Properties of stem cells
Stem cells have several key properties that distinguish them from other cells. Firstly, they are able to divide and make copies of (renew) themselves numerous times. Secondly, they are able to mature (differentiate) into one of the 200 or so specialised cell types found in the body, such as muscle cells and nerve cells.
Although stem cells can produce more than one type of cell, their potency - the capacity to differentiate into specialised cells - varies between different types.
Totipotent cells - the most potent - are found in the earliest stages of development, before the blastocyst stage. The 'total potential' of these cells means that they can become any kind of cell in the body and are able to produce an entire organism, including the 'extraembryonic' tissues, such as the placental cells that do not become part of the fetus.
Plant cells retain totipotency throughout life, so a cutting from a mature plant can give rise to a whole new plant. The same is not true for humans. After around five days of development a pre-embryonic ball of cells called a blastocyst is formed. The inner cell mass of this blastocyst contains embryonic stem cells, which can give rise to all cells in the body, but cannot produce the extraembryonic tissues needed to create an entire organism. Embryonic stem cells are known as pluripotent.
Less potent still are multipotent cells, which can give rise to a range of cells. Multipotent adult stem cells reside in the body to produce the set of cells needed to maintain and repair particular organs and tissues. ‘Unipotent’ stem cells (e.g. skin cells) give rise to only one type of cell but are able to renew themselves.
Strictly speaking, only totipotent and pluripotent cells are true stem cells as they can give rise to any type of body cell. However, the progenitors (multipotent and unipotent cells) are often referred to as stem cells.
Image: Human embryonic stem cell; Annie Cavanagh, Wellcome Images
