Every one of the millions of cells of the human body, with the exception of germ cells, contains the same DNA sequence, that is, the same instructions. In an embryo, at the initial steps of development, all cells are equivalent and can give rise to any type of cell. These so-called embryonic stem cells maintain this property indefinitely; this is why it is said they are totipotent (they can give rise to all types of tissue). How a full organism, formed by different types of cells, can be generated from a single cell is precisely what is studied in developmental biology. Stem cells are dividing constantly, giving rise to daughter cells which, at a given time, will activate their cellular differentiation program – marked by the activation of specific genes- and will become a specific type of cell: a cardiac cell, a skin cell, etc.
Embryonic stem cells are totipotent, which means they can give rise to all types of tissues.
It is the totipotency of stem cells what interests scientists and the base of their future use in regenerative medicine, which aims to regenerate tissues, such as muscle, or to cure diseases such as Parkinson’s and Alzheimer’s. The idea is to transplant stem cells into the damaged tissue, and that these cells then generate new cells of the type that is needed.
Working with embryonic cells is an ethically controversial subject, and so scientists from all over the world are looking for new sources of stem cells. One possibility are the stem cells that can be found in the bone marrow. These adult stem cells are not able to generate all types of cells, but can differentiate into a variety of blood cells during their life. Scientists hope to be able to reprogram these adult stem cells so that, instead of generating only blood cells, they are able to generate any type of cells, as embryonic stem cells do.
There are two other ways that scientists are trying to create pluripotent cells:
- Transdifferentiation: a process by which one somatic adult cell is directly transformed into another type of somatic adult cell;
- iPS cells (induced Pluripotent Stem cells): a type of cells that can be obtained directly from differentiated adult cells with a technique developed in 2006.