Embryonic stem cells store pre-activated Bax (red) in the Golgi, near the nucleus (blue). Stem cells have the remarkable potential to develop into many different cell types in the body during early life and growth. In addition, in many tissues they serve as a sort of internal repair system, dividing essentially without limit to replenish other cells as long as the person or animal is still alive. When a stem cell divides, each new cell has the potential either to remain a stem cell or become another type of cell with a more specialized function, such as a muscle cell, a red blood cell, or a brain cell. Embryonic stem cells are derived from embryos. Most embryonic stem cells are derived from embryos that develop from eggs that have been fertilized in vitro - in an in vitro fertilization clinic - and then donated for research purposes with informed consent of the donors. They are not derived from eggs fertilized in a woman's body. Research is underway to develop various sources for stem cells, and to apply stem cell treatments for neurodegenerative diseases and conditions, diabetes, heart disease, and other conditions. In more recent years, with the ability of scientists to isolate and culture embryonic stem cells, and with scientists' growing ability to create stem cells using somatic cell nuclear transfer and techniques to created induced pluripotent stem cells, controversy has crept in, both related to abortion politics and to human cloning.