A study of mechanisms for formation of thalamocortical projectic

在大脑发育过程中，轴突停止生长并形成分支。丘脑皮质连接是研究这一问题的合适系统，因为丘脑感觉轴突主要投射到新皮质的第四层，具有良好的细胞结构特征。迄今为止，发育研究表明，丘脑轴突可以通过皮质板层特有的线索识别靶层，但其分子机制几乎仍不清楚。我们试图揭示导致丘脑皮质轴突层特异性终止和分支的可能因素的特征。为此，本文对丘脑皮质轴突的层特异性终止和分支机制进行了研究以大鼠胚胎丘脑在化学固定的新皮质脑片上的轴突延伸为实验模型系统。在这种条件下，可以分析膜结合成分的作用而不受皮质组织释放的可溶性因子的污染。丘脑轴突甚至延伸到固定的皮质上，并且在深层(第5层和第6层)比上层(第1层到第4层)生长得更远。培养前对皮质切片的酶处理进一步表明，轴突生长的层间差异主要是由于存在于上层的PI-PLC敏感成分。另一方面，丘脑轴突在深层(第5层和第6层)的生长比上层(第1层到第4层)更远。进一步证明，培养前对皮质切片的酶处理进一步表明，轴突生长的层间差异主要是由于存在于上层的PI-PLC敏感成分。另一方面，丘脑轴突在深层(第5层和第6层)比上层(第1层到第4层)生长得更远。进一步证明，轴突生长的层间差异主要是由于存在于上层的PI-PLC敏感成分。另一方面，轴突分支主要发生在固定的皮质切片的第四层，神经氨酸酶处理的皮质切片中，这种分支形成的板层定位略有减弱，分支的数量增加。这些发现表明，GPI锚定的分子和含唾液酸的分子分别通过抑制活性来调节层特异性丘脑轴突的生长和分支的形成。

In the development of the brain axons stop growing and form branches upon reaching their targets.The thalamocortical connection is a suitable system for studying this issue, because sensory thalamic axons project primarily to layer 4 of the neocortex, which is well characterized by cytoarchitectonic structure.To date developmental studies have suggested that thalamic axons can recognize the target layer through lamina-specific cues in the cortex, but the molecular mechanism has almost remained unknown.We attempted to reveal characteristics of possible factors that are responsible for layer-specific termination and branching of thalamocortical axons.For that purpose, axonal extension from rat embryonic thalamus on postnatal neocortical slices which had been fixed chemically was used as an experimental model system.Under these conditions the role of membrane-bound components can be analyzed without contamination of soluble factors released from cortical tissues.Thalamic axons extended even on fixed cortex and exhibited farther growth in the deep layers (layers 5 and 6) than the upper layers (layers 1 through 4).Enzymatic treatment to cortical slices prior to culturing further demonstrated that the laminar difference in axonal outgrowth was primarily due to PI-PLC-sensitive components which are present in the upper layers.On the other hand, axonal branching occurred mostly in layer 4 of the fixed cortical slices.This laminar localization of branch formation was slightly weakened in neuraminidase-treated cortical slices with an increase in the number of branches.These findings suggest that GPI- anchored and sialic acid-containing molecules regulate layer-specific thalamic axon growth and branch formation, respectively, through inhibitory activity.