What are stem cells?
Our tissues are largely made up of specialised cells and there are about 200 different types. Some of the most well known include: red blood cells, neurons, adipocytes, fibroblasts or osteocytes. These cells have a shorter lifespan than our lifespan and they must therefore be replaced when they die.
Stem cells are cells which have not yet been differentiated or specialised to perform a permanent and specific function. They have both the capacity to renew and to differentiate themselves into several types of cells depending on the requirements of our body.
Embryonic stem cells and adult stem cells
Stem cells are not all the same – there is a fundamental distinction between embryonic stem cells and adult stem cells.
Embryonic stem cells, present in the embryo of one week, are capable of producing all types of different cells which are found in our bodies (totipotent stem cells). Their use is not ethical as the extraction of an embryonic stem cell leads to the destruction of the embryo itself.
Whereas adult stem cells, present in certain tissues and organs, can give rise to different specific cell types (multipotent or pluripotent stem cells) or give rise to a specific type of cell (unipotent stem cells). Contrary to the use of embryonic stem cells, cells of adult stem cells do not pose any ethical problem as they are not derived from destroyed human embryos.
Haematopoietic stem cells and mesenchymal stem cells
Adult stem cells are differentiated into two main types: haematopoietic and mesenchymal.
- Adult haematopoietic stem cells specialise into blood cells: white blood cells, red blood cells and platelets.
- Adult mesenchymal cells specialise into a multitude of cells constituting for example muscular, cartilaginous, boney, connective or adipose tissue.
Where are the stem cells found?
There are different types of stem cells present in nearly all tissue. Nevertheless, some parts of the human body are particularly interesting sources for medicine.
The three main sources are:
- bone marrow,
- umbilical cord,
- adipose tissue.
Bone marrow was the first source discovered and it has been used for 60 years.
At the end of the eighties, umbilical cord blood proved in its turn to be an extremely rich source of rich haematopoietic stem cells which present very high therapeutic qualities and incomparable accessibility.
The collection of umbilical cord stem cells is in fact totally devoid of risk and is carried out in a simple manner. This explains why over the last 30 years this alternative to bone marrow is being used more and more.
Furthermore, in comparison with haematopoietic stem cells of bone marrow donors, haematopoietic stem cells of the umbilical cord blood seem to cause less immune rejection or a graft-versus-host disease complication. Therefore, cord blood does not need to be perfectly compatible with the patient, contrary to bone marrow.
Other stem cells which present different properties in addition to those of the umbilical cord have also since been discovered and are starting to be used as for example adipose tissue and Wharton’s jelly enveloping the umbilical cord tissue (mesenchymal stem cells).