Callaway E M
Systems Neurobiology Laboratory, The Salk Institute for Biological Studies, La Jolla, California 92037, USA.
J Neurosci. 1998 Feb 15;18(4):1505-27. doi: 10.1523/JNEUROSCI.18-04-01505.1998.
Previous studies have demonstrated that axonal arbors specific for the four main cortical layers - 2/3, 4, 5, and 6 - develop precisely from the outset using activity-independent cues. In macaque primary visual cortex (V1), layer 2/3 is subdivided into layers named 2/3A, 3B, 4A, and 4B, and layer 4 is subdivided into 4Calpha and 4Cbeta. Individual neurons in V1 of mature macaques have axonal arbors that are highly specific for these sublayers. We have studied the prenatal development of laminar and sublaminar specificity of local circuits in macaque V1. Two-hundred thirty-eight neurons were labeled intracellularly in living brain slices prepared from V1 of five prenatal macaque monkeys aged 100 to 145 d postconception (E100-E145). Axonal and dendritic arbors of labeled neurons were reconstructed to assess their relationships to the cortical layers. We find that developing spiny neurons in layers 2-4B and layer 5 specifically target superficial and deep layers without forming "incorrect" branches in layer 4C. Similarly, layer 6 pyramidal neurons that target layer 4C do not form "incorrect" branches in layer 5. These results indicate that specific projections to the main cortical layers develop with a high degree of selectivity, as in other species. However, the development of sublayer-specific projections was not always precise from the outset. Unlike postnatal animals, axons of some prenatal layer 4Cbeta spiny neurons branch in layer 4B. At similar ages, many pyramidal neurons in the upper half of layer 6 have axonal branches in layer 4Calpha as well as 4Cbeta; these projections are specific for 4Cbeta in more mature animals. Also, there is similar "exuberance" in axonal arbors of other layer 6 cell types. Transient projections were also observed in the subplate and to the white matter for cells from all layers, except 4Cbeta. These observations indicate that at least some sublayer-specific projections emerge by elimination of exuberant axonal branches and suggest that they may use activity-dependent mechanisms to identify "correct" target layers. Such cues could be provided by laminar differences in the patterns of spontaneous prenatal activity in the retino-geniculo-cortical network.
先前的研究表明，特定于四个主要皮质层（2/3层、4层、5层和6层）的轴突分支从一开始就利用与活动无关的线索精确发育。在猕猴初级视觉皮层（V1）中，2/3层又细分为2/3A层、3B层、4A层和4B层，4层细分为4Cα层和4Cβ层。成年猕猴V1中的单个神经元具有对这些亚层高度特异的轴突分支。我们研究了猕猴V1中局部回路的层状和亚层特异性的产前发育。在从五只受孕后100至145天（E100 - E145）的产前猕猴的V1制备的活脑切片中，对238个神经元进行了细胞内标记。重建标记神经元的轴突和树突分支以评估它们与皮质层的关系。我们发现，2 - 4B层和5层中正在发育的棘状神经元特异性地靶向浅层和深层，而不在4C层形成“错误”分支。同样，靶向4C层的6层锥体神经元不在5层形成“错误”分支。这些结果表明，与其他物种一样，向主要皮质层的特定投射以高度的选择性发育。然而，亚层特异性投射的发育从一开始并不总是精确的。与出生后的动物不同，一些产前4Cβ层棘状神经元的轴突在4B层分支。在相似的年龄，6层上半部分的许多锥体神经元在4Cα层以及4Cβ层都有轴突分支；在更成熟的动物中，这些投射对4Cβ层是特异的。此外，6层其他细胞类型的轴突分支也有类似的“过度生长”现象。对于除4Cβ层之外所有层的细胞，在中间板和白质中也观察到了短暂投射。这些观察结果表明，至少一些亚层特异性投射是通过消除过度生长的轴突分支而出现的，并表明它们可能利用与活动相关的机制来识别“正确”的目标层。这种线索可能由视网膜 - 膝状体 - 皮质网络中产前自发活动模式的层状差异提供。
