Introduction: What are stem cells, and why are they important?

Stem cells have great potential to become many different cell types in the body during early life and growth. Moreover, in many tissues that serve as a kind of internal repair system, which essentially divide 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 so much potential to remain a stem cell or become another type of cell with a more specialized function, such as a muscle cell, red blood cell or a brain cell. 

Stem cells are distinguished from other cell types by two important features. First, they are unspecialized cells, capable of renewing themselves through cell division, sometimes after long periods of inactivity. Second, under physiological or experimental conditions which can be induced to become any tissue or cell specific organs with special functions. In some organs, such as the intestine and bone marrow, stem cells divide to regularly repair and replace worn or damaged tissues. In other organs, however, as the pancreas and heart, stem cells divide only under special conditions.

Until recently, scientists worked mainly with two types of stem cells in animals and humans: embryonic stem cells and not embryonic "somatic" or "adult" stem cells. The functions and characteristics of these cells described in this document. Scientists discovered ways to obtain embryonic stem cells from early mouse embryos for almost 30 years in 1981. The detailed study of the biology of mouse stem cells led to the discovery in 1998 of a method for obtaining human embryonic stem cells and grow the cells in the laboratory. These cells are called human embryonic stem cells. The embryos used in these studies were created for reproductive purposes through in vitro fertilization procedures. When no longer required for that purpose, which were donated for research with informed consent from donors. In 2006, researchers made another breakthrough in identifying the conditions that allow some specialized adult cells to be "reprogrammed" genetically to assume a stem cell-like state. This new type of stem cells, 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-embryo, called a blastocyst, the inner cells give rise to the whole body of the organism, including all of the many types of specialized cells and organs such as heart, lung, skin, semen, eggs andother tissues. In some adult tissues like bone marrow, muscle and brain, discrete populations of adult stem cells generate replacements for cells that are lost through normal wear and tear, injury or illness.

Given its unique regenerative abilities, stem cells offer new possibilities for treating diseases like diabetes and heart disease. However, much work remains to be done in the laboratory and clinic to understand how to use these cells for cell-based therapies to treat the disease, also known as reparative or regenerative medicine.

Laboratory studies of stem cells allows scientists to learn about the essential properties of cells and what makes them different from the types of specialized cells. Scientists are already using stem cells in the laboratory to identify new drugs and development of model systems to study normal growth and determine the causes of birth defects.

The stem cell research continues to advance the knowledge of how an organism develops from a single cell and how healthy cells replace damaged cells in adult organisms. Stem cell research is one of the most fascinating areas of contemporary biology, but, like many expanding fields of scientific research, research on stem cells raises scientific questions as rapidly as it generates new discoveries.