Stem Cells (HL IB Biology)

• A stem cell is a cell that can divide (by mitosis) an unlimited number of times
• Each new cell (produced when a stem cell divides) has the potential to remain a stem cell or to develop into a specialised cell such as a blood cell or a muscle cell (by a process known as differentiation)

• After differentiation, a stem cell is no longer considered a stem cell
• However, some stem cells do remain in specific locations in the human body, this is called the stem cell niche
• Their presence gives the capacity for these tissues to regenerate and repair
• Some of the tissues which retain stem cells within a niche include
  • Bone marrow:
    • Bone marrow provides a niche for stem cells which are used to replace red blood cells, white blood cells and platelets
    • This is important for continual production of these cells which are required indefinitely
  • Hair follicle:
    • This niche is located at the root of the hair where the hair is anchored into the skin
    • Stem cells here promote continual hair growth
• The environment provided by the niche must have
  • The ability to maintain an inactive state of the stem cells
  • The ability to stimulate stem cell proliferation and differentiation

• The ability of stem cells to differentiate into more specialised cell types is known as potency
• There are four types of potency:
  • Totipotency – totipotent stem cells are stem cells that can differentiate into any cell type found in an embryo, as well as extra-embryonic cells (the cells that make up the placenta)
    • The zygote formed when a sperm cell fertilises an egg cell is totipotent
    • Also the embryonic cells up to the 16-cell stage of human embryo development are totipotent
  • Pluripotency - pluripotent stem cells are embryonic stem cells that can differentiate into any cell type found in an embryo but are not able to differentiate into extra-embryonic cells (the cells that make up the placenta)
  • Multipotency -  multipotent stem cells are adult stem cells that can differentiate into closely related cell types 
    • For example bone marrow stem cells differentiate into different blood cells
  • Unipotency - unipotent stem cells are adult cells that can only differentiate into their own lineage
    • For example heart muscle cells (cardiomyocytes) can generate new cardiomyocytes through the cell cycle to build and replace heart muscle. Most cells in animal bodies are unipotent

Stem Cell Potency Diagram

There are different levels of potency that cells can have. Totipotent cells have the highest potency and can therefore differentiate into any type of cell. Unipotent cells have the lowest potency, only being able to divide into one cell type.