Cell culture lines are cells, taken from donor tissue, that have been divided and separated over and over and over — providing researchers with reliably identical "families" of cells that can be used to biomedical research. Some, like the now-famous HeLa line, are derived from cancerous tissue and replicate indefinitely. Others, like WI-38, will only divide a set number of times (in the case of WI-38, it's 50), but new cells can be frozen at any point and stored. When you thaw them out later, they'll pick back up dividing from the point in the 50-division cycle where they were when frozen.
WI-38 is a particularly important cell culture line. Used extensively in the development of vaccines, these are the cells that helped create the vaccine for Rubella, a disease that, just a few decades ago, used to kill and maim many fetuses whose mothers' became infected. Between 1962 and 1965, it's estimated that rubella infections caused 30,000 stillbirths and left 20,000 children with life-long disabilities.
But WI-38 is controversial. That's partly because the cells that founded the line came from the lung tissue of a fetus that was legally aborted during the fourth month of pregnancy by a woman in Sweden in 1962. At Nature News, Meredith Wadman has a fascinating long read about the moral and ethical issues surrounding WI-38. This isn't just about the abortion question. Also at issue: Did the fetus' mother consent to tissue donation? And are we okay with the fact that she and her family have never received compensation, despite the money that's been made off selling WI-38 cell cultures?
Medical Research: Cell Division by Meredith Wadman in Nature News