Cell depletion (cell loss without equivalent replacement) happens in some of our most important tissues -- particularly the heart and some parts of the brain. It also happens in our muscles. Sometimes the gaps where the cells were are filled up by nearby cells getting bigger (as in the heart), sometimes by replacement by other types of cell or by fibrous acellular material (this happens in the brain and the heart) and sometimes not at all -- the tissue just shrinks (this happens in muscle).

Cell depletion can be fixed in three main ways. One is by "natural" stimulation of cell division; this is how exercise increases the size of muscles, though some muscles are much harder to stimulate in this way than others. The second way is by the artificial introduction (e.g., by injection) of growth factors that stimulate cell division; this works well in muscle and may also work in the thymus, an important part of the immune system.

Both natural and artificial stimulation of cell division have their limitations, however, partly because as a defence against cancer cells have a variety of blocks against dividing excessively. So, for example, there is evidence that stem cells in continuously renewing tissues like the blood tend to get depleted late in life in some mouse strains. This is the main reason why we'll almost certainly need the third way to fix cell depletion, which is to introduce new whole cells that have been engineered into a state where they will divide to fix the tissue even though cells already present within the body were not doing so. This is what stem cell therapy is. I guess I don't need to write any more about stem cell therapy: if you're reading this website, you're probably interested enough in medical progress that you know plenty about it already.

We need more work in all these three areas, even though they are all progressing very encouragingly.

Talks on exercise at IABG 10:
Ji      Powers      Tidball      Goto

Talks on growth factors at IABG 10:
Rosenthal      Aspinall      Brockes      Goldspink

Talks on growth factors at SENS2:
Conner

Talks on stem cells at IABG 10:
West      Svendsen      Sefton      Kirkwood      Van Zant      Haseltine