Not the cells, but the growth medium. You need a growth medium to grow cells in a culture, because there's no support from the rest of the animal for growth. Normally the animal would provide blood, etc. to the tissue, but there is no animal in this case.

It's the medium, not the genes. Like someone else mentioned, it's called fetal bovine serum (presuming GP meant "cow" and not "mare") and it's used in large quantities.

Ah, that makes sense! Thank you!

I’m much less skeptical now