Are you talking about the spell "scrying"? I'm going to assume yes.
I wouldn't allow a single cell to count. How would you even know you had one? I would require that you have enough of a sample to see and handle without much effort.
The spell says body part, lock of hair, bit of nail, etc. Body part, maybe an eye, finger, toe, or some other body part. Lock of hair, so maybe a few dozen strands at least an inch long. Bit of nail, finger nail or toe nail. It certainly lowers the bar on body part stuff. At this point, I would accept a still bloody knife used to stab the individual earlier that day. Or a handkerchief that has dried blood.
