After animal cells differentiate into tissues and organs, some tissues retain a group of undifferentiated cells to replace that tissue’s damaged cells or replenish its supply of certain cells, such as red and white blood cells. When needed, these adult stem cells (ASCs) divide in two. One cell differentiates into the cell type the tissue needs for replenishment or replacement, and the other remains undifferentiated.
Embryonic stem cells (ESCs) have much greater plasticity than ASCs because they can differentiate into any cell type. Mouse embryonic stem cells were discovered and cultured in the late 1950s. The ESCs came from 12-dayold mouse embryo cells that were destined to become egg or sperm (germ cells) when the mouse matured. In 1981, researchers found another source of mouse ESCs with total developmental plasticity—cells taken from a 4-dayold mouse embryo.
In the late 1990s researchers found that human ESCs could be derived from the same two sources in humans:
primordial germ cells and the inner cell mass of 5-day-old embryos. Scientists also have been able to isolate pluripotent stem cells from human placentas donated following
normal, full-term pregnancies. Under certain culture conditions, these cells were transformed into cartilagelike
and fat-like tissue.
Maintaining cultures of ESCs and ASCs can provide answers to critical questions about cell differentiation: What factors determine the ultimate fate of unspecialized stem cells?
How plastic are adult stem cells? Could we convert an ASC into an ESC with the right combination of factors? Why do stem cells retain the potential to replicate indefinitely? Is the factor that allows continual proliferation of ESCs the same factor that causes uncontrolled proliferation of cancer cells?
If so, will transplanted ESCs cause cancer?
The answers to these questions and many more will determine the limits of the therapeutic potential of ESCs and ASCs. Only when we understand the precise mix of factors controlling proliferation and development will we be able to reprogram cells for therapeutic purposes. Using stem cell cultures, researchers have begun to elaborate the intricate and unique combination of environmental factors, molecular signals and internal genetic programming that decides a cell’s fate. Israeli scientists directed ESCs down specific developmental pathways by providing different growth factors. Others discovered that nerve stem cells require a dose of vitamin A to trigger differentiation into one specific type of nerve cell, but not another.
In the late 1990s researchers found that human ESCs could be derived from the same two sources in humans:
primordial germ cells and the inner cell mass of 5-day-old embryos. Scientists also have been able to isolate pluripotent stem cells from human placentas donated following
normal, full-term pregnancies. Under certain culture conditions, these cells were transformed into cartilagelike
and fat-like tissue.
Maintaining cultures of ESCs and ASCs can provide answers to critical questions about cell differentiation: What factors determine the ultimate fate of unspecialized stem cells?
How plastic are adult stem cells? Could we convert an ASC into an ESC with the right combination of factors? Why do stem cells retain the potential to replicate indefinitely? Is the factor that allows continual proliferation of ESCs the same factor that causes uncontrolled proliferation of cancer cells?
If so, will transplanted ESCs cause cancer?
The answers to these questions and many more will determine the limits of the therapeutic potential of ESCs and ASCs. Only when we understand the precise mix of factors controlling proliferation and development will we be able to reprogram cells for therapeutic purposes. Using stem cell cultures, researchers have begun to elaborate the intricate and unique combination of environmental factors, molecular signals and internal genetic programming that decides a cell’s fate. Israeli scientists directed ESCs down specific developmental pathways by providing different growth factors. Others discovered that nerve stem cells require a dose of vitamin A to trigger differentiation into one specific type of nerve cell, but not another.