says. Skeletal muscle consists of thousands of muscle fibers, each controlled by one motor neuron whose cell body lies in the brain or spinal cord.

Connections between muscle and nerve cells are established embryonically when newborn neurons extend axons to ‘wire’ the appropriate muscle fiber. The wiring process is highly orchestrated –each motor neuron has already pledged allegiance to a particular muscle fiber before it reaches out to connect with its predetermined partner. But until now, scientists could only speculate how the invisible bond was formed. ‘The question was how these motor neurons know where to go,’ says Pfaff. ‘It would be a disaster if you wanted to move your arm and instead bent your back.’ Earlier studies suggested that muscles lining the spine sent out chemical cues as a siren song for specific motor neurons known as MMCm cells. But when attempts to identify the enticing substance failed, many started to doubt its existence.

After screening numerous candidates, the Pfaff team found not only that FGF is expressed in target muscle, but that FGF ‘sensors,’ known as FGF receptors, are expressed in MMCm motor neurons. Furthermore, MMCm axons could not ‘hear’ their muscle partner's call and failed to reach their destination in mouse mutants lacking the sensor molecule. Finally, using mice engineered to express a fluorescent protein in MMCm neurons, the investigators demonstrated that only the glowing neurons extended axons in the direction of target cells expressing FGF. ‘After a lot of hard work, we narrowed it down to FGFs and showed that they were indeed the long sought-after mysterious substance,’ says Pfaff. Neural stem cells can now be coaxed to develop into motor neurons in a test tube.

In that artificial environment, explains Pfaff, ‘Most external cues that guide immature motor neurons during embryonic development will be missing.’ Hence the need to identify axon guidance factors. He continues, ‘It is not enoug
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