Somatic stem cells are often also described as Adult Stem Cells (although they are found in both children and adults). These are natural cells found in the human body which are undifferentiated (i.e. not committed to a permanent tissue type). The name adult stem cells can be misleading since both children and adults have somatic stem cells. Somatic stem cells serve two functions:

1. The replacement of dying cells (a continuous requirement for organs such as the epidermis).

2. The repair of damaged tissues (as required in response to trauma or disease).

Many type of somatic cells are not actually true stem cells, but rather progenitor cells since many have limitations on their abilities of differentiation and replication.  Although they share many aspects in common with true, totipotent stem cells they do not have the same level of universal adaptability.

Somatic stem cells occur in a variety of different tissue types:

1. Haematopoietic stem cells – these blood-forming stem cells are found in the bone marrow as well as the umbilical cord of newborn babies.
2. Stromal stem cells – located in the bone marrow these cells can differentiate into cartilage, fat / adipocytes and bone.
3. Neural stem cells – these stem cells can differentiate into various neural cells including neurons and the myelin-sheath producing oligodendrocytes.

Under normal physiological conditions somatic (adult) stem cells are usually quiescent, existing in small quantities in specific areas of each tissue.  They may remain inactive in these tissues for  long periods until injury or disease necessitates their activation. In addition to the three main physiological sources of somatic stem cells listed above,  other somatic stem cells have been identified in even more tissue and organ systems:

	epidermal tissues (skin)
	skeletal muscles
	hepatic (liver) tissues

Regardless of source, stem cells all have potential to replicate and differentiate to some degree. The figure below demonstrates how stem cells recovered from the bone marrow have a diverse range of cell types into which they can differentiate:

http://media.nasaexplores.com/lessons/04-211/images/stem_cell_diagram.jpg

Harvest and Growth of Adult Stem Cells

Somatic / adult stem cells are a product of the patient's own cells and a donor egg (undifferentiated) which has had it's nucleus (genetic material) removed. The genetic information from a somatic cell of the patient is inoculated into the donor egg (to replace the nucleus) such that the cell will grow to have complete biocompatibility with the patient's tissues and immune system. A schematic diagram of this process is provided below:

Significant research is being conducted into improving and standardizing the isolation and propagation of somatic stem cells via tissue culture. The ability to extract and propagate large numbers of these cells could provide novel medical therapies for diseases such as:

1. Diabetes - to re-establish the production of insulin by replacing dysfunctional pancreatic cells.
2. Multiple Sclerosis / Parkinsons  - located in the brain, neural stem cells can differentiate to create both neurons as well as related, non-neural cells (astrocytes and oligodendrocytes) to support .
3. Cardiac conditions - replacement of damaged cardiac tissues injured through stroke or disease.
4. Blood conditions - hematopoietic stem cells create the various blood cell types: (erythrocytes, monocytes, B and T lymphocytes, natural killer cells, platelets etc.).
5. Many other conditions - for example, bone marrow cells  give rise to a variety of cell types including  bone, cartilage, fats / lipids and other  connective tissues. Other examples sources of somatic stem cells include intestinal epithelial cells (digestive tract) and epidermal stem cells.

(additional information on the medical uses of stem cells is provided in the section on therapeutics.)

Pluripotent Somatic Stem Cells

Many somatic / adult stem cells can differentiate into multiple different cell types and are thus considered pluripotent.  Examples of pluripotent somatic stem cells are diverse and include:

1. Blood / Hematopoietic stem cells can become:

   	Neural cells
   	skeletal muscle cells
   	cardiac muscle
   	hepatic / liver cells

2. Bone marrow somatic cells can become:

   	cardiac muscle cells
   	skeletal muscle cells

3. Brain stem cells can become:

   	blood cells
   	skeletal muscle cells

   This is summarized in the schematic diagram below:

http://stemcells.nih.gov/info/basics/basics4.asp

Exactly which mechanisms are responsible for triggering somatic stem cell proliferation and division are not yet known.  Clearly there is great interest in these signal mechanisms, as they may not only provide key data on stem cells properties and applications, but may also hold the key to understanding unrestricted cell proliferation in diseases such as cancer.  

The fact that somatic / adult stem cells exist throughout an individual's lifespan means that there may well be a continuous stockpile of donor cells for medical therapies. As the techniques for isolating and growing these cells in vitro continues,  there are parallel increases in the number of medical disorders that may be one treated by somatic cell therapy. (Additional information is available in the section on therapeutics).