The group behind the tapes accuses Planned Parenthood of selling fetal tissue for profit — which is illegal and which Planned Parenthood denies doing. House Republicans plan to investigate. This may be just one more battle in the nation’s long war over abortion, but the dispute has raised questions about who the buyers and sellers are, what fetal tissue is used for and what the law allows.

Scientists at major universities and government labs have quietly been using fetal tissue for decades. They say it is an invaluable tool for certain types of research, including the study of eye diseases, diabetes and muscular dystrophy. Nevertheless, some agree to talk about it only if their names and their universities’ names are withheld, because they have received threats of violence from abortion opponents. Companies that obtain the tissue from clinics and sell it to laboratories exist in a gray zone, legally. Federal law says they cannot profit from the tissue itself, but the law does not specify how much they can charge for processing and shipping.