Whereas a skin cell or a liver cell, for instance, can reproduce only more skin or liver, stem cells, which are the most robust at the beginning of human development, are “master” cells. As a fertilized egg turns into an embryo, and then a fetus, those initial stem cells somehow show their daughter cells how to become skin, liver, eye, or bone.
干细胞之所以特殊是因为它能够产生有别于自身的形式。例如，皮肤细胞或者肝脏细胞只能复制更多的皮肤或者肝脏。作为人类发育开端中最为活跃的细胞，干细胞是“主”细胞。受精卵变成胚胎，然后变成胎儿，这些最初的干细胞不知怎的告诉他们的子细胞如何成为皮肤、肝脏、眼睛以及骨头。我们需要了解这一转变是如何被激活的。