 His team was the first in the world to isolate a new type of stem cell from adult mouse testes (male sex glands), called spermatagonial stem cells.It was able to show that some of these stem cells, called multipotent adult germline stem cells (maGSCs), turned into heart, muscle, brain and other cells.Prof Nayernia and his team proposed that similar cells could be extracted from men using a simple testicular biopsy.