Stem cells are undifferentiated cells that have the ability to form different types of cells. For example adult stem cells from the bone marrow can form many types of blood cells. Embryonic stem cells can differentiate into any type of cell and are therefore called pluripotent cells. They have the capability to form nerve cells or heart muscle cells; these cells cannot normally regenerate. For example people who damage the nerve cells in their spinal cord are usually paralysed below the level of their injury because the nerve cells cannot repair and brain damage after a stroke is permanent.
Pluripotent stem cells can differentiate into any type of cells and so may be used to repair or replace nerves or even organs. They have until recently have had to be taken from dead embryos and because of this many people have ethical objections.

John Gurdon showed that adult cells can be re programmed to behave like egg cells. That the nucleus of any cell has the information to make any other type of cell. He transplanted the nucleus of an adult frog cell into an egg cell of the same species which grew into a healthy adult. This led to the possibility of cloning and paved the way for Dolly the sheep. Professor Yamanaka from Japan built on John Gurdon’s work and showed in 2006 that mature cells could be ‘unwound’ and regressed to form stem cells. Not only does this evade the ethical problems of using embryos but it can mean that it may be possible to use a persons own skin cells to grow a replacement organ and because it is formed from his own cells there is no possibility of organ rejection.

Organs have been produced by removing dead organs such as a heart then stripping it of heart cells using a detergent like substance leaving a collagen frame. Stem cells were then injected into the frame and they self differentiated into heart cells and multiplied. The hearts produced even started pumping, but were too weak to be used.
So far stem cells have been used to form new retinas and have been successfully used in retinal transplants. The Lancet [Nov 23 2011] reported that a patient in Sweden had received the worlds first trachea produced from stem cells. The patient had a cancer in his wind pipe [trachea]. An artificial trachea was created by using a glass model to produce an artificial scaffold. Then stem cells were inserted into the scaffold then the glass removed.

Stem cells are producing much excitement in the field of regenerative medicine. If you have not already, have a look at the 10 min TED talk by Professor Aubrey Grey I recommended a few months ago. He believes that humans may possibly live for ever! He is a crazy looking guy but his ideas have some merit and were discussed in the BMJ. Since he did the talk the Methusela mouse prize he talks about has been won . Scientists actually managed to make a mouse younger not just stop aging![p46 of my book] You may also want to read P45 -50 ‘Medicine in the future’ from my book Medical School Interviews The Knowledge to help answer the common question ‘What do you consider to be the most exciting field in medicine.’