Human stem cell A colony of embryonic stem cells, from the H9 cell line.

Seemingly not a day goes past when stem cells do not feature somewhere in our media, heralded as the Golden Bullet or Magic Cure that will rid us of all ails. While such aims are the goal of the field of regenerative medicine, the barrage of headlines obscures the truly amazing impact that the discovery of embryonic stem (ES) cells has had on the way we as researchers can begin to understand the complexities of the human body, to determine what goes wrong in disease and ultimately to develop strategies to repair it.

In 1981, Martin Evans together with his colleague, Kaufman, described the isolation of ES cells from early stage mouse embryos that could be grown in culture. By genetically modifying these cells and implanting them into adult female mice Evans was able to create genetically modified offspring, which led to the creation of transgenic mice.

Now, almost every aspect of mammalian physiology can be studied by such gene targeting. We can switch genes on or off at specific time points, allowing us to dissect the role of these genes both during development and in the adult organism. We can change expression throughout the whole organism or just within certain cells in a given organ or tissue. We can introduce mutations known to cause disease in humans allowing us to study the disease process and to develop strategies to prevent disease or repair any damage done. We can even introduce reporters, such as green fluorescent protein, under the control of a given gene or its promoter, allowing us to label all the cells within the organism that express that gene. Suddenly, we are able to sort specific populations of live cells from one another, to track the expression of a given gene in development and even to follow cells transplanted from one organism to another.

My own work examines the potential for repairing the diseased retina by transplanting healthy photoreceptors and transgenic animals are crucial to my research. By using mice engineered to carry mutations known to cause disease in humans, we are able to accurately model human retinal disease. We use donor cells from transgenic animals that have been modified to carry reporter constructs, which allow us to track the transplanted cells within the diseased recipient and now we are beginning to take stem cells to the next level; to use them as a renewable source of cells to generate donor photoreceptors.

Evans' work not only gave rise to the discovery of stem cells but a wealth of tools and the beginnings of regenerative medicine itself. 

Dr Rachael Pearson is a University Research Fellow at the University College London Institute of Ophthalmology. Her work has helped develop a novel therapeutic approach to restoring vision, by replacing the cells lost through degeneration with functioning, light-sensitive cells.

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