神经系统高级功能的实现有赖于神经环路的精确建立。神经元是神经环路的基本组成单位，不同的神经元之间依靠神经纤维（轴突）形成突触联系。同一类群的神经纤维在投射过程中紧密地组织在一起成为神经纤维束。脑内存在着类型各异的神经纤维束，纵横交错，广泛地联系着不同的脑区，使之成为一个具有高级整合功能的有机体。现有研究表明，在神经系统发育过程中，Sema3A对神经纤维束的正确形成起着至关重要的作用。我们初步的实验结果发现，下调Rab5能够影响皮层Ⅱ/Ⅲ层神经元轴突的成束化，并且在体外培养的皮层神经元中，Rab5的活性受Sema3A的调节。因此，在皮层投射神经纤维的发育中，Rab5可能是一个重要的调控分子。为了深入阐明Rab5的作用机制，我们将利用分子生物学、细胞生物学、生物化学、胚胎电转和光学成像等方法，进一步研究Rab5调节轴突成束化的分子和细胞学机制。本研究将为皮层发育机制的认识提供新的科学依据。

The nervous system's functions rely on the exact connections of neural circuits. Neurons that the basic units of the neural circuits are connected by nerve fibers (axons). A group of nerve fiebers with the same origin usually cling together and become axon tracts, axon fasciculation is the typical feature of the axon tracts. There are many types of axon tracts in brain, which work together to make the brain most integrated structures. Some research works reveal that Sema3A is essential for the axon fasciculation. Our primary research data show that Rab5 could be activated by Sema3A in cultured cortical layers Ⅱ/Ⅲ neurons, and down regulation of Rab5 impeded axon fasciculation of layers Ⅱ/Ⅲ projection neurons. Collectively, these primary data suggest that Rab5 play an important role in axon fasciculation during cortical development. In order to reveal the functional mechanism of Rab5 further, we will take advantage of a series of methods, such as molecular & cellular method, biochemical method, in uterus electroporation, optical imaging and so on, to explore the molecular and cellular mechanisms of Rab5 underlying axon fasciculation. This project will not only widen our knowledge about the role of Rab5 in axon fasciculation, but also provide new evidences to understand the mechanisms of cortical development.