Stem cells will likely play a central role in future medicine, due to their unique ability to become any type of cell stored in its complete genetic data. Every cell of your body carries a complete copy of all the genes, including those coding for at least 200 different types of human cells (which actually pales to the number of different symbiotic and neutral bacteria living in us). The difference between a “stem cell” and an adult cell is that not all genes are able to be expressed (or active) in an adult/differentiated cell: the majority of the genome/transcriptome lies silent in order for this specialized cell to do its job, and only the genes this cell needs tend to be “turned on” or up-regulated.
What this means is that an unfertilized or newly fertilised egg is a stem cell, containing the two “mixed” chromosomes (one from each parent, and each of those representing a mix of the genomes they inherited from their parents). For each gene, one copy (sometimes both) is selected and “expressed”, which genes are selected are dependant on cell type, determining its specific pattern of differentiation from the “pluripotent” stem cell. Pluripotent means it can turn into any cell type, such as blood, bone, skin, neurons etc…
The first research with stem cells required using fertilised eggs (embryos), but advances soon allowed the use of unfertilized eggs around 2007. More recently, scientists have managed to induce already adult cells into retaking a pluripotent form, including from skin cells and more recently even fat cells. This means that doctors can theoretically, and will eventually literally, reprogram your own cells to compensate for cells which have developed incorrectly or are damaged. This may include merely transplanting other healthy cells with a functional genome, or correcting any genetic problems and then reintroducing a more functional version. This has been shown to be useful in terms of recovering cognitive capacity after brain trauma and implicates the future use of stem cells in many problems (including healing broken bones, missing limbs, scars etc).
Future casts may not just shield your wound from the outside world, but may contain stem cells which themselves aid in your recovery by supplying the raw resources your body needs to, for instance, recover a lost hand. By allowing people to repair cell damage or disabilities, or even “improve” their genome, this technology may change the path of human evolution. It may even lead the way to a form of immortality, as seen in the jellyfish Turritopsis dohrnii, opening the door to a whole new world of ethical questions.
Research even shows that crippled mice, when given stem cells, develop muscles stronger than they were before. The opportunities this technology offers for the individual is huge: both for the ill and the healthy. But in the end, it doesn’t make sense to focus on extending the life of the individual while ignoring the survival of the species.