He proposes using a live mouse as a 'host' to make human sperm.

Once taken off the compound, the mice continued to make sperm.

A healthy male may be able to make sperm well into his 80's or longer.

It allows the cells that make sperm to be put on ice and grown up in another species many years later.

Some men, he said, start to make sperm, but then the process halts.

And in some species, individuals make both eggs and sperm (Box 1).

These added genes, not surprisingly, all have functions involved in making sperm.

Boys begin making sperm when they start growing into men, at a time of life called puberty.

If a man's testicles are kept more warm than usual, they make less sperm.

The testicles have two main jobs: to make hormones and sperm.