An axon often has to navigate great distance to reach its target cell (ie, another neuron or muscle fiber) and establish the synaptic connection. This process is axonal pathfinding. The axon does not navigate its natural pathway due to any single cue, but via combinations of signals. This redundancy makes axonal pathfinding extremely complex. Axonal pathfinding is extremely specific: a particular axon always contacts the same (or at least very similar) set of neurons or muscles.
Neurons have an intrinsic polarity that is always the same for a specific type of neuron. At one end of this polarity, a localized increase in cell membrane activity occurs (with ruffles, aka lamellipodium often visible) and fine filopodia of the cell cytoplasm extend and withdraw. The structure of filopodia is maintained by microfilaments, while axonal strucure is maintained by microtubules. Eventually, a filopodium forms a growth cone, an actively motile region at the far end.
The growth cone travels based on cues from a substrate called the extracellular matrix (ECM). The ECM contains laminin, collagen and fibronectin (the first being the most important) that adhere to cadherins and integrins in the growth cone membrane. Also, the ECM contains unique molecules (ie, nerve growth factor and retinoic acid) that guide the growth cone, sometimes across a gradient and other times along a narrow path. These unique molecules are deposited: by tissues over which the axons grow (ie, epithelia, somites and blood vessels); by cells at crucial migratory points; and even by the target tissue itself.
Once the growth cone reaches its specific target (ie, muscle fibers or another neuron) it must stop growing and establish a synaptic connection. The high specificity of axonal pathfinding is determined by any of three ways:
|Timing||There may a single cell (or group of cells) mature enough to form a synapse with the incoming axon. Therefore, specificity is determined by the timetable of donor and receiver neuronal maturation.|
|Chemoaffinity||The growth cone and target cell may express matching recognition molecules that restrict interactions with other cells. Once in the target region, the growth cone locates its target cell by unique molecular tags that bind to recognition molecules on the target cell.|
|Pruning||Axons may bind target cells in a somewhat non-specific manner; improper connections are recognized via neuronal activity or trophic factors produced by the target cell. Incorrect connections are eliminated via cell death or selective retraction of certain axonal branches.|
Commisural neurons are interneurons in the (usually dorsal) spinal cord that extend their axons across the midline. Commisural projects project ventrally toward the floor plate, and then cross the midline ventral to the floorplate of the spinal cord. Recombination experiments have shown that the floor plate secretes netrin, an attractive signal critical for commisural axons to migrate to the floor plate and cross the midline. In netrin-/- mutants, commisural axons fail to grow to the floor plate.
Ephrins are membrane-bound ligands that bind to membrane-bound ephrin-receptors called ephs. When an ephrin and an eph bind, a signal is generated in both cells -- this means the signal is bi-directional. Ephins are repulsive signals in axon migration, with ephrin/eph signaling responsible for setting the retino-tectal topographic map. In a topographic map, the spatial organization of neurons in one region (retina) is replicated in a connected region (tectum). The retina and tectum contain inverse gradients of ephs and ephrins; axons extend from the retina to the tectum until reaching a signaling threshold (inversely proportionate to their starting point) that inhibits further navigation. Thus, axons from a retinal region with low levels eph will target a tectal region with high levels of eph. In the figure below, axons from a given retinal quadrant will target the tectal quadrant of matching color.
All in all, ephrin/eph signaling is critical for:
- motor neurons extending only through the anterior of a somite;
- setting the retino-tectal topographic map;
- inhibiting mixing -- rhombomeres, compartments in the hindbrain across which cells do not mix, express ephrins and ephs.