Li Q, Martin J H
Columbia University Center for Neurobiology and Behavior and New York State Psychiatric Institute, New York 10032, USA.
Exp Brain Res. 2000 Sep;134(2):187-98. doi: 10.1007/s002210000454.
The primary motor cortex of cats, monkeys, and humans has distinct rostral and caudal subregions. In the cat, projections from the caudal subregion terminate predominantly in laminae 4-6 and, from the rostral subregion, in laminae 6-8. The purpose of this study was to determine if these distinctive termination patterns are present during early postnatal development, when corticospinal axons are establishing connections with spinal neurons, or if there was postnatal refinement of the distribution of terminations. We used the anterograde tracer biotinylated dextran amine (BDA) to label selectively projections from the two subregions in immature (postnatal days 25, 35 at time of analysis) and mature animals. We compared the distribution of spinal gray matter labeling from caudal and rostral motor cortex. In immature animals, substantial bilateral terminations were present after tracer injection into either subregion. Partial axon reconstructions revealed that individual axons terminated bilaterally. The dorso-ventral laminar distribution of contralateral labeling after caudal motor cortex injections was significantly more extensive for immature than mature animals. In immature animals, most of the labeling was present in laminae 5-7 (dorsal portion). with lesser amounts in laminae 1-4 and 7 (ventral portion), 8, and 9. In mature animals, there were significant reductions in the amount of label in laminae 7-9, resulting in contraction of the labeled territory. The distribution of dorsal horn-ventral horn labeling shifted from 41% and 59% in immature animals to 77% and 23% in maturity. The distribution of contralateral labeling after rostral motor cortex injections also was different in immature and mature animals, but the changes were less extensive than for the caudal motor cortex. In immature animals, the distribution of labeling was similar to that after caudal motor cortex injections in animals of the same age. In mature animals, there was a significant reduction in the amount of labeling in laminae 1-4 and a smaller reduction in 7 (ventral)-9. The overall dorsal horn-ventral horn distribution, however, remained largely unchanged, from 40% and 60% in immature animals to 44% and 56% in maturity. In immature animals after rostral motor cortex injection, corticospinal terminations were present within the lateral motor nuclei. Thus, the distinctive spinal termination patterns of caudal and rostral motor cortex in maturity each reflected postnatal refinement of the distribution of axon terminations. It is plausible that the dorso-ventral refinement of corticospinal terminations relies on activity-dependent competition between caudal and rostral motor cortex, similar to activity-dependent refinement of the laterality of terminations. Our results, however, suggest that the developmental program for achieving corticospinal connectional specificity by the two subregions is different because there was large-scale refinement of the terminations of caudal motor cortex but only local refinement of rostral motor cortex terminations.
猫、猴和人类的主要运动皮层有明显的嘴侧和尾侧亚区。在猫中，来自尾侧亚区的投射主要终止于第4 - 6层，而来自嘴侧亚区的投射则终止于第6 - 8层。本研究的目的是确定在出生后早期发育阶段，当皮质脊髓轴突与脊髓神经元建立联系时，是否存在这些独特的终止模式，或者终止分布是否在出生后得到细化。我们使用顺行示踪剂生物素化葡聚糖胺（BDA）选择性标记未成熟（分析时为出生后第25天、35天）和成熟动物中两个亚区的投射。我们比较了来自尾侧和嘴侧运动皮层的脊髓灰质标记分布。在未成熟动物中，将示踪剂注射到任一亚区后都出现了大量双侧终止。部分轴突重建显示单个轴突双侧终止。与成熟动物相比，未成熟动物在尾侧运动皮层注射后对侧标记的背腹层分布明显更广泛。在未成熟动物中，大多数标记出现在第5 - 7层（背侧部分），在第1 - 4层和第7层（腹侧部分）、第8层和第9层中的量较少。在成熟动物中，第7 - 9层中的标记量显著减少，导致标记区域收缩。背角 - 腹角标记的分布从未成熟动物中的41%和59%转变为成熟动物中的77%和23%。嘴侧运动皮层注射后对侧标记的分布在未成熟和成熟动物中也有所不同，但变化程度比尾侧运动皮层小。在未成熟动物中，标记分布与同年龄动物尾侧运动皮层注射后的分布相似。在成熟动物中，第1 - 4层中的标记量显著减少，第7层（腹侧） - 9层中的减少量较小。然而，背角 - 腹角的总体分布基本保持不变，从未成熟动物中的40%和60%变为成熟动物中的44%和56%。在未成熟动物嘴侧运动皮层注射后，皮质脊髓终止出现在外侧运动核内。因此，成熟时尾侧和嘴侧运动皮层独特的脊髓终止模式各自反映了轴突终末分布的出生后细化。皮质脊髓终末的背腹细化可能依赖于尾侧和嘴侧运动皮层之间的活动依赖性竞争，类似于终末偏侧性的活动依赖性细化。然而，我们的结果表明，两个亚区实现皮质脊髓连接特异性的发育程序不同，因为尾侧运动皮层的终末有大规模细化，而嘴侧运动皮层终末只有局部细化。
