cancer biology and growth factor mechanisms. - (Feb/07/2012 )

I am a beginner biology student and I was wondering if there is the possibility that cells can differentiate between secreting a growth factor and not secreting a growth factor. For example, if a cell encounters another cell that needs a growth factor and another cell that doesn't need the growth factor, can it only secrete the growth factor when it encounters the first cell but not the second. If this is possible, would anyone please provide reference papers so I can understand the process better? Thank you!

you should find examples on the level of immuno cells...

you may refer to the regulation of hedgehog/norch pathway during development.