A study of mechanisms for formation of thalamocortical projectic

在大脑发育过程中，轴突在到达目标时停止生长并形成分支。丘脑皮质连接是研究这一问题的一个合适的系统，因为感觉丘脑轴突主要投射到新皮层的第4层，该层具有很好的细胞结构特征。迄今为止的发育研究表明，丘脑轴突可以通过皮层中的层特异性信号识别目标层，但其分子机制几乎仍然未知。我们试图揭示可能的因素的特点，负责层特异性终止和分支丘脑皮质轴突。以大鼠胚胎丘脑轴突延伸在化学固定后的新生皮层切片上作为实验模型系统。在这些条件下，可以分析膜结合成分的作用，而不受皮层组织释放的可溶性因子的污染。丘脑轴突甚至在固定皮层上延伸，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。在培养前对皮质切片进行酶处理进一步表明，轴突生长的层状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皮层切片的第4层。在神经氨酸酶处理的皮层切片中，随着分支数量的增加，这种分支形成的层状定位略有减弱。这些发现表明GPI锚定分子和含唾液酸分子分别通过抑制活性调节丘脑特定层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.