Stem cells are immature cells that have the potential to convert into bone, muscle, blood vessels, nerve fibers, and other body cells and tissues. However, older stem cells are not as robust as young ones.  Xiao-Dong Chen, from the University of Texas Health Science Center San Antonio (Texas, USA), and colleagues report the successful rejuvenation of old stem cells via placement in a young microenvironment.  Positing that giving stem cells a youthful environment for growth would cause them to regenerate faster, the team extracted mesenchymal stem cells from the bone marrow of 3-month-old mice and 18-month-old mice. The researchers also obtained extracellular matrix (ECM) from mice of both ages. ECM is a scaffold of connective tissue, such as collagen, which constitutes a majority of the body’s structure.  The team then seeded half of the older stem cells on ECM from the 3-month-old mice and half on ECM from the 18-month-old mice. Likewise, half of the young stem cells were seeded on the young ECM and half were seeded on the old ECM. Young and old cells showed a 16.1-fold and 17.1-fold expansion, respectively, when grown on ECM from young mice, compared to a 4.1-fold and 3.8-fold expansion when grown on ECM from old mice.  The study authors write that: “We concluded that aging negatively affects the formation of an ECM that normally preserves [mesenchymal stem cell] function, and aged [mesenchymal stem cells] can be rejuvenated by culture on a young-ECM.”