Embyronic stem cell lines, which were originally produced from the inner core of human blastocysts as well as those of other mammals, are now widely established in the research community using traditional in vitro culture. The cell lines preserve their undifferentiated state and normal nuclei during subculture and they remain capable of differentiating into virtually any type of tissue. Omnipotent embyronic stem cells can be turned into pluripotent stem cells (such as neuronal stem cells, muscle stem cells, vascular endothelial stem cells, and hematopoietic stem cells) by the application of specific culture conditions, and then accordingly differentiate into neurons, muscle cells, vascular endothelial cells and blood cells. However, unlike the fertilised egg, a cluster of embyronic stem cells cannot independently develop into a human being (or other animal).