The optic chiasm as a midline choice point.

The mouse optic chiasm is a model for axon guidance at the midline and for analyzing how binocular vision is patterned. Recent work has identified several molecular players that influence the binary decision that retinal ganglion cells make at the optic chiasm, to either cross or avoid the midline. An ephrin-B localized to the midline, together with an EphB receptor and a zinc-finger transcription factor expressed exclusively in the ventrotemporal retina where ipsilaterally projecting retinal ganglion cells are located, comprise a molecular program for the uncrossed pathway. In addition, the mechanisms for axon divergence in the optic chiasm are discussed in the context of other popular models for midline axon guidance.