Re: Fetal stem cells transformation

The variety of cell types which can develop from fetal stem cells usually does not arise spontaneously. Stem cells develop and differentiate under the influence of so called growth (or survival) factors that push the cell into a developmental direction by changing its metabolism or gene expression pattern, i.e they determine when and where specific genes are active. Growth factors or comparable signals may be either cell-surface molecules (which means cells have to be in contact for the signal to be relayed) or soluble factors.

Researchers try to imitate the conditions in the body or certain aspects of these conditions by using culture media with different combinations of growth factors and checking what happens to the cells they study when they change the balance of the growth factors. Changes can be either apparent (as morphological changes are) or less conspicuous, e.g. changes in the amount and distribution of certain proteins, mRNA, etc. in the cell.

As stem cells progress along a certain developmental path their 'potency' to develop into other cell types becomes ever more restricted, e.g. a precursor for blood cells will usually not develop into a nerve cell but may develop into a variety of different blood cells, e.g. red blood cells, some lymphocytes (immune cells), etc.

What is called 'spontaneous' might as well be explained as a lack of understanding on the researchers' side. It is virtually impossible to describe a cell's (metabolic/biochemical or other) status at a specific point of development completely. Cells might vary only slightly, e.g in their enzyme levels or number of receptors on their cell surface, with this difference being virtually undetectable. Thus, cells are sometimes not the same (even when they look the same) and do not react the same way when exposed to 'identical' outside conditions. This is because they are already veering a certain developmental direction.

I hope this answer helps a little.
If you have further questions, send me an email.

P.S. An overview (2 pages) which is not too terribly technical and gives an outlook on the more practical applications of human stem cell biology can be found in Nature Medicine: Keller G and Snodgrass HR , Nature Medicine Feb 1999; vol. 5, number 2, pp. 151-152.