small- form spontaneously but are limited by diffusion of metabolites
• Provide models of development and disease
• Can be long lived

Question 8

How do we know if a cell is a stem cell?

Answer 8

• Self-renewal: a single cell can repopulate a tissue and give rise to differentiated progeny as well as more stem cells
• Identified in transplantation assays with marked cells or evidence for lineage tracing of what they can become
• Descendants of stem cell are shown to be functional