stem cells

• All cells begin as undifferentiated cells- they do not have a specific function.

  these undifferentiated cells are stem cells

  these can divide by mitosis
• Stem cells are able to undergo cell division exponentially.

  Therefore they are a source of new cells for:

  • Growth
  • Development
  • Repair
• The ability to differentiate is known as potency.

  Greater the number of cells it can differentiate into, the greater potency -

  • pluripotent
  • totipotent
  • multipotent
• totipotent - can differentiate into any cell type
• pluripotent - can form all tissues but not whole organisms
• multipotent - only form a range of cells within a set tissue
• Totipotent cells-

  • These can differentiate into any cell for the organism and any extra embryonic tissues i.e. Umbilical cord.
  • The fertilised egg is Totipotent and remains so until around day 7 when the blastocyst forms.
  • After this the cell remains in a pluripotent state (can form all tissue types but not whole organisms)

  
• Multipotent cells-

  • Adult stem cells
  • Can differentiate into cells within the same tissue.
  • An example is Haemopoietic stem cells in the bone marrow.
  • These form the different types of blood cell.
• Erythrocytes have a lifespan of 120 days

  They have no nucleus - this means we need to continually produce them from their lineage - this is erythropoiesis
• Neutrophils typically survive for 6 hours. They are an essential part of the human immune system response so need to be continually replaced.
• The vascular cambium are secondary meristems that form in stems and roots after the tissues of the primary plant body have differentiated.

  The cambium is essentially meristem tissue which sits between phloem and xylem. Stem cells at the inner surface of the cambian differentiate into xylem ,whilst the outer surface become phloem.

  Meristem cells are pluripotent.
• • The xylem and phloem are formed from stem cells that are found in a meristem tissue between them, known as the cambium
  • The stem cells at the inner edge of the cambium differentiate into xylem cells. They lose their cytoplasm, deposit lignin in their cell walls and lose their end cell walls.
  • The stem cells at the outer edge of the cambium differentiate into phloem cells. They lose some of their cytoplasm and organelles, and develop sieve plates at the ends of the cells.
  • This cell differentiation is stimulated by hormones.
• Many diseases are a result of genetic defects. These diseases may be inherited or acquired. There is currently a lot of research into use of stem cells for curing these diseases- stem cell replacement therapy. This can be therapies utilising embryonic stem cells (more controversial) or using adult stem cells
• We do currently use stem cells in some capacity in medicine:

  • Burns treatment- quicker than making grafts, relatively successful
  • Drug trials- cultures of stem cells or of tissues produced from stem cells are used to test the effectiveness of certain drugs or to test for potential side effects/toxicity of drugs prior to human and animal trials
  • Study of developmental biology- stem cells are grown and monitored to see how and why they develop into set cell types
• There is growing research into iPSCs induced pluripotent stem cells

  These are usually derived from somatic cells, especially skin or blood cells, but have been reprogrammed and signalled back into a pluripotent state- they can then develop into many more cell types.

  Benefits:

  • From adult cells
  • Patient specific
  • Genes can be edited or added (in comparison to embryonic
• Problems -

  • Tissue rejection (this does not apply if using the patients own cells)
  • Issues with appropriate differentiation and adult stem cells are more likely to have acquired mutations.
  • Treated patients have a suspected potential to develop certain cancers (iPSCs use cell signals such as Cox2 and there is a potential link with cancer development)