LYON, France — A French startup working with a top government lab said it has developed in-vitro human sperm, claiming a breakthrough in infertility treatment sought for more than a decade.

Researchers with Kallistem had announced the discovery previously, but they and French government lab CNRS described how it works for the first time Thursday after taking out a patent on the process.

They have developed sperm from immature cells known as spermatogonial cells, which are present in all males, and under normal conditions develop into sperm cells once puberty starts. The technology must now be clinically tested.

Philippe Durand, the chief Kallistem researcher, said the genesis of the research was indications that male fertility was declining, which he said could be attributed to environmental factors.