Richards L J
Department of Anatomy and Neurobiology, and the Program in Neuroscience, School of Medicine, University of Maryland, Baltimore, MD 21201, USA.
Braz J Med Biol Res. 2002 Dec;35(12):1431-9. doi: 10.1590/s0100-879x2002001200004.
The corpus callosum is a large fiber tract that connects neurons in the right and left cerebral hemispheres. Agenesis of the corpus callosum (ACC) is associated with a large number of human syndromes but little is known about why ACC occurs. In most cases of ACC, callosal axons are able to grow toward the midline but are unable to cross it, continuing to grow into large swirls of axons known as Probst bundles. This phenotype suggests that in some cases ACC may be due to defects in axonal guidance at the midline. General guidance mechanisms that influence the development of axons include chemoattraction and chemorepulsion, presented by either membrane-bound or diffusible molecules. These molecules are not only expressed by the final target but by intermediate targets along the pathway, and by pioneering axons that act as guides for later arriving axons. Midline glial populations are important intermediate targets for commissural axons in the spinal cord and brain, including the corpus callosum. The role of midline glial populations and pioneering axons in the formation of the corpus callosum are discussed. Finally the differential guidance of the ipsilaterally projecting perforating pathway and the contralaterally projecting corpus callosum is addressed. Development of the corpus callosum involves the coordination of a number of different guidance mechanisms and the probable involvement of a large number of molecules.
胼胝体是连接左右大脑半球神经元的大型纤维束。胼胝体发育不全（ACC）与许多人类综合征相关，但对于ACC发生的原因知之甚少。在大多数ACC病例中，胼胝体轴突能够向中线生长，但无法穿过中线，而是继续生长成称为普罗布斯特束的大型轴突漩涡。这种表型表明，在某些情况下，ACC可能是由于中线轴突导向缺陷所致。影响轴突发育的一般导向机制包括由膜结合或可扩散分子呈现的化学吸引和化学排斥。这些分子不仅由最终靶点表达，还由沿途的中间靶点以及作为后来到达轴突导向的先驱轴突表达。中线神经胶质细胞群体是脊髓和大脑（包括胼胝体）中连合轴突的重要中间靶点。本文讨论了中线神经胶质细胞群体和先驱轴突在胼胝体形成中的作用。最后，探讨了同侧投射的穿孔通路和对侧投射的胼胝体的差异导向。胼胝体的发育涉及多种不同导向机制的协调以及大量分子可能的参与。
