What is my fate?
The fate of cells is governed by the genes that are active within them. If nose-cell genes are active in a cell, its fate is sealed. But earlier on in its existence, it might not have been so committed; it might have the option of being a nose cell or a cheek cell. At some point it was tipped down a pathway that led to the nose and there was no way back.
Even earlier on, when it left the neural crest, it had more options still. In fact, it was a stem cell - able to form a range of different cell types. But it was not pluripotent (able to turn into any kind of cell type).
So cells end up specialising. Gradually, as their gene programmes change, their options become more and more limited until they take on their final differentiated form. It's a bit like training for a job: when someone leaves school or university, there are many possible career routes open. After a period of training, a pilot cannot do what a teacher does (or vice versa).
So could this process of differentiation be reversed - the clock rolled back so that the cell regains its flexibility? This reprogramming could create stem cells able to replenish cells lost due to disease or injury. The Holy Grail would be to wind the cell all the way back to a state in which it was pluripotent.
That remains a dream - though one that could become a reality. Researchers have made great progress recently in reprogramming adult cells. Some of the key genes needed to make a cell pluripotent are now known - and, remarkably, it only takes a handful to be active for a cell to have a whole world of developmental opportunities opened up to it. Medically, this has enormous potential, as a way of generating new cells to replace those that have been lost due to disease or injury.
Image: Six-day-old human embryo implanting into the lining of the uterus; Yorgos Nikas, Wellcome Images