Types of stem cells
Types of Stem Cells
Stem cells are undifferentiated cells with the ability to divide for long periods. They have two main characteristics 1) they can self-renew, producing more stem cells and 2) they can differentiate, that is, they can potentially turn into a variety of cell types, in response to environmental signals.
Stem cells can be classified based on their potency, the ability to differentiate into one or more tissues. Under normal physiological conditions, a totipotent cell can turn into ANY type of cell of the human body; a totipotent cell can be found only in the very earliest stage of life (zygote). A pluripotent cell is the one able to produce all types of cells of the body, but unable to differentiate into extra-embryonic cells (for example, placenta cells). Multipotent cells are limited to generate cells of a certain type of tissue and, unipotent cells are those restricted to turn into a single cell type.
Another classification for stem cells can be done on the basis of their source:
Embryonic Stem Cells (ESCs)
ESCs are pluripotent stem cells derived from a cell mass called the blastocyst, formed 3 to 5 days after an egg is fertilized. As shown in the picture below, ESCs derive from the Inner Cell Mass which can specialize into any non-embryonic tissue of the body.
For research purposes, ESCs are obtained from the inner cell mass of in-vitro fertilized eggs donated for that purpose.
Somatic or Adult Stem Cells (ASCs)
ASCs are multipotent cells, originated from different tissues. Their principal function is to replenish damaged or dead tissue. Therefore, ASCs can be found almost in every tissue of the human body, throughout the majority of our lifetime. Common ASCs sources are the umbilical cord, bone marrow, muscle, brain, fat tissue, skin, gut, blood, spinal cord, dental pulp, cornea, etc. A big limitation of this type of cells is that they are scarce and in some cases difficult to obtain.
There are different types of adult stem cells:
- Hematopoietic Stem Cells (Blood Stem Cells)
- Mesenchymal Stem Cells
- Neural Stem Cells
- Epithelial Stem Cells
- Skin Stem Cells
ASCs have been used broadly in clinical research to evaluate their effect in the treatment of a broad range of human pathologies.
Induced Pluripotent Stem cells (iPSCs)
This classification was added recently since iPSCs were discovered in 2007. iPSCs are pluripotent cells created in the laboratory. To obtain them, differentiated cells (tissue-specific cells, like skin cells) are engineered to behave like an ESC.
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