According to the Longevity Meme (2007), the type of stem cells which is more controversial is the embryonic stem cells which are found in embryos. These stem cells have the potential to replace tissue, and to be used as regenerative medicine, but a problem with all stem cells is preventing transplant rejection. Embryonic stem cells are very controversial because in order to create an embryonic stem cell line, the destruction of a human embryo is required.

Figure 8: A Human Embryo
Source: http://www.scienceclarified.com/images/uesc_04_img0230.jpg