All organisms have stem cells. At the earliest stage of human development, when the embryo is about 3-5 days old (a blastocyst), there are numerous stem cells that transform or change into different specialized cells of the heart, the lungs, the skin, and other body organs. But despite the existence of stem cells since the dawn of man, (of all animals, for that matter) the intensive study of stem cells has begun only in the late 1990s. Human stem cells were first isolated in 1998, less than a decade ago.

There are three distinct characteristics of stem cells that make them unique from other kinds of human cells. First, stem cells, as they are, seem to perform no unique function. Unlike nerve cells that transmit electrochemical signals, and unlike skin cells that aim at protecting the body, the stem cells appear to just exist as they are, doing no specific function. This is why stem cells are described as “unspecialized”.

Second, stem cells can divide and produce more stem cells. Unlike muscle cells that usually don’t divide by itself, the stem cell has the ability to replicate itself over and over again. And unlike blood cells that eventually die after a few months, the stem cell is capable of self-renewal. These replication and self-renewal processes are described as “proliferation”. With proliferation, every human being is assured that he will always have stem cells as long as he is alive.

The third characteristic of the stem cells is probably the most fascinating one that probably surprised and excited many biologists. The unspecialized stem cells have the ability to change into different specialized cells. That is, a stem cell can become a brain cell, a muscle cell, a blood cell, or any other cell that performs a specific function. This process in which the stem cell changes into a specialized cell is called “differentiation”.

For example, the stem cell in the bone marrow, called the hematopoietic stem cell, can change or differentiate to form red blood cells, or white blood cells, or platelets. But these are not the only specialized cells that the hematopoietic stem cell can differentiate into. This stem cell can differentiate into other cells such as the nerve cells in the brain or the cardiac muscle cells of the heart. This ability of one type of stem cells to differentiate into a cell of a different tissue is called “plasticity”.

The differentiation and plasticity of the stem cells implies that these stem cells can replace any cell that had been damaged. Stem cells can be used to treat diseases in a method called “regenerative or reparative medicine”.

The task now of scientists is to determine how exactly can a stem cell differentiate to become a specific cell. Some scientists have theorized that a combination of factors is involved, such as the nature of the neighboring specialized cells and the chemical environment. At the moment, researches about stem cells are being accelerated.