Some people are calling it the greatest medical breakthrough so far this century. Surgeons in Spain have successfully carried out the world’s first organ transplant using new stem cell technology.

But what are stem cells? Most cells in our bodies are designed to serve specific purposes – for example, a liver cell develops to work in the liver and cannot become a heart cell.

But stem cells are different. They are very young, and in the laboratory scientists can grow them into different types of cell.

Claudia Castillo needed a new windpipe after contracting tuberculosis. Scientists from the University of Bristol in the UK took a donor windpipe, or trachea, from someone who had recently died.

They used strong chemicals to remove the donor’s cells, leaving a tissue scaffold. This was repopulated with cells from Ms Castillo’s windpipe and nose, and stem cells from her bone marrow. After four days the cells had grown sufficiently for the trachea to be transplanted into Ms Castillo.

Currently, transplant patients have to take drugs for the rest of their lives to prevent their bodies rejecting the new organs. These drugs can have bad side-effects, and do not always prevent rejection.

But by using Ms Castillo’s own cells, doctors were able to trick her body into thinking the new trachea was her own organ. Five months on, Claudia Castillo is in perfect health.

This ground-breaking procedure could be used in other transplant operations in the future. Scientists also believe stem cells might be used to treat Parkinson's disease, Alzheimer's disease, heart disease, stroke, arthritis, diabetes, burns and spinal cord damage.

However, stem cell research is extremely controversial. The most effective stem cells do not come from adults but from embryos created in laboratories and which are just a few days old. Many people have religious or ethical objections to growing embryos, even if they can be used to cure diseases.