Embryonic stem cells are pluripotent cells derived from the embryo of humans and other animals during the blastocyst stage of development, before implantation. In human embryos this stage is reached 4–5 days after fertilization, at which time they consist of 50–150 cells. The cells are the subject of considerable research into potential therapeutic use, including the treatment of diabetes and heart disease. They are also studied for clinical therapies, models of genetic disorders, and cellular/DNA repair. However, adverse effects in the research and clinical processes such as tumours and unwanted immune responses have been reported. Isolating the inner cell mass results in destruction of the blastocyst, a process which raises ethical issues, including whether or not embryos at the pre-implantation stage should have the same moral considerations as embryos in the post-implantation stage of development.Photograph: Ryddragyn