What are the two main types of stem cells?

Stem cell research is a topic that I want to get into in more detail soon, but this question is a fantastic way to kick off.

Stem cells may be defined as cells that exhibit two characteristics:

• Self-renewal - Stem cells can divide and divide and divide and still potentially remain undifferentiated.


• Potency - In this context, 'potency' refers to the potential of the stem cells to differentiate into specialised tissue.

Totipotent cells have the potential to become any cell at any stage in our development, including both adult cells and any extraembryonic (literally, 'outside the embryo') tissue, such as the placeta. A fertlised ovum (zygote) as well as all the cells up to the morula phase of development are totipotent.

Pluripotent cells have the potential to become any adult cell type but not any extraembryonic cells. Thus you wouldn't be able to 'grow' a human being with them alone.

Lastly, multipotent cells are somewhere in between pluripotent and mature cells in terms of specialising. They are capable of differentiating into mature cells of only a limited variety of types (e.g. just the various types of cells found in the heart).

The two main classes of stem cells that you've asked about are embrionic stem cells and adult stem cells.

Embrionic stem cells, as their name suggests, come from an embryo. Specifiically, they are the pluripotent cells taken from the inner cell mass of a blastocyst. This involves destroying the embryo, which is a cause for ethical concern in certain quarters (more on this topic in the near future). Amongst the scientific difficulties of this class of stem cells are the formidable challenges of guiding a pluripotent cell down a particular useful cell line, and making sure that any therapeutic stem cells aren't simply rejected by the potential patient's body.

By contrast, adult stem cells are stem cells taken from a developed organism (including, confusingly, a child!). They are generally mutipotent, although scientists are working on ways of reversing this partial specialisation and making them pluripotent. Also, they are few in number, making them sometimes hard to isolate. Lastly, they are more likely to have abnormal DNA, having had a substantially more prolonged contact with mutagens like ultraviolet radiation and free radicals. However, since embryos don't have to be destroyed in the process, they do bypass the moral objections that some people construct to oppose embrionic stem cell research.