A developing baby's brain is a seething mass of chemical signals, instructing nerve cells to seek out new connections with each other. One of these signals is called semaphorin, named after the flag-waving communication system semaphore. Researchers have now discovered that semaphorin also plays an important role in the adult brain, stimulating nerve cells to produce sex hormones. The red blob on the right is a cluster of hormone-producing nerve cells from a rat, which have been exposed to semaphorin signals produced by a particular part of the brain. This makes them shoot out tiny tendrils that release a hormone triggering egg production (ovulation) in females. On the left is a clump of the same cells that have been treated with a drug that stops the signal getting through – no signal means no tendrils, and no hormone production – showing that semaphorin plays a crucial part in controlling ovulation.
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