Membrane molecule guides nerve growth.

Nerve cells in the visual system of a developing embryo wire themselves properly by avoiding a specific molecule on the outer membranes of other nerve cells, new studies of chicks and fetal mice indicate.

The long, tail-like axons of nerve cells in the eye's retina plug into the correct sockets in developing animal's optic tectum -- the brain region that coordinates visual stimuli -- by seeking out the lowest concentration of a chemical named "repulsive guiding molecule" (RGM), asserts Friedrich Bonhoeffer of the Max Planck Institute for Developmental Biology in Tubingen, Germany. Bonhoeffer reports that the axons of retinal nerve cells grown in the laboratory avoided high concentrations of membrane-bound RGM, preferring regions of a culture dish with low RGM levels.

He speculates that cells bearing RGM form one of two chemical gradients that sprouting retinal axons use to orient themselves. Bonhoeffer and his colleagues have used labeled antibodies to show that RGM levels increase steadily from the front to the back of an animal's optic tectum. He hypothesizes that a second, unidentified chemical may increase from the back to the front of the optic tectum, forming an opposite gradient.

By finding their optimal positions between these two gradients, retinal axons replicate in the optic tectum the same visual pattern that their cell bodies create in the retina itself, Bonhoeffer suggests. In this way, retinal cells convey a visual stimulus from a specific part of the retina to its corresponding part of the tectum, which allows an animal to tell where in its field of vision a given stimulus originated.

Bonhoeffer says it's too early to tell whether RGM might help reprogram the retinal axons of people with some vision disorders. But, he adds, "it would be interesting to see."