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 def ining the midline and posterior boundaries of the future optic chiasm. These neurons express L1, an immunoglobulin superfamily molecule know n to promote retinal axon outgrowth, and CD44, a cell surface molecule that we find inhibits embryonic retinal axon growth in vitro. Incomin g retinal axons do not penetrate this L1/CD44 neuron array, but turn t o establish the characteristic X-shaped optic chiasm along the anterio r border of this array. These results suggest that L1/CD44 neurons may serve as an anatomical template for retinal axon pathways at the embr yonic mouse ventral diencephalon.