About this document
This primer on stem cells is intended for anyone who wishes to learn more about the biological properties of stem cells, the important questions about stem cells that are the focus of scientific research, and the potential use of stem cells in research and in treating disease. The primer includes information about stem cells derived from embryonic andnon-embryonic tissues. Much of the information included here is about stem cells derived from human tissues, but some studies of animal-derived stem cells are also described. The National Institutes of Health (NIH) developed this primer to help readers understand the answers to questions such as: f f f f What are stem cells? What are the different types of stem cells, and where do they come from? Whatis the potential for new medical treatments using stem cells? What research is needed to make such treatments a reality?
This document provides basic information about stem cells. More detailed discussion is available from the NIH stem cell reports at http://stemcells.nih.gov/info/. Quick answers to specific common queries can be found on the Frequently Asked Questions page. Throughout “StemCell Basics,” many technical terms appear in bold, underlined maroon type. Click the term to see its definition in the Glossary at the end of the primer.
I. Introduction: What are stem cells, and why are they important?
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 ofinternal 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. Stem cells are distinguished from other cell types by twoimportant characteristics. First, they are unspecialized cells capable of renewing themselves through cell division, sometimes after long periods of inactivity. Second, under certain physiologic or experimental conditions, they can be induced to become tissue- or organ-specific cells
with special functions. In some organs, such as the gut and bone marrow, stem cells regularly divide torepair and replace worn out or damaged tissues. In other organs, however, such as the pancreas and the heart, stem cells only divide under special conditions. Until recently, scientists primarily worked with two kinds of stem cells from animals and humans: embryonic stem cells and non-embryonic “somatic” or “adult” stem cells. The functions and characteristics of these cells will be explained in thisdocument. Scientists discovered ways to derive embryonic stem cells from early mouse embryos nearly 30 years ago, in 1981. The detailed study of the biology of mouse stem cells led to the discovery, in 1998, of a method to derive stem cells from human embryos and grow the cells in the laboratory. These cells are called human embryonic stem cells. The embryos used in these studies were created forreproductive purposes through in vitro fertilization procedures. When they were no longer needed for that purpose, they were donated for research with the informed consent of the donor. In 2006, researchers made another breakthrough by identifying conditions that would allow some specialized adult cells to be “reprogrammed” genetically to assume a stem cell-like state. This new type of stem cell,called induced pluripotent stem cells (IPSCs), will be discussed in a later section of this document. Stem cells are important for living organisms for many reasons. In the 3- to 5-day-old embryo, called a blastocyst, the inner cells give rise to the entire body of the organism, including all of the many specialized cell types and organs such as the heart, lung, skin, sperm, eggs and other...