Embryonic stem cells are harmful to the recipient because they tend to grow tumors, and safer alternatives that do not use embryos are available, but many [[abortion]] supporters still insist on using
'''Embryonic stem cells''' are taken from a young embryo. They are generally taken from the inner cell mass of a [[blastocyst]]. They may also be derived from early blastomeres taken from a "morula", which is an embryo of 4 to 32 cells. An embryo reaches the blastocyst stage about 4-5 days after [[fertilization]]. At that point they contain about 50-150 cells. However, implantation (attaching to the uterine wall) does not occur until at least 9 days after fertilization. Neurulation (primordial development of the central nervous system) occurs about a week later.<ref>http://9e.devbio.com/</ref>
Embryonic stem cells can differentiate into the three "germ layers": [[ectoderm]], [[endoderm]] and [[mesoderm]]. The 220 types of cells in humans are all based on these germ layers. Some scientists have shown effectively that embryonic stem cells have greater differentiation potential, and divide into greater numbers, than [[adult stem cells]] can.