Human embryonic stem cells were differentiated into cells like those found in the pancreas (blue), which give rise to insulin-producing cells (red). When implanted in mice, the stem cell-derived pancreatic cells can replace the insulin that isn't produced in type 1 diabetes. 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.