| In the present study, we investigated expression and function of several molecules in the development of the mouse optic chiasm. Shh was found in the chiasm as early as E12 before the arrival of retinal axons. At later development stage, Shh was found in optic stalk, chiasm and optic tract, demonstrating a defined pattern. At E14 and E15, Shh expression was detected in the optic stalk, and was downregulated when axons approached the midline of the chiasm. After crossing the midline, Shh expression was upregulated again. In the tract, Shh staining was obvious in deeper region of optic axon layer. However, Shh expression was not detected on axons and growth cones in culture of retinal explants. Patch, the receptor of Shh and Smoothened, another membrane protein involved in the signaling of Shh, were found on growth cones. Disturbing the Shh function with cyclopamine and 5E1, the antibody to Shh generated abnormal axon crossing and optic tract-finding, suggesting the important role of Shh in signaling the growth of optic axons.; Sema3a and its receptor neuropilin also showed a particular pattern in optic axons in our study. At E13, very few axons crossed the midline after Sema3a antibody treatment. While axons grew normal when brain slices were treated with anti-neuropilin antibody. This suggested that Sema3a might signal axon growth not only through neuropilin. Last, we examined the functions of polysialylated form of neural cell adhesion molecule (PSA-NCAM) and heparin on axon routing in the mouse chiasm. Treatment of the E14 and E15 brain slices with 5A5, an antibody against PSA-NCAM, produced a defasciculation of axons when they crossed the midline of the chiasm, but had no obvious effect on the routing of the uncrossed axons. Although exogenous heparin caused axon defasiculation at E14, it could prevent the defasciculation induced by 5A5 at this developmental stage. Furthermore, heparin treatment produced fewer uncrossed axons at E15 but not at E14. All these results suggested that PSA-NCAM and heparin might play an important role in guiding axon growth during the development of chiasm. |
