Higashi S, Yamazaki M, Murakami F
Department of Biophysical Engineering, Faculty of Engineering Science, Osaka University, Japan.
J Comp Neurol. 1990 Sep 15;299(3):312-26. doi: 10.1002/cne.902990305.
Morphological changes in individual corticorubral fibers and the pattern of crossed and uncrossed corticorubral projections were studied during the postnatal development of cats in order to understand cellular mechanisms for restriction of corticorubral projections with development. The anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) was injected into restricted areas of the pericruciate cortex in kittens and PHA-L-labeled axons in the red nucleus were examined at postnatal days (PND) 7-73. In accordance with our previous study (Murakami and Higashi, Brain Res. 1988; 447:98-108), a crossed corticorubral projection was observed in addition to the uncrossed one in every experimental animal. During the early period of development (PND 7-8), swellings of irregular shape were observed along the entire course of the axons and they were often interconnected with extremely fine axonal segments. These axons bifurcated only infrequently and often ended as growth cones. These features were common to both uncrossed and crossed corticorubral axons. At later stages of development (PND 28 or later), the total number of swellings decreased and axonal swellings with smooth contours became dominant. A quantitative examination of axonal branches indicated that axons on the ipsilateral side branch occurred more frequently at later stages of development. However, there was no substantial change in branching frequency for the crossed corticorubral fibers during development. In parallel with morphological changes in individual axons, the crossed projection that was initially relatively abundant was reduced during development. Since a PHA-L injection can be confined to a small region of cortex, topographic projections can easily be detected. At PND 7-8 there was no well-defined topographic order in the ipsilateral corticorubral projection. Adult-like topography was first discernible at PND 13. These observations suggest that the unilateral uncrossed corticorubral projection in the adult cat is achieved at least in part by the formation of axonal arbors in the uncrossed projection. This was accompanied by the failure of crossed fibers to form complex arbors. It is possible that a similar mechanism also operates in the formation of topographic maps.
为了了解皮质红核投射在发育过程中受到限制的细胞机制，研究了猫出生后发育过程中单个皮质红核纤维的形态变化以及交叉和不交叉皮质红核投射的模式。将顺行示踪剂菜豆白细胞凝集素（PHA-L）注射到小猫十字周皮质的受限区域，并在出生后第7至73天检查红核中PHA-L标记的轴突。根据我们之前的研究（村上和东，《脑研究》，1988年；447:98 - 108），在每只实验动物中，除了不交叉的皮质红核投射外，还观察到了交叉的皮质红核投射。在发育早期（出生后第7 - 8天），沿轴突全长观察到不规则形状的肿胀，它们常与极细的轴突段相互连接。这些轴突很少分叉，且常以生长锥的形式终止。这些特征在不交叉和交叉的皮质红核轴突中都很常见。在发育后期（出生后第28天或更晚），肿胀的总数减少，轮廓光滑的轴突肿胀占主导。对轴突分支的定量检查表明，同侧轴突分支在发育后期更频繁地出现。然而，交叉皮质红核纤维在发育过程中的分支频率没有实质性变化。与单个轴突的形态变化同时，最初相对丰富的交叉投射在发育过程中减少。由于PHA-L注射可局限于皮质的一个小区域，因此可以很容易地检测到拓扑投射。在出生后第7 - 8天，同侧皮质红核投射中没有明确的拓扑顺序。类似成年动物的拓扑结构在出生后第13天首次可辨。这些观察结果表明，成年猫中单侧不交叉的皮质红核投射至少部分是通过在不交叉投射中形成轴突分支实现的。这伴随着交叉纤维未能形成复杂的分支。类似的机制也可能在拓扑图的形成中起作用。
