CONTROL OF NEURON POPULATION SIZE

拟议研究的目标是研究不同的 可能影响神经元种群大小的因素 正常发育。一种流行的解释，准确的 对神经元及其靶点中神经元数量的控制 与此相匹配的是一个涉及神经元和 选择性神经细胞死亡。非洲爪蛙中的非洲爪蛙 Levis，运动神经元数量的个体差异 腰椎侧方运动柱与身体大小高度相关 自然产生的二倍体和实验产生的二倍体 三倍体动物。神经元中这种与大小相关的变异 数字是由涉及某些特征的竞争过程产生的 这些运动神经元支配的后肢肌肉的情况尚不清楚 由于运动神经元数量和数量的显著相关 在二倍体动物中观察到的肌肉纤维数量在 他们的三倍体兄弟姐妹。然而，初级肌管，一种前体 肌肉纤维，可能是靶点的相关特征 限制运动神经元在二倍体和三倍体中的存活。 初级肌管平均数与平均肌管数的对应关系 细胞死亡后运动神经元的平均数量已被注意到 但运动神经元之间的关系 个体动物的数量和肌管数量从来没有 下定决心。使用非洲爪哇兄弟姐妹进行的两组实验是 建议澄清初级肌管可能在 限制运动神经元在正常发育期间的存活。第一, 在确定了初级肌管形成的时间之后 有代表性的后肢肌肉是完整的，肌管和 相同动物中的运动神经元将被计数以测量 这些细胞群体中的个体变异并确定如何 在运动神经元死亡发生之前，这些种群是相关的。 第二，因为自由生活的非洲爪哇幼虫正常生长 后肢截肢后，肌肉中的肌管 被截肢的肢体将被计算并与 运动神经元(逆行标记)支配 细胞死亡后残肢的肌肉。假设 肌管数量与运动神经元存活有明确的关系 在正常发育过程中可以进行测试。总体而言， 信息与了解哪些过程可能相关 产生神经元数量的自然变化和惊人的 正常后观察到神经元数量的双侧对称性 发展。

The objectives of the proposed research are to study various factors which may affect the size of a neuron population during normal development. A prevailing explanation for the precise control of neuron numbers in that neurons and their targets are matched by a process involving competition between neurons and selective neuronal cell death. In the African clawed frog, Xenopus laevis, individual variation in the number of motoneurons in the lumbar lateral motor column is highly correlated with body size in both naturally occurring diploid and experimentally produced triploid animals. Whether this size-related variation in neuron number is produced by a competition process involving some feature of the hindlimb muscles innervated by these motoneurons is unclear since the significant correlation between motoneuron number and muscle fiber number observed in diploid animals is not observed in their triploid siblings. However, primary myotubes, a precursor of muscle fibers, may be the relevant feature of the target that limits motoneuron survival in both diploids and triploids. Correspondences between the average number of primary myotubes and the average number of motoneurons after cell death have been noted by other investigators, but the relationship between motoneuron number and myotube number in individual animals has never been determined. Two sets of experiments using Xenopus siblings are proposed to clarify the role that the primary myotubes may have in limiting motoneuron survival during normal development. First, after determining when primary myotube formation in a representative hindlimb muscle is complete, both the myotubes and the motoneurons in the same animals will be counted to measure the individual variation in these cell populations and to determine how the populations are related before motoneuronal cell death occurs. Second, because the free-living Xenopus larvae grow normally following hindlimb amputation, the myotubes in the muscle of an amputated limb will be counted and compared with the number of motoneurons (identified by retrograde labelling) innervating the muscle of the remaining limb after cell death. The hypothesis that myotube number is specifically relevant to motoneuron survival during normal development can be tested. In general, the information is relevant to understanding what processes are likely producing the natural variation in neuron number and the striking bilateral symmetry in neuron number observed after normal development.