Stem cells are stirring up great excitement in medical research. What are they and why are scientists so intrigued by them?
Most of the 300 trillion cells that make up the human body are fully specialised for particular functions in organs such as the heart or the brain, or in tissues like muscle, fat and bone. Others play a supply or defensive role in the blood or immune system.
Each cell type has a specific lifespan and function, which is dependent on the desired activity of the cell. Some cells are replaced, others live for the duration of a person’s life. For example blood cells only live for up to a few months, and are replaced at a rate of several billion each day. Whereas brain cells may last a lifetime. Stem cells are the foundation of normal growth and development of any organism and serve as a biological repair system for the body.
Stem cells are found in the early embryo, the foetus, placenta, umbilical cord, and in many different tissues of the adult body. Different types of stem cells have different levels of potential. A stem cell that can become every type of cell in the body is called pluripotent and a stem cell that can become only some types of cells is called multipotent.
Stem cells are different from other cells in the body in three main ways:
- Stem cells are unspecialised. They have not developed into cells that perform a specific function.
- Stem cells can differentiate. This means they can divide and produce cells that have the potential to become other more specific cell types, tissues or organs. These new cells and tissues are used to repair or replace damaged or diseased cells in the body. Once cells have differentiated, they have less capacity to form multiple different cell types, and become ‘committed’ to becoming a particular cell type. Skin stem cells, for example, give rise to new skin cells when needed, to assist regeneration after damage and as part of the normal ageing process.
- Stem cells are capable of self-renewal. Stem cells are able to divide and produce copies of themselves, which leads to self-renewal. Once a cell has become specialised (has differentiated) to a particular tissue or organ, it has a very limited capacity to self-renew (produce new stem cells) but instead produces only cells relevant to that organ.