Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA 02138, USA.
Neuron. 2012 Jun 7;74(5):816-29. doi: 10.1016/j.neuron.2012.04.017.
Using light and serial electron microscopy, we show profound refinements in motor axonal branching and synaptic connectivity before and after birth. Embryonic axons become maximally connected just before birth when they innervate ∼10-fold more muscle fibers than in maturity. In some developing muscles, axons innervate almost every muscle fiber. At birth, each neuromuscular junction is coinnervated by approximately ten highly intermingled axons (versus one in adults). Extensive die off of terminal branches occurs during the first several postnatal days, leading to much sparser arbors that still span the same territory. Despite the extensive pruning, total axoplasm per neuron increases as axons elongate, thicken, and add more synaptic release sites on their remaining targets. Motor axons therefore initially establish weak connections with nearly all available postsynaptic targets but, beginning at birth, massively redistribute synaptic resources, concentrating many more synaptic sites on many fewer muscle fibers. Analogous changes in connectivity may occur in the CNS.
使用光镜和电镜，我们发现在出生前后运动轴突的分支和突触连接有明显的改善。胚胎轴突在出生前达到最大连接，此时它们支配的肌纤维数量是成熟时的 10 倍。在一些发育中的肌肉中，轴突几乎支配每一个肌纤维。在出生时，每个运动终板是由大约 10 个高度交织的轴突共同支配的（而成年人只有一个）。在出生后的头几天中，大量的终末分支发生凋亡，导致树突稀疏，但仍然覆盖相同的区域。尽管大量修剪，但随着神经元轴突的伸长、变粗以及在剩余的靶标上增加更多的突触释放部位，每个神经元的轴浆总量会增加。因此，运动轴突最初与几乎所有可用的突触后靶标建立弱连接，但从出生开始，它们就会大量重新分配突触资源，将更多的突触部位集中在更少的肌纤维上。类似的连接变化可能也发生在中枢神经系统中。
