During fetal development, cells of the embryonic testis are organized to form tubular structures. The organizer ‘nurse’ cell (the Sertoli cell, shown in blue) arranges cells that will ultimately become sperm (germ cells, shown in red) to create the germ cell niche. During embryonic testicular development, germ cells enter a quiescent mitotic state. Cells that remain proliferative are associated with testicular dysgenesis and cancer. To test if germ cells have undertaken this developmental step, we regularly assess the proliferative state of germ cells (red) within the embryonic testis.