Stem Cell, a cell with the capacity to divide repeatedly with little change for long periods of time. They can give rise to cells specialized for specific types of tissue. Stem cells form at a very early stage in the life of a multicellular organism. They continue to exist in mature organisms, where they are crucial in the development and maintenance of an organism's many types of tissue.

Researchers have identified two main kinds of stem cells. Embryonic stem cells form in an embryo within a few days after it comes into existence. They can give rise to most of the different kinds of specialized cells of the body. Adult stem cells are found in specialized tissue. They can give rise to the specialized kinds of cells of the tissue from which they come. For example, the stem cells found in blood can develop into platelets, or white or red blood cells. Adult stem cells have been found in the liver, skin, bone marrow, and other tissues. They serve as sources of fresh cells for these tissues.

In 1998, scientists, for the first time, isolated human embryonic stem cells and were able to grow them in the laboratory. The isolation of such stem cells involves the destruction of the embryos. Scientists also discovered techniques for guiding both embryonic and adult stem cells to develop into specific kinds of human tissue by exposing them to specific stimuli. These achievements raised hopes for the possibility of using stem cells to repair or replace organs and tissues in the human body.

In 2001, the United States banned the use of federal funds for creating new sources of embryonic stem cells. By 2005, researchers in the United Kingdom and South Korea had created cloned human embryos as sources of embryonic stem cells. In 2008, physicians in Spain replaced a woman's trachea, or windpipe, with one that was grown using her own stem cells.