Stem Cells Sphere - stem cells (May/31/2011 )

Why do stem cells form spheres? what's the gold standard test for examining self renewal property of stem cells?

i would highly appreciate if somebody drops his reply here.

Thanks.

have you read this?
http://www.ncbi.nlm.nih.gov/pubmed?term=21549325

have you seen this?
http://www.ncbi.nlm.nih.gov/pubmed?term=21549325