Introduction

In the vertebrate visual system, retinal ganglion cell (RGC) axons from each eye grow toward the brain and meet at the midline of the ventral diencephalon. Here they establish an X-shaped pathway, the optic chiasm. This major brain decussation, or crossing, carries fibers from the retina to targets in the thalamus (the lateral geniculate nuclei, LGN) and the midbrain (superior colliculus, or optic tectum in lower vertebrates). The projection through the chiasm establishes connections in central targets for an orderly topographic map of the retina. During development of the optic chiasm, RGC axons from each eye diverge from one another to grow to the optic tract on the same and opposite sides of the brain, a projection pattern that subserves binocular vision in higher animals.

Here we will review optic chiasm development, including the axon paths and behaviors of RGCs during their growth as they form the chiasm, and the relationships of RGC growth cones to specialized cells positioned in and around the optic chiasm. Where relevant, we will highlight differences across species in terms of degree of binocularity and the developmental principles underlying the plan of retinal projection. We will then discuss the recent progress on uncovering the cellular and molecular mechanisms directing chiasm formation. Finally, we will discuss aberrations in chiasm development, such as in albinos, in which reduced ocular pigment is linked to a decrease in uncrossed projections, and in a rarer condition in which complete failure of optic chiasm development occurs.