Ribak Charles E, Shapiro Lee A
Department of Anatomy & Neurobiology, University of California at Irvine, Irvine, CA 92697-1275, USA.
Brain Res Rev. 2007 Oct;55(2):390-4. doi: 10.1016/j.brainresrev.2006.12.005. Epub 2006 Dec 23.
Ramon y Cajal described the fundamental morphology of the dendritic and axonal growth cones of neurons during development. However, technical limitations at the time prevented him from describing such growth cones from newborn neurons in the adult brain. The phenomenon of adult neurogenesis is briefly reviewed, and the structural description of dendritic and axonal outgrowth for these newly generated neurons in the adult brain is discussed. Axonal outgrowth into the hilus and CA3 region of the hippocampus occurs later than the outgrowth of dendrites into the molecular layer, and the ultrastructural analysis of axonal outgrowth has yet to be completed. In contrast, growth cones on dendrites from newborn neurons in the adult dentate gyrus have been described and this observation suggests that dendrites in adult brains grow in a similar way to those found in immature brains. However, dendrites in adult brains have to navigate through a denser neuropil and a more complex cell layer. Therefore, some aspects of dendritic outgrowth of neurons born in the adult dentate gyrus are different as compared to that found in development. These differences include the radial process of radial glial cells acting as a lattice to guide apical dendritic growth through the granule cell layer and a much thinner dendrite to grow through the neuropil of the molecular layer. Therefore, similarities and differences exist for dendritic outgrowth from newborn neurons in the developing and adult brain.
拉蒙·伊·卡哈尔描述了神经元在发育过程中树突和轴突生长锥的基本形态。然而，当时的技术限制使他无法描述成体大脑中新生神经元的此类生长锥。本文简要回顾了成体神经发生现象，并讨论了成体大脑中这些新生神经元树突和轴突生长的结构描述。轴突向海马体门区和CA3区的生长比树突向分子层的生长要晚，并且轴突生长的超微结构分析尚未完成。相比之下，成体齿状回中新生神经元树突上的生长锥已被描述，这一观察结果表明，成体大脑中的树突生长方式与未成熟大脑中的相似。然而，成体大脑中的树突必须在更密集的神经毡和更复杂的细胞层中穿行。因此，与发育过程中相比，成体齿状回中新生神经元的树突生长在某些方面有所不同。这些差异包括放射状胶质细胞的放射状突起作为一个晶格，引导顶端树突穿过颗粒细胞层，以及一条细得多的树突穿过分子层的神经毡。因此，发育中和成体大脑中新生神经元的树突生长既有相似之处，也有不同之处。
