Stem cell therapies are one theoretical path towards therapies for sarcopenia, the loss of muscle mass and strength with age. Here, researchers have discovered "the mechanism that causes stem cells in the embryo to differentiate into specialised cells that form the skeletal muscles of animals' bodies. ... The field has the potential to revolutionise medicine by delivering therapies to regenerate tissue damaged by disease or injury. Differentiation happens soon after fertilisation, when embryonic cells are dividing rapidly and migrating as the animal's body takes shape. ... The scientists investigated the effect of a known signalling pathway called NOTCH on muscle differentiation. They found that differentiation of stem cells to muscle was initiated when NOTCH signalling proteins touched some of the cells. These proteins were carried by passing cells migrating from a different tissue - the neural crest - the progenitor tissue of sensory nerve cells. Muscle formation in the target stem cells occurred only when the NOTCH pathway was triggered briefly by the migrating neural crest cells. ... This kiss-and-run activation of a pathway is a completely novel mechanism of stem cell specification which explains why only some stem cells adopt a muscle cell fate. ... the team would now focus on unravelling the mechanisms of embryonic muscle cell differentiation at the molecular level as a necessary step to regulating regeneration of the muscles in human patients."