Stem cells are a group of primary, non specialised cells that can transform into any cell of a human body. Stem cells have almost limitless potential to divide. Their primary function in a human body is to exchange old ‘worn out’ cells with new ones, fully developed and functional.
How can stem cells be classified?
Stem cells can be classified depending on where they come from:
- Embryonic stem cells–arrive only in the earliest stages of human development, that is in the embryonic stage. It is in them that the genetic information containing instruction of human development is coded.
- Somatic stem cells–can be found in almost any tissue of human body. They have the ability to produce specialised cells or start the cell lines which at the further stages of development produce target cells.
Stem cells can also be classified depending on their ability to differentiate:
- Totipotential cells – the only cells in a body that can divide and produce all types of cells in a given organism, in other words that are able to create the entire organism. The example of the most primary totipotential cell is the fertilized cell, zygote, or subsequent cells developed as a result of the zygote cleavage, that is blastomeres.
- Pluripotential cells – subsequent to blastomeres cells that arrive during the next stage of the zygote development. These are the cells that are able to differentiate into each of the three types of components present in a human during the pre-natal stage of development, the so-called germ layers: mesoderm, ectoderm and endoderm. These cells can also start each type of cells, they only cannot transform themselves back into totipotential cells that are present at the earliest stage of the human development.
- Multipotential cells – arriving only in one germ layer. They can transform into any type of cells only within the germ layer they originate from. For example they can start bone marrow, blood or muscle cells within mesoderm; they include also hematopoietic stem cells which originate from human umbilical cord blood.
- Unipotential cells – the last to develop, they can differentiate only into one, strictly defined type of mature cells (e.g. epithelial cells); they differ from mature cells in that they maintain their ability to divide.