Embryonic stem cells

These stem cells are extracted from an early stage embryo a few days after fertilisation. They are the most powerful type of stem cell as they have the potential to grow into every single type of cell in the body. But they are also the most controversial. Embryonic stem cells either come from “spare” embryos from fertility treatments or from cloning adult cells. Either way, human eggs are required, and they are in short supply. Many people object to this type of research on ethical grounds

Foetal stem cells

These come most commonly from umbilical cord blood but can be obtained from amniotic fluid and the placenta. They are not as adaptable as embryonic stem cells but are more adaptable than adult stem cells.

Adult stem cells

It is now clear that most, if not all, tissues in the body contain stem cells that produce more of that tissue. Heart stem cells will produce a range of heart cells. Liver, liver cells and so on. They are partially differentiated in that they can only produce a small subset of tissue types. There is evidence, though, that bone marrow cells in particular might be able to produce a much wider range of cells than previously thought. Adult stem cells are reasonably easy to obtain by biopsy and carry none of the controversy of embryonic cells. They are, though, the least plastic (adaptable) and the least potentially useful.

Induced pluripotent stem cells

A very recent development and perhaps the holy grail of stem-cell research. These adult stem cells have been persuaded to revert to an embryonic state. They can produce every single tissue type but their production does not involve embryos. At present there is a problem with using them for treating patients. The process that reverts them to an embryonic state involves introducing four genes, one of which is a cancer-causing gene. While useful for research, other techniques that do not include this gene will be needed before studies in human beings can begin.

Differentiation

This is the process by which stem cells become other types of tissue. For example, bone marrow stem cells differentiate into the many different types of white blood cells in the body. Working out the triggers that causes them to differentiate, or change, in this way is a central element of stem-cell research. Embryonic stem cells are the most undifferentiated cell type whereas mature organs such as heart, liver and brain, contain fully differentiated cells.

Plasticity

The degree to which a stem cell gives rise to other types of cell.

Autologous

Stem cells grown from a patient’s own tissue ready for transplant back into the patient. They will not be rejected as foreign by the immune system.