We all have some cells in our body that are able to rebuild and replenish lost cells and thus keep those tissues and cells which are constantly changing and with short lifetime, such as skin and blood tissue. Cells with the ability are “stem cells”.
To be called a stem cell, the following two abilities are essential. One is the ability to produce various cells that make up our body such as skin, red blood cells and platelets, which is the differentiation ability. And the other ability to divide into cells with exactly the same ability as itself, which is called self-replicating ability.
Stem cells can be roughly divided into two categories. One is stem cells that continuously make substitutes for died out cells in fixed tissues or organs such as skin or blood. This type of stem cells is called “tissue stem cell”. Tissue stem cells are not able to “become everything”, their functions are determined. For example, hematopoietic stem cells, which produce blood, are able to derive blood cells only; and neural stem cells, which form the nervous system, are able to derive neural cells only.
The other category is the “Pluripotent stem cell”, which is able to produce any kind of cells in our body, the ES cell (embryonic stem cell) is one of the pluripotent stem cells. That is to say, the pluripotent stem cells can also produce various tissue stem cells in our body. The iPS cells (induced pluripotent stem cells) are pluripotent stem cells artificially made from normal cells.
Using such nature of “stem cells”, researches are going on in fields like taking cells itself as a medicine to treat injury or disease, which is a new therapy called “regenerative medicine”, or reproducing the condition of cells in our body to investigate the mechanism of diseases.