Here is a picture of a teratoma, a type of germ cell tumor, which has often been found associated with the use of embryonic stem cells in animal studies. These teratomas can contain mature processes such as limbs, hair, muscle and bone.
Today a story in Nature News provides an article entitled "Tumours spark stem cell review". In the Nature News article, they discover that a young boy who had received treatments of fetal neural stem cells for a neurodegernerative disease (ataxia telangiectasia) in Moscow was found riddled with tumors in his brain and spinal cord. The tumors came from his fetal neural stem cell treatment.
These types of dangers from embyronic stem cell treatments are widely known within the scientific community, but often are not discussed. Persons entering into future clinical trials with embryonic stem cells should understand the risks invovled. They are substantial.