It’s a big day for sperm science. In the journal Nature today, scientists from Japan describe how they were able to mature mouse sperm in a petri dish outside of the testis.
The sperm assembly line is a complicated series of steps that takes about two months from start to finish. Sperm start off as big, round immature cells called “spermatogonia” whose main job is to multiply. At some point, cells decide to turn into full-fledged sperm, becoming cells known as “spermatocytes”. A spermatocyte splits into halves, becoming a “spermatid” in a process called meiosis, which will allow its precious genetic cargo ultimately to combine with its complementary other half waiting in an egg. The spermatid half cells finally transform their shape, growing propellers, outboard motors and egg-digesting caps in becoming “spermatozoa”.
Until now, scientists were unable to get immature sperm cells to grow outside the body into those exquisitely shaped torpedo half cells. That’s where Takuya Sato and fellow scientists have succeeded. By carefully controlling the conditions for the growing sperm’s bed, these scientists discovered how to make mature sperm outside the body.
The practical implications are enormous. Sperm can be frozen before chemotherapy to spare a man’s fertility, but only from an adult man already making mature sperm. We don’t yet have a similar way to preserve the future fertility of a boy who has yet to go through puberty, and this discovery may someday allow doctors to freeze a small piece of testis from a boy about to have chemotherapy and then mature his sperm in a petri dish later in his life. For men with “maturation arrest“, where the sperm assembly line stops midstream, it might be possible to grow their sperm to completion in a petri dish, and use the grown sperm in in-vitro fertilization.
The distance between Sato and his fellow scientists’ discovery and practical use is not small. Men are different than mice, and trying to shepherd science into the doctor’s office always brings unforeseen challenges. But this discovery is a big leap for sperm science.