Embryonic neurons of the developing optic chiasm express L1 and CD44, cell surface molecules with opposing effects on retinal axon growth.

The first retinal ganglion cell axons arriving at the embryonic mouse ventral diencephalon encounter an inverted V-shaped neuronal array defining the midline and posterior boundaries of the future optic chiasm. These neurons express L1, an immunoglobulin superfamily molecule known to promote retinal axon outgrowth, and CD44, a cell surface molecule that we find inhibits embryonic retinal axon growth in vitro. Incoming retinal axons do not penetrate this L1/CD44 neuron array, but turn to establish the characteristic X-shaped optic chiasm along the anterior border of this array. These results suggest that L1/CD44 neurons may serve as an anatomical template for retinal axon pathways at the embryonic mouse ventral diencephalon.