Abstract: Objective To understand the function of Sonic hedgehog in chiasm development in mouse embryos of embryonic day 13(E13)to E15.Methods Brain slices of E13-E15 mouse embryos containing the optic pathway from the eyes to the optic tract were prepared and cultured in DMEM/F12 in the presence of 10% fetal bovine serum at 37 ℃ in a rolling incubator for 5 h.The antibody to Shh was added into the culture medium of the slices in the treatment group,while no additional chemical or only normal mouse IgG was added in the control groups.After culture,the brain slices were fixed and a DiI granule was inserted into the optic disc in one eye.Seven days later,the tissue overlying the chiasm was removed to expose the DiI-labeled chiasm for observation under confocal microscope,and the images were analyzed by METAMORPH software.Results Shh antibody treatment produced a reduction of crossing of the earliest retinal axons at the midline of E13 chiasm,and the uncrossed axons were also influenced by Shh antibody at E15.Conclusion Shh executes a transient but important function in axon decussation in the early stage of mouse optic chiasm development and signals axon turning in the later stage.