Specifically, they are derived from the inner cell mass at the stage before the blastocyst would implant itself on the uterine wall. The embryonic stem cell is pluripotent and can self-replicate indefinitely.
Embryonic stem cells proliferate extensively in the embryo, and can be isolated and grown in vitro (in a laboratory setting), where they will continue to replicate and also show the potential to differentiate. For research purposes, scientists have developed criteria to help further define the embryonic stem cell. They must be capable of long-term self-renewal and maintain a stable, complete karyotype (set of chromosomes).
Several techniques have been used to demonstrate the pluripotency of embryonic stem cells in mice. One test involves the injection of embryonic stem cells into histocompatable or immunodeficient (to prevent tissue rejection) adult mice. In the host animal, the cells develop into benign tumors called teratomas. These tumors contain cell types derived from all three embryonic germ layers. Microscopic examination typically reveals "gut-like" structures of epithelial tissue and smooth muscle, neural tissue, skeletal or cardiac tissue (which may spontaneously contract), cartilage and osseous tissue. This proves that embryonic stem cells maintained in vitro can behave like pluripotent cells in vivo.
Teratomas greatly resemble teratocarcinomas, which are testicular tumors that arise from human germ cells. Interestingly, these tumors contain the same tissue structures as teratomas, and therefore exhibit pluripotency. Teratocarcinomas are the only source of adult pluripotent stem cells that have been discovered.
Another technique used to demonstrate pluripotency is to allow mouse embryonic stem cells in vitro to differentiate by the introduction of growth factors into the growth medium. With the removal of the feeder layer (a layer of cells within the culture that maintain the undifferentiated state of the pluripotent cells) and the introduction of leukemia inhibitory factor (LIF), the embryonic stem cells aggregate and may form embryoid bodies.