The human body is an elaborate mass of cells, each with a defined role and organised into tissues and organs. Every cell, whether in liver, heart or brain takes on its function by growing into a particular cell type with a characteristic shape, structure and purpose. The cells grow into a particular cell type through a process called cell differentiation. Differentiation begins early in embryonic development and eventually leads to over 200 diverse cell types each with a different activity from the human body.
A stem cell is the popular name for a cell that is undifferentiated or immature. If a cell is undifferentiated, it has not yet begun to develop to maturity - to differentiate - into one of more than 200 types of tissue found in the human body, e.g. blood, bone, fat, brain etc.
Stem cells have been studied since the 1960s when they were first discovered of being capable of regenerating several types of specialised cells. This means that stem cells are able to transform into any type of cell in the body. This capacity of stem cells allows them to replenish dead and damaged cells and thus have generated a considerable amount of scientific and medical research.
Due to their renewal and regenerative potential, certain stem cells hold great promise for the treatment of diseases in which cells and tissues are diseased or damaged. This is the case in conditions such as Parkinson's and Alzheimer's diseases, stroke and heart disease, diabetes and arthritis
Stem cells are found in different locations throughout the body and are present from just after conception until the death of the organism. Stem cells are most commonly defined by the stage of development of the organism from which they are derived and so they are typically referred to as either embryonic, foetal or adult stem cells, depending on the source of these cells.
Stem cells are also classified into three main categories according to their ability to differentiate into specialised cell types. They are said to be totipotent, pluripotent or multipotent, depending on how many cell types they can give rise to. Some stem cells are able to develop into an entire organism and are, therefore, referred to as totipotent. Those referred to as pluripotent are able to differentiate into all of the specialised cell types of the body but cannot generate an entire organism on their own. Finally, multipotent stem cells can only differentiate into a particular subset of specialised cell types.