Stem cells are a special type of cell found in multicellular organisms, such as plants and animals, which are yet to develop traits that distinguish specific tissues from one another.

While these cells exist in both fully developed humans and their embryos, each contains a slightly different type of stem cell with different tasks.

The cluster of cells in a five day old embryo, for instance, is made up of pluripotent stem cells, which can develop into any of the hundreds of cell types making up an adult body. Their purpose is to differentiate as they reproduce, generating the body's structures in stages.

Once fully developed, a body maintains a supply of stem cells that can be regarded as multipotent. These 'undecided' cells have lost their ability to grow into most tissues, but can still develop characteristics of a variety of related cell types.

An example can be found in bone marrow and the blood stream. Hematopoietic stem cells are capable of transforming into both red and white blood cells, as well as platelets.

Unlike embryonic stem cells, these adult (or somatic) stem cells assist in growth and repair, replacing lost cells and fixing damaged tissue.

In 2006, researchers showed that by adding a handful of specific genes to adult cells, called fibroblasts, they can turn embryonic stem cells into pluripotent cells.

These induced pluripotent stem cells, or iPSCs, have a wide range of potential uses in medical treatment and biological research.

How are stem cells used to treat disease?

Thanks to their regenerative properties, stem cells have attracted attention in research as a potential treatment for a wide variety of conditions.

Transplanting the multiplurant adult stem cells in closely-matched bone marrow can help treat blood cancers or immune-related disorders such as multiple sclerosis.

Using embryonic stem cells, or inducing adult tissues to be pluripotent, could open the way to replacing an even greater variety of damaged or dysfunctional tissues in the body, from nerve cells to pancreatic tissue. If the induced stem cells come from the person receiving the treatment, they have a far lower chance of rejecting the transplanted material.

Observing how stem cells develop into other tissues in the lab, or respond to various hormones and drugs, assists in the understanding of disease and pathways for new therapies.

Why is stem cell therapy controversial?

Stem cell treatments might, or might not, have the potential many claim, with promising therapies still being tested for efficacy and safety. This hasn't stopped an unregulated stem cell industry from promoting services in countries such as China.

Human embryonic stem cells are derived from a small cluster of freshly divided cells just a few days old, raising ethical questions over the nature of personhood and body autonomy.

Different nations and authorities have imposed local laws and policies dictating who can produce embryonic stem cell lines and the research that can be conducted with them.

For example, US laws currently restrict federal funding for research to existing cell lines. An executive order passed in 2009 by Barack Obama opens opportunities to fund research on existing private embryonic cell cultures.

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