For the past 25 years, men whose sperm can’t manage the arduous swim across a Petri dish have had the option of injecting a single sperm cell directly into an egg. But that method still left some sterile men out in the cold.
Human testes are masters of mass production, spitting out sperm at a rate of 200 million per day. But that doesn’t mean the process is fast–it takes 64 days to make a sperm. The organ keeps the count high with an assembly-line anatomy that scales up sperm development from a trickle to a flood.