STEM CELL

Stem cells are biological cells found in all multicellular organisms, that can divide (through mitosis) and differentiate into diverse specialized cell types and can self-renew to produce more stem cells. In mammals, there are two broad types of stem cells: embryonic stem cells, which are isolated from the inner cell mass of blastocysts, and adult stem cells, which are found in various tissues. In adult organisms, stem cells and progenitor cells act as a repair system for the body, replenishing adult tissues. In a developing embryo, stem cells can differentiate into all the specialized cells (these are called pluripotent cells), but also maintain the normal turnover of regenerative organs, such as blood, skin, or intestinal tissues.

There are three sources of autologous adult stem cells: 1) Bone marrow, which requires extraction by harvesting, that is, drilling into bone (typically thefemur or iliac crest), 2) Adipose tissue (lipid cells), which requires extraction by liposuction, and 3) Blood, which requires extraction through pheresis, wherein blood is drawn from the donor (similar to a blood donation), passed through a machine that extracts the stem cells and returns other portions of the blood to the donor. Stem cells can also be taken from umbilical cord blood. Of all stem cell types, autologous harvesting involves the least risk. By definition, autologous cells are obtained from one's own body, just as one may bank his or her own blood for elective surgical procedures.

Highly plastic adult stem cells are routinely used in medical therapies, for example bone marrow transplantation. Stem cells can now be artificially grown and transformed (differentiated) into specialized cell types with characteristics consistent with cells of various tissues such as muscles or nerves through cell culture. Embryonic cell lines and autologous embryonic stem cells generated through therapeutic cloning have also been proposed as promising candidates for future therapies.^[1] Research into stem cells grew out of findings by Ernest A. McCulloch and James E. Till at the University of Toronto in the 1960s.^[2][3]

Properties

The classical definition of a stem cell requires that it possess two properties:

• Self-renewal: the ability to go through numerous cycles of cell division while maintaining the undifferentiated state.
• Potency: the capacity to differentiate into specialized cell types. In the strictest sense, this requires stem cells to be either totipotent or pluripotent—to be able to give rise to any mature cell type, althoughmultipotent or unipotent progenitor cells are sometimes referred to as stem cells. Apart from this it is said that stem cell function is regulated in a feed back mechanism.

Self-renewal

Two mechanisms to ensure that a stem cell population is maintained exist:

1. Obligatory asymmetric replication: a stem cell divides into one father cell that is identical to the original stem cell, and another daughter cell that is differentiated
2. Stochastic differentiation: when one stem cell develops into two differentiated daughter cells, another stem cell undergoes mitosis and produces two stem cells identical to the original.

Potency definitions

otency specifies the differentiation potential (the potential to differentiate into different cell types) of the stem cell.^[4]

• Totipotent (a.k.a. omnipotent) stem cells can differentiate into embryonic and extraembryonic cell types. Such cells can construct a complete, viable organism.^[4] These cells are produced from the fusion of an egg and sperm cell. Cells produced by the first few divisions of the fertilized egg are also totipotent.^[5]
• Pluripotent stem cells are the descendants of totipotent cells and can differentiate into nearly all cells,^[4] i.e. cells derived from any of the three germ layers.^[6]
• Multipotent stem cells can differentiate into a number of cells, but only those of a closely related family of cells.^[4]
• Oligopotent stem cells can differentiate into only a few cells, such as lymphoid or myeloid stem cells.^[4]
• Unipotent cells can produce only one cell type, their own,^[4] but have the property of self-renewal, which distinguishes them from non-stem cells (e.g., muscle stem cells).