A sperm cell attempts to penetrate an ovum coat to fertilize it.

A French laboratory has announced that it has obtained complete human sperm in vitro - a world first, although experts have reacted cautiously to an apparent success that scientists have been chasing for 15 years.

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The Kallistem laboratory, in the central French city of Lyon, announced this week that its researchers obtained "complete human spermatazoa in vitro" from cells taken from "immature" testicles this week.

The research, which will not be published in a peer-reviewed journal before 23 June, "paves the way for innovative therapies to preserve and restore male sterility," the laboratory said, adding that this is "a major issue [because] numbers of spermatozoa have declined by 50 per cent over the last 50 years.”

“If it works, this procedure opens great prospects,” said Nathalie Rives head of a hospital centre for medically assisted procreation, although she remained cautious about the fact that adults suffering from a complete lack of sperm could also have “genetic anomalies which would prevent in vitro spermatogenesis”.

The procedure is preferable to reproductive cloning, Professor Israel Nisand, co-founder of the European Bioethics Forum, told Le Figaro newspaper.

“If this is true, it is a considerable step forward in treating male sterility,” he said.

Kallistem will hold preclinical trials until 2016 and clinical trials in 2017, aiming to market its technologies by 2020.

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