There are so many factors for a stem cell to consider when deciding to remain a stem cell or develop into a specific cell type.  One factor, age, has not been explored thoroughly—thankfully, a recent paper from the journal Development discusses how the age of a stem cell affects its future.

Neurons and glial cells are two major cell types in the nervous system – neurons transmit the information of the nervous system, while glial cells provide support, protection, and nutrients to neurons.  Neurons and glial cells both come from the many divisions of neural stem cells (NSCs).  A recent paper describes the use of the simple nervous system in fruit flies to understand how NSCs decide to form neurons or glial cells.  A protein called Gcm drives the development of glial cells from NSCs, and Flici and colleagues found that introducing very high levels of this protein in NSCs forced a complete conversion of NSCs to glial cells.  In addition, the decision of NSCs to divide into glial cells drops as the stem cells get older.  Interestingly, after NCSs fall into a quiescent state at old age (meaning no more cell divisions), high levels of Gcm were no longer able to force glial cell formation.  In the images above, fly embryos are stained to show neurons (green) and glial cells (purple).  Control embryos (left) have few glial cells, while embryos with high levels of Gcm (right) have many glial cells.  The longer the embryos have increased levels of Gcm, the more glial cells are formed at the expense of neurons (top is early, bottom is late).