PC12 cells-passages

[Marek Treiman]

Hello netters,
What does the passage number for a batch of PC12s really mean? If you 
received a batch of cells from someone designated "passage 10", do you 
keep increasing this number or do you start from the beginning? The 
latter alternative seems to make little sense biologically - but on the 
other hand, if a continuing, "absolute" passage count were to be 
maintained, it would have to go all the way back to Greene and Tischler's 
work establishing PC12 line! Or do people set the number to "0" each time 
they freeze the cells - which again would not reflect their true "age"?!
Enlightened and enlightening comments very appreciated!! Cheers, Marek