What are stem cells?
Stem cells are cells that have the potential to develop into many different or specialized cell types. Stem cells can be thought of as primitive, "unspecialized" cells that are able to divide and become specialized cells of the body such as liver cells, muscle cells, blood cells, and other cells with specific functions. Stem cells are referred to as "undifferentiated" cells because they have not yet committed to a developmental path that will form a specific tissue or organ. The process of changing into a specific cell type is known as differentiation. In some areas of the body, stem cells divide regularly to renew and repair the existing tissue. The bone marrow and gastrointestinal tract are examples of areas in which stem cells function to renew and repair tissue.
The best and most readily understood example of a stem cell in humans is that of the fertilized egg, or zygote. A zygote is a single cell that is formed by the union of a sperm and ovum. The sperm and the ovum each carry half of the genetic material required to form a new individual. Once that single cell or zygote starts dividing, it is known as an embryo. One cell becomes two, two become four, four become eight, eight become sixteen, and so on, doubling rapidly until it ultimately grows into an entire sophisticated organism composed of many different kinds of specialized cells. That organism, a person, is an immensely complicated structure consisting of many, many, billions of cells with functions as diverse as those of your eyes, your heart, your immune system, the color of your skin, your brain, etc. All of the specialized cells that make up these body systems are descendants of the original zygote, a stem cell with the potential to ultimately develop into all kinds of body cells. The cells of a zygote are totipotent, meaning that they have the capacity to develop into any type of cell in the body.
The process by which stem cells commit to become differentiated, or specialized, cells is complex and involves the regulation of gene expression. Research is ongoing to further understand the molecular events and controls necessary for stem cells to become specialized cell types.