On May 27, 1995, Christopher Reeve, the iconic portrayer of Superman, broke his spine after falling from a horse; he never regained the use of his lower limbs. Injuries that damage the spinal cord often have devastating effects because the cells in your spinal cord, called neurons, have stopped growing by the time you reach adulthood. Unlike blood vessels or liver cells, for example, which can regenerate in damaged areas, neurons don’t have the ability to regrow after injuries. That means spinal or brain injuries are often permanent. But how is it that some types of cells constantly regenerate (you can safely donate pints of blood throughout your life without depleting your blood supply), while other cells are fixed from childhood for your whole life?
Part of the answer has to do with a topic that has generated plenty of controversy in the last decade: stem cells. The single-celled egg out of which a human being grows is a stem cell. That one cell has the ability to divide myriad times, and stem cells can “grow up” into any type of cell in your body – muscle cells, neurons, hair follicles, blood vessels etc. This “growing up” is a forward process. Just like a boy’s choice to quit playing soccer and focus on engineering means that he cannot be a professional athlete a decade later, stem cells that become muscle cells can’t suddenly turn into, say, heart cells.
Most of the cells in your body are “adult” cells with a specialized job. However, some cells need to maintain their “stemness” in order to regenerate cells that we regularly lose, such as skin cells and blood cells. Unfortunately, these stem cells aren’t true stem cells. The cells that regenerate your blood are blood-specific stem cells, and can only grow into one of the many types of cells found in your blood; the stem cells that regrow skin cells are skin-specific, too. Neither of these can be prodded into turning into other cell types. So how do we get true stem cells in order to grow new neurons for people with injuries like Christopher Reeve’s?
Scientists are finding that cells are more reflective of the human adults they comprise than we originally thought. Let’s go back to our engineer who blew his chance to play soccer by quitting when he was younger. What if he got in crazy good shape, quit his job, and started training with the best soccer coaches in the world? He might get good enough to play soccer…even if it wasn’t for the number one team. Similarly, scientists have figured out how to reprogram an adult cell into a stem cell, which enables them to study stem cells without having to resort to the controversial embryonic stem cells that were originally used in the 1990s. (So no more ethical dilemmas – we can go back to focusing on the science.)
Eventually, scientists will figure out how to dictate the path a stem cell follows as it grows up, so that they can grow entire organs to replace a failing kidney or heart. They’ll be able to regenerate damaged muscle tissue and develop better treatments for severe burn victims. We don’t fully understand yet how to influence the choices cells make about what type of cell to become, but once we do the possibilities could be endless. One day stem cells will even enable patients with spinal injuries to regain the ability to walk.
More detail about different types of cells that can regrow in the human body: https://www.hhmi.org/askascientist/answers/the_human_body_is_constantly_replacing_and_regenerating_cells_to_maintain_proper_and_efficient_funct.html
Reprogramming adult cells into stem cells won the Nobel prize last year: (complete with a longer description of how the reprogramming works)