What is the most controversial source of stem cells?

Embryonic stem cells are recognized as the most controversial source of stem cells due to the ethical issues surrounding their derivation. These cells are obtained from embryos, typically created through in vitro fertilization, which raises significant moral and ethical concerns regarding the status and rights of the embryo. The process of harvesting embryonic stem cells often results in the destruction of the embryo, leading to debates about when life begins and the ethical implications of using embryonic material for research and medical treatment.

In contrast, adult stem cells, induced pluripotent stem cells, and pluripotent stem cell lines (which are derived from reprogrammed adult cells) generally do not provoke the same level of controversy because their collection does not involve the same ethical dilemmas associated with embryos. While pluripotent stem cell lines can be created using embryonic cells, the ethical concerns primarily pertain to the original source of those cells rather than the lines themselves. Thus, embryonic stem cells stand out as the most controversial option due to the deep ethical and moral questions they raise.