How neurons grow

Cultured hippocampal neurons

How do neurons know exactly where they're supposed to go? How do they connect to the correct parts of the brain, and don't just wander aimlessly? Well, let's use the retina of tadpoles as an example, being rather unique as when they become frogs their vision changes from monocular to binocular.
An axon from each retina cell is grown toward the optic area of the brain - seemingly pulled along by a growth cone at its tip, which then determines the direction the axon grows. Unsurprisingly, this direction of growth appears to be affected mostly by chemicals which attract and repel the growth cone, directing it to where it needs to be. When it reaches the optic part, the axons from each eye cross over each other, such that the right half of the brain corresponds to the left eye, and the left half to the right eye.
But once the metamorphosis into frog-y-ness starts, the nerves have to change, relatively quickly. In order for the frog to be able to see with both eyes together, the right and left halves of each eyes' axons must end up together.
So new neurons are once again grown - but they end up going to different places. How they do this was discovered by investigations from Christine Holt and Shin-ichi Nakagawa. They found that ephrin B, a gene which opposes the growth cone, is activated. The reason then that only half of the growth cones are affected is because only half express the gene receptor for ephrin B - so half are repelled, and half simply grow regardless of the gene, meaning that the frog can now correctly interpret its binocular vision.
So ephrin B acts as a signal for axons - and you guessed it! There are others for other axons, however perhaps fewer than you might think. So far four main types have been found - ephrins,semaphorins, slits, and netrins (although netrins are slightly different as they usually attract neurons while the rest repel them.) And although there may not seem to be enough, it is likely that these are actually all that are required - as scientists are finding them in many parts of the brain, for many many different animals. This is a rather good example of how only a few genes/proteins, can work in different ways to create many dissimilar things, which on the surface appear so different but are actually remarkably similar.
The practically-exactly-the-same small things make deceptively different big things.


Source: Nature via Nurture by Matt Ridley