Stem cells are a class of undifferentiated cells that are able to differentiate, or mature, into specialised cell types. Commonly, stem cells come from two main sources: embryos which are formed during the blastocyst phase of embryological development (embryonic stem cells), and adult tissue (adult stem cells). Both types are generally characterised by their potency or potential to differentiate into different cell types, such as skin, muscle or bone1.
As well as the ability to ‘differentiate’ into another cell type with a specialised function, stem cells are also characterised by the fact that they are able to divide and multiply to form more cells of the same kind.
These two distinct properties mean stem cells can serve as an internal repair system, dividing without limit to replenish other cells as long as the person is still alive.
Stem cells differ from other cells in the body due to the following characteristics:
- they are immature or undifferentiated; that is they have not yet developed to a mature cell type or taken on a function that a mature cell fulfils
- when cultured in a laboratory they can divide and produce an almost indefinite number of copies of themselves
- in the human body, on the other hand, the division of the stem cells are very tightly controlled. In the adult being, they often divide to make a copy of themselves plus a daughter cell that goes on to differentiate into a mature cell of the body. Thus their numbers remain controlled
- they are capable of ‘differentiating’ and developing into different tissues within the body, if the right stimuli are provided – for example into skin, blood or bone
- they can colonise and repair damaged or sick tissues or organs, replacing any defective cells with healthy ones, including in the gut and bone marrow. However, in other organs such as the heart and the pancreas, stem cells only divide under special conditions.
Stem cells are central to the normal growth and development of animals and humans. There are various types of stem cells found at different stages of human development and in different parts of the body, all of which are of interest to researchers:
-
umbilical cord blood: during pregnancy cells from foetal bone marrow move into the umbilical cord and placenta. Umbilical cord blood represents a potential source for blood stem cell transplantation. However, despite the many cord blood banks that have been established, they are not widely used and cord blood stem cells are most commonly utilised in paediatric settings. Much effort is focused on expanding stem cells from cord blood for transplantation
-
embryonic: these stem cells are found in fertilised eggs that are only a few days old and are the only cells that can develop into any cells within the body
-
foetal: during embryonic development there is a gradual shift from stem cells to more mature cell types as we form parts of our body. The abundance of stem cells therefore decreases as the embryo grows
-
specialised adult or tissue-specific: the tissues and organs in our bodies consist predominantly of specialised cells which have specific functions according to the part of the body in which they are found. These cells are kept healthy and repaired by resident unspecialised adult stem cells which are still in the early stages of development and can evolve into the types of cells required
-
bone marrow: these stem cells are found in our bone marrow and bones
-
induced Pluripotent Stem Cells (iPS cells): in 2006 scientists discovered how to ‘reprogramme’ specialised cells in the laboratory so that they would act like embryonic stem cells. These are known as induced pluripotent (iPS) cells. Like embryonic stem cells, iPS cells have the ability to develop into the cells of any organ or tissue, although they sometimes behave slightly differently. The techniques used to create iPS cells still need refinement before they can be used for safe and effective therapies but considerable advances are being made in this area.
-
www.medicalnewstoday.com
