I am culturing CCD18Co colon fibroblast with DMEM + 10% FBS + 1% P/S. I used DMEM with 0.5% FBS on the subculture day and replace it with 10%FBS on the following day. However, our lab members suggest using this cell line for experiments for only 4 passages (stock: P16 --> use until P20 only). Is this the common thing when you culture fibroblast or is it specific for CCD18Co? What changes should I make in the protocol so that I can use this cell line for a higher number of passages?