CONTROL OF NEURON POPULATION SIZE

本研究的目的是研究各种 可能影响神经元群体大小的因素 正常发展。 一种流行的解释， 控制神经元数量，因为神经元及其目标是 与之相匹配的是神经元之间的竞争过程， 选择性神经细胞死亡 在非洲爪蟾中， 左心房运动神经元数量的个体差异 腰椎外侧运动柱与身高高度相关， 无论是自然产生的二倍体还是实验产生的 三倍体动物 这种与神经元大小相关的变异 数字是由涉及某些特征的竞争过程产生的 受这些运动神经元支配的后肢肌肉的数量尚不清楚 由于运动神经元数量与 在二倍体动物中观察到的肌纤维数量在 他们的三倍体兄弟姐妹 然而，初级肌管， 肌肉纤维的，可能是目标的相关特征， 限制了二倍体和三倍体中运动神经元的存活。 初级肌管平均数量与 已经注意到细胞死亡后运动神经元的平均数量 但运动神经元和神经元之间的关系 个体动物的数量和肌管数量从未被 测定 使用爪蟾兄弟的两组实验是 提出澄清的作用，初级肌管可能有 限制正常发育过程中运动神经元的存活。 第一、 在确定了初级肌管形成的时间后， 代表性的后肢肌肉是完整的，肌管和 将对相同动物中的运动神经元进行计数以测量 这些细胞群体中的个体变异，并确定如何 这些群体在运动神经元细胞死亡发生之前是相关的。 第二，因为自由生活的爪蟾幼虫生长正常 后肢截肢后，肌肉中的肌管 将对截肢进行计数，并与 运动神经元（通过逆行标记识别）支配 细胞死亡后剩余肢体的肌肉。 的假设 肌管数量与运动神经元存活率有特殊关系 在正常的发展过程中，可以进行测试。 总体上 信息与理解哪些过程可能 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.