Domains of regulatory gene expression and the developing optic chiasm: correspondence with retinal axon paths and candidate signaling cells.

In mammals, some axons from each retina cross at the optic chiasm, whereas others do not. Although several loci have been identified within the chiasmatic region that appear to provide guidance cues to the retinal axons, the underlying molecular mechanisms that regulate this process are poorly understood. Here we investigate whether the earliest retinal axon trajectories and a cellular population (CD44 and stage-specific embryonic antigen 1 [SSEA] neurons), previously implicated in directing axon growth in the developing chiasm (reviewed in Mason and Sretavan [1997] Curr. Op. Neurobiol. 7:647-653), correlate with the expression patterns of several regulatory genes (BF-1, BF-2, Dlx-2, Nkx-2.1, Nkx-2.2, and Shh). These studies demonstrate that gene expression patterns in the chiasmatic region reflect the longitudinal subdivisions of the forebrain in that axon tracts in this region generally are aligned parallel to these subdivisions. Moreover, zones defined by overlapping domains of regulatory gene expression coincide with sites implicated in providing guidance information for retinal axon growth in the developing optic chiasm. Together, these data support the hypothesis that molecularly distinct, longitudinally aligned domains in the forebrain regulate the pattern of retinal axon projections in the developing hypothalamus.