Stem cells are cells with the unique potential to become multiple different types of cell within the body.
Most of your cells are equipped to accomplish a specific job, whether carrying oxygen in your blood or transmitting messages to and from your brain. These specialists are known as differentiated cells.
Stem cells, on the other hand, have the flexibility to specialise into a variety of cell types. And unlike most differentiated cells, they can replicate many times, giving rise to both more stem cells and to specialised cells.
The most versatile stem cells are found in embryos just a week old. Embryonic stem cells (ESCs) transform the embryo from a tiny ball of unspecialised cells into a baby, generating all of the 250-odd cell types in the human body. A biological blank slate, their vast – and highly coveted – potential is known as pluripotency.
After birth, stem cells continue to play a vital role as your body’s maintenance and repair kit, taking up residence in tissues such as the brain, bone marrow, liver, heart muscles, skin and gut. Adult stem cells are less flexible than their embryonic counterparts, generating a more limited range of cell types. The haematopoietic stem cells found in bone marrow, for example, are dedicated solely to producing blood cells.
When it comes to researching stem cells and the therapies that rely on them, getting hold of these cells is a major obstacle. ESCs are taken from donated embryos from IVF procedures, but this stirs up thorny ethical issues.
Although challenging to work with, adult stem cells dodge some of these ethical quandaries, leading many to store their offspring’s stem cell-rich umbilical cord blood. Furthermore, tissues that have been generated from a patient’s own stem cells don’t risk rejection by their immune system.
Stem cell milestones
It’s still early days, but stem cells show every intention of keeping their promises. Pioneering surgeon Paolo Macchiarini, based at Sweden’s Karolinška Institute, carried out the first organ transplant using a windpipe grown from adult stem cells in 2008. Since then, he has built newtracheas for several patients using a synthetic scaffold.
Research into therapy for type-1 diabetes has also made impressive progress. Sufferers’ lymphocytes (a key part of the body’s immune system) attack the pancreas, preventing the production of insulin. Exposing them to healthy lymphocytes grown from cord blood stem cells, however, appears to ‘re-educate’ them, limiting their harmful behaviour.
Induced pluripotent stem cells (otherwise known as iPSCs) obtained by manipulating mature specialised cells could well resolve the ethical controversy which currently restricts embryonic stem cell research. This year might well see the first trials of iPSCs in humans by US biotech firm Advanced Cell Technology (ACT). Initially experimenting with healthy volunteers, they hope to eventually provide blood platelets for patients with cancer and other blood disorders.
Stem cells to the rescue
Allowing researchers to watch cell specialisation unfold before their eyes, stem cells deliver unprecedented insight into many diseases and birth defects. Stem cells share many traits with cancer cells and could therefore reveal some of their secrets; some speculate that cancer may even be driven by out-of-control stem cells.
Many future treatments aim to harness stem cells’ regenerative properties. Healthy cell and tissue transplants could patch up patients with a variety of different complaints, from diabetes to Parkinson’s. Recent trials suggest, for instance, that injecting failing hearts with stem cells could grant them a new lease of life.
Tissues made from stem cells may also enable new medications to be tested on human cells in the early stages of drug development. One day, entire organs might be grown in the lab from patients’ own stem cells, dramatically cutting waiting lists for organ donors. In the meantime, scientists need plenty more time to research the finer details of controlling cell differentiation.
Who first discovered stem cells?
The concept of stem cells was first mentioned by Valentin Haecker and Theodor Boveri in the 19th century. In parallel, Artur Pappenheim, Alexander Maksimov, Ernst Neumann and others used it to describe a proposed origin of the blood system. As the field progressed, the term ‘stem cell’ has been used to describe the capacity of stem cells for self-renewal as well as the ability to give rise to all cell types that make up our bodies.