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Axonal pathfinding is the process of finding the right targets by growing axons. For most axons, finding the correct target is not trivial and requires covering long distances, while making a number of correct turns and sometimes even going backwards. Guidance cues play an essential role in this process by serving as road signs and telling the axon where to go (attractive molecules) and where not to go (repellent molecules).

One example in the development of the mammalian nervous system where guidance cues are used is the crossing of the midline by the spinal commissural axons. Slit proteins acting as ligands for their receptors Robo ensure that (1) axons that should not cross the midline never do so, and (2) those that have to cross, do so only once and do not re-cross.

Select the BEST mechanism of how Slit/Robo can be utilized during midline crossing.


Robo receptors are expressed on all spinal commissural axons, and Slit is an attractant molecule expressed in the midline. Slit becomes a repellent molecule for the axons that crossed the midline after crossing.


Robo receptors are expressed only by the axons that intend to cross the midline, and Slit is an attractant molecule expressed in the midline.


Slit is a repulsive cue expressed on the surface of axons that do not cross. Axons that cross the midline express Robo receptors, and they are repelled from the periphery and are forced to cross the midline.


Slit is a repulsive cue expressed in the midline. Robo receptors are expressed by axons staying on the same side at all times, while axons that do cross the midline only begin to express Robo after crossing.

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