15.6: Embryonic Stem Cells
Embryonic stem (ES) cells are undifferentiated pluripotent cells, meaning they can produce any cell type in the body. This gives them tremendous potential in science and medicine since they can generate specific cell types for use in research or to replace body cells lost due to damage or disease.
ES cells are present in the inner cell mass of an embryo at the blastocyst stage, which occurs at about 3–5 days after fertilization in humans before the embryo is implanted in the uterus. Human ES cells are usually derived from donated embryos left over from the in vitro fertilization (IVF) process.
The cells are collected and grown in culture, where they can divide indefinitely—creating ES cell lines. Under certain conditions, ES cells can differentiate—either spontaneously into a variety of cell types, or in a directed fashion to produce desired cell types. Scientists can control which cell types are generated by manipulating the culture conditions—such as changing the surface of the culture dish or adding specific growth factors to the culture medium—as well as by genetically modifying the cells. Through these methods, researchers have been able to generate many specific cell types from ES cells, including blood, nerve, heart, bone, liver, and pancreas cells.
Regenerative medicine concerns the creation of living, functional tissues to replace dead, diseased or malfunctioning ones. Given their ability to differentiate into any cell type, ES cells are used in regenerative medicine. While this field is still in the early stages, several potentially beneficial cell types have been produced from ES cells, and clinical studies have begun to test their safety and effectiveness in patients. Some initial results have been promising—for instance, paralyzed patients regained some movement after receiving ES-derived nervous system cells. Additionally, ES cells can be used to study early events in human development—which is otherwise difficult—and provides a source of specific cell types which can be used in drug testing and other scientific research.