Stem Cells
- 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)
- This 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, as are the embryonic cells up to the 16-cell stage of human embryo development
- 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 (e.g. bone marrow stem cells differentiate into different blood cells)
- Unipotency - unipotent stem cells are adult cells that can only differentiate into their own lineage, e.g. 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
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.
Examiner Tip
Remember the two key properties of stem cells are that they can self-renew (capacity to divide) and can differentiate.Make sure you learn the three levels of potency of stem cells described above, and what range of cell types these stem cells can differentiate into.Don’t forget, while still classed as stem cells (as they can divide any number of times), only a limited range of specialised cells can be formed from adult stem cells as they have already partially differentiated. For example, stem cells in bone marrow can only produce cells that differentiate into the different types of blood cells.