Why are stem cells shiny?

I'm in an undergraduate SC bio class/lab. One of our most fundamental criteria for recognizing ES cells is them being small and shiny. No one i've spoken with, however, seems to know WHY they are so shiny.... any ideas? Someone hypothesized its something to do with the high nucleus:cytoplasm size ratio, but I don't see how this could work. Thanks!

 I'm assuming your referring to embryonic stem cells or iPS cells grown on feeder layers (generally mouse embryonic fibroblasts).  When viewed under phase contrast microscopy, the differences in refractive index of cells and their various parts make some objects brighter than others.  The size, shape and makeup of ES cells makes them phase bright compared to the flat fibroblasts that they are growing on.  

If you google phase contrast microscopy, you will get a lot of hits that explain the what is going on in terms of wavelengths etc.

ESC, without a feeder layer--just media and LIF.  And I'm not absolutely possitive, but none-the-less pretty sure, that we're NOT using phase contrast microscopes.

But I think you're on to something about the structural aspects... when the ES cells are attached to gelatin layer, their podia/podia-like extensions stop appearing "shiny"... so maybe its something specific to the z-axis structure.

More details on 'shiny': the perimeter of the cells have almost a white glow; a general brightness at their periphery.