What Are Stem Cells?

Stem cells are very special, powerful cells found in both humans and non-human animals. They have been called the centrepieces of regenerative medicine – medicine that involves growing new cells, tissues and organs to replace or repair those damaged by injury, disease or aging.

Stem cells are the precursors of all cells in the human body. What makes stem cells special is that they are regenerative and malleable. They have the ability to replicate themselves and to repair and replace other tissues in the human body. Some tissues, like skin, need constant renewal, which could not take place without skin stem cells. Other stem cells repair damage to the body’s tissues, for example, rebuilding damaged or degenerating muscle tissue. New research also indicates that stem cell malfunction or damage may be responsible for certain cancers and even muscular-degeneration diseases like Muscular Dystrophy. Research on stem cell functioning is therefore a critical avenue to finding treatments for these and other diseases.

What is a stem cell? Narrated by Dr. Jim Till from Stem Cell Network on Vimeo.

What are embryonic stem cells? Narrated by Dr. Janet Rossant from Stem Cell Network on Vimeo.

What are induced pluripotent stem cells? Narrated by Dr. Mick Bhatia from Stem Cell Network on Vimeo.


Sources of stem cells

Stem cells differ according to their source and their malleability. Adult stem cells can be found in specific tissues in the body and include neural stem cells, skin stem cells, and blood (hematopoietic) stem cells. Hematopoietic stem cells make blood cells and can be found in bone marrow or in cord blood from umbilical cords. Fetal stem cells are taken from discarded fetal tissue, and embryonic stem cells (ES cells) are derived from 5 day-old blastocysts – precursors to embryos.

There is a continuing debate over the properties possessed by adult human and ES cells. Research into adult stem cells continues to evaluate their ability to differentiate which may be more limited than ES cells. Another difference between specialized (adult) stem cells and ES cells is their behaviour in culture. It is easier to get ES cells to replicate in culture and to keep those cells alive for a very long time, an attribute that is very useful in creating cell lines to study diseases or cell functioning. Regardless of their differences, both avenues of research will likely yield valuable and different information for the development of clinical therapies.

Recent breakthroughs

In 2007 the announcement that Japanese researcher Shinya Yamanaka had created pluripotent stem cells using skin cells and not embryos caused great excitement in the stem cell community due to the possibility that they may have the same properties and benefits as ES cells. These stem cells are called “induced pluripotent stem cells” (also known as iPS and iPSC) and are created from adult cells – often skin cells. Typically this involves the reprogramming of some of the cells through the introduction of genes commonly found in stem cells. Yamanaka introduced the reprogramming genes through viral vectors using retroviruses that transcribe the genes into the host cell DNA, which raised concerns about the effects of the retroviruses on the cell.

In 2009, a Canadian-Scottish research team led by Andras Nagy was the first of several teams to announce a method to create iPS cells without the use of retroviruses. This is an important development as it could lead to greater application of iPS cells, which are a less controversial source of stem cells. Continuing research using iPS cells is needed, in particular because they may be an extremely powerful tool for creating stem cell lines as disease models.

Other resources

A Stem Cell Story

An award-winning short film produced by EuroStemCell that provides an introduction to stem cells, though does not include the discovery of iPS (induced pluripotent stem cells).

Stem Cell Facts

A downloadable brochure that outlines stem cell research and provides a brief glossary of terms.


A series of short videos on a wide range of topics related to stem cells, produced by the California Institute of Regenerative Medicine.

Fireside Chat with James Till and Janet Rossant

James Till, who along with research partner Ernest McCulloch were the first people to describe stem cells, sits down with Janet Rossant at StemCellTalks 2010 in Toronto to talk about his work, and how he got into the field of stem cell science.