So. What is a Stem Cell?
The term usually conjures up thoughts of embryo derived cells, but there are actually multiple kinds of stem cell, the majority of which are not from embryos. These can broadly be categorised into three groups:
· Embryonic Stem Cells (ESCs) (these actually are derived from embryos)
· Induced Pluripotent Stem Cells (iPSCs)
· Adult Stem Cells (ASCs)
The defining characteristics of all types of stem cell are that they can multiply themselves (they can “proliferate”) and they can subsequently turn into other kinds of cells (they can “differentiate”). ESCs sit at the beginning of the body’s development, which means they ultimately give rise to all types of cell (skin cells, muscle cells, gut cells, brain cells… et cetera). ASCs, which also arise from the differentiation of ESCs, are important for the human body after its initial development from a fertilized egg. They exist in children and adults to replace tissue-specific cells that naturally die off after a certain period of time. ASCs are more restricted in their ability to differentiate than ESCs, and exist to replenish specific kinds of cells. A good example of this kind of stem cell is hematopoietic stem cells (i.e. bone marrow stem cells); these can turn into any type of blood cell (of which there are around 7 types), and importantly, hematopoietic stem cells can self-replicate and divide, thus maintaining a source for new blood cell production. The terms used to describe these two different differentiation capacities are:
· Pluripotent (cells that have the potential to differentiate into any kind of cell, e.g. embryonic stem cells)
· Multipotent (cells that have the potential to differentiate into tissue specific cells, e.g. adult stem cells including haematopoietic stem cells)
As the name suggests, induced pluripotent stem cells are also pluripotent, but we will cover this stem cell type in a separate post.
The differentiation capacity is the reason for the huge interest in stem cells as a treatment for disease. Since almost any desired type of cell can be created using pluripotent stem cells, the basic idea is that they could be used to replace cells lost in degenerative diseases. Degenerative diseases are characterised by irreplaceable cell loss, often of a very specific kind of cell. For example, in Parkinson’s disease specific brain cells (neurons) die off and cannot be replaced, which is the cause for impaired movement in affected people. We already know that pluripotent stem cells can give rise to the neurons that are lost in Parkinson’s patients. So, the only thing left to do is to differentiate stem cells into neurons in a lab, transplant these into a patient, and hope that these neurons replace the patient’s lost cells… Right?
Things are not as simple as they might sound, and before this kind of stem cell therapy can become a reality there are many obstacles to overcome.
