MSC in culture - stop growing? - Mesenchymal stem cells in culture randomly stop expanding (Jun/07/2011 )

I have rat mesenchymal stem cells derived from bone marrow in culture. They expand very well, but lately they have randomly stopped expanding. I am using the same culture media and split ratio as always, but sometimes after a split the cells just don't grow. They are all alive, and morphology looks correct, but they can sit in culture for greater than 2 weeks and never divide.
This has happened at various passage number, it has happened at pass 5, 10, and recently transduced cells (GFP+) at pass 23 stopped expanding.
Does anyone have experience with this? What could be causing it, and can I kick start them again?
Thanks!

what culture are you using for the MSC anyways? what did you do to isolate the MSC?

MSC is somewhat sensitive to contact inhibition.
If you make MSC culture confluent, they often stop growing even passage 5.
so it might be the reason??