Bruce L L
Division of Anatomy, School of Medicine, Creighton University, Omaha, NE 68178.
Brain Res Dev Brain Res. 1993 May 21;73(1):47-61. doi: 10.1016/0165-3806(93)90045-c.
The corticotectal projection in adult cats has a precise topographic and laminar organization. Yet this projection initially grows beyond these adult targets. To begin to understand how the growing cortical axons achieve this precision, the morphological development of axons growing from the posteromedial lateral suprasylvian area (PMLS) to the superior colliculus was studied by injecting the anterograde tracer biocytin into the PMLS of cats between postnatal day (P0) and adulthood. The labeling patterns showed that (1) axons grow independently towards the colliculus and (2) the first axons from the PMLS arrive in the colliculus by P1 and continue to arrive over several days. Labeled growth cones were seen within the colliculus up to P15. (3) After reaching the colliculus, the axons undergo several morphological changes. Initially, they are unbranched and beaded, then short side branches are formed and finally extensive arborizations appear. Comparing the timing of these events with results from electron microscopic and electrophysiological studies suggests that the appearance and increase in labeled axons with short side branches roughly coincides with the appearance and increase in number of synapses in the colliculus, whereas the elaboration of extensive arbors (and hence a corresponding increase in synapses) is well underway before visual cortical influences on the colliculus can be measured. Thick sinuous axons are also labeled during maturation, usually in areas of the colliculus where they would be considered exuberant and may represent degenerating axons. (4) A coarse topography develops as the axons grow into the colliculus and becomes more precise in the following weeks. Initially, some axons extend well beyond their correct terminal zone, growing into the contralateral colliculus, caudally into the inferior colliculus and reaching all laminae of the ipsilateral superior colliculus. Similar targeting 'errors' have been reported during the growth of retinotectal axons, suggesting that cortical, retinal and perhaps other sources as well, may use the same extracellular cues to establish an initial coarse topography within the colliculus.
成年猫的皮质顶盖投射具有精确的拓扑和分层组织。然而，这种投射最初会生长到超出这些成年目标的范围。为了开始理解生长中的皮质轴突是如何实现这种精确性的，通过将顺行示踪剂生物素注入出生后第0天（P0）至成年期猫的后内侧外侧上薛氏区（PMLS），研究了从该区域生长到上丘的轴突的形态发育。标记模式显示：（1）轴突独立地向上丘生长；（2）来自PMLS的第一批轴突在出生后第1天（P1）到达上丘，并在接下来的几天内持续到达。直到出生后第15天（P15），在上丘内都能看到标记的生长锥。（3）到达上丘后，轴突会经历几种形态变化。最初，它们没有分支且呈串珠状，然后形成短的侧支，最后出现广泛的分支。将这些事件的时间与电子显微镜和电生理研究的结果进行比较表明，带有短侧支的标记轴突的出现和增加大致与上丘中突触的出现和数量增加相吻合，而广泛分支的形成（以及相应的突触增加）在视觉皮层对上丘的影响可以测量之前就已经在顺利进行。在成熟过程中，粗而蜿蜒的轴突也会被标记，通常在上丘中被认为是过度生长的区域，可能代表正在退化的轴突。（4）随着轴突生长进入上丘，会形成粗略的拓扑结构，并在接下来的几周内变得更加精确。最初，一些轴突延伸到远远超出其正确终末区的范围，生长到对侧上丘，向后生长到下丘，并到达同侧上丘的所有层。在视网膜顶盖轴突的生长过程中也有类似的靶向“错误”报道，这表明皮质、视网膜以及可能其他来源，可能使用相同的细胞外信号来在上丘内建立初始的粗略拓扑结构。
