Male fertility is in the bones

Researchers have found an altogether unexpected connection between a hormone produced in bone and male fertility. The study in the February 18th issue of Cell, a Cell Press publication, shows that the skeletal hormone known as osteocalcin boosts testosterone production to support the survival of the germ cells that go on to become mature sperm.The findings in mice provide the first evidence that the skeleton controls reproduction through the production of hormones.

Bone was once thought of as a "mere assembly of inert calcified tubes," the researchers explained. But in the last ten years, scientists have gained a much more dynamic picture of bone as a bona fide endocrine organ with links to energy metabolism and reproduction.

Prior work on skeletal ties to reproduction had focused primarily on the reproductive organs as a regulator of bone remodeling. That led researchers to wonder whether the influence might go the other way as well. Given the links between menopause and osteoporosis, his team anticipated such a connection would more likely turn up in females. But that's not what they found at all.

They found that the bones do control reproduction, but only in males. That was obviously a surprise, but that's the finding.

Osteocalcin produced by the bone-building cells known as osteoblasts induces testosterone production by the testes, but fails to influence estrogen production by ovaries, the researchers report.

Matings between normal females and osteocalcin-deficient males produced smaller and less frequent litters than those between typical males and females. Males lacking a second gene that inhibits osteocalcin's endocrine functions showed just the opposite: larger (though not significantly larger) and more frequent litters.

The researchers further showed that osteocalcin works through a receptor found on testosterone-producing Leydig cells in the testes.
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