Human organs grown in chimeras.

Dr. Hiromitsu Nakauchi is a stem cell biologist and professor of genetics at Stanford University:

"If we just add human stem cells to an early embryo (of an animal), we will make a chimera in which all tissues, cells, organs are a mixture of human and animal cells. But one trick that I used is to use a host animal that has been genetically modified in such a way that this animal cannot form, for example, a heart, that is, organogenesis has been disrupted. When they grow up, only human cells derived from stem cells can create a heart, and host cells cannot. Therefore, although it is a mixture of two different types of cells, two different origins, the heart should consist entirely of cells derived from human stem cells. Thus, the body is a mixture of human and animal cells, but the heart is only made of human stem cells."