Stem cells have been an important part of medicine since their isolation from mice in 1981, but in actual fact they have been used far longer than this. Bone marrow transplants for leukaemia and skin grafts for burn victims both rely on the principles of stem cells and regeneration. Even in ancient Greece they were imagined as an essential part of human biology, shown by the story of Prometheus who, as punishment for stealing Zeus’ fire and giving it to man, was bound to a rock and had his liver pecked out by a giant eagle every single day, just to have it grow back each night. Nice.
The term stem cell actually covers different types of cells, and is arguably thrown around a little too often nowadays. They can divide forever and generate new cell types, much like an oak tree can keep on growing, throwing out new branches. This power has made them exciting with respect to repairing damaged organs and for use in developing new drugs.
Embryonic stem cells are defined as being pluripotent, that is to say that if you took one embryonic stem cell it has the capacity to become any other cell type of the human body. These are the roots and trunk of the oak tree, shooting branches off in any direction it needs, whilst each root can make a new oak tree. During gestation, these are the cells that build us; the fertilized egg generates a shell in which the embryonic stem cells grow, they then divide and follow paths to different fates, for example a neuron, or a heart cell. This ability allows one original cell to go on to produce all the cells that we are made of. The same principle applies for all life; we all start off from the same building blocks.
Adult stem cells are pretty much what it says on the tin. They are stem cells that continue to stay in the body even into adulthood. Adult stem cells are the branches of the great oak; they do not under normal conditions make a new tree, but continually sprout new leaves and acorns necessary for the ongoing life of the original oak. They are found in brain, bone marrow, blood vessels, muscle, skin, teeth, heart, gut, liver, ovaries and testicles. The main difference between these cells and embryonic stem cells is their ability to make new cell types. Essentially, adult stem cells are restricted in what cell type they can make, only creating cells down a certain path, for example the neural path. The body does not want teeth filling our arteries, nor intestines sprouting out the top of our head, so cell types are kept limited.
Induced pluripotent cells – or IPS cells for short – are slightly different matter. They are the acorns and cuttings which when replanted can generate a new oak tree. Scientists have found that they can take cells from our skin; force expression of a combination of genes and this reverses the path the cell has taken, reverting it back to a pluripotent stem cell i.e. a cell that can then generate any other cell type. This demonstrates an incredible progress in our understanding of stem cells.
In the next two parts I will give a slightly more detailed introduction to why and how stem cells are used, and the major points of controversy that arise. Hopefully, it will give an insight into the lives of scientists that work on them and help you decide your attitude towards the subject.
Stem Cell Oak Tree – I.S.