Shepherd Gordon M G, Raastad Morten, Andersen Per
Department of Physiology, University of Oslo, Post Box 1103 Blindern, 0317 Oslo, Norway.
Proc Natl Acad Sci U S A. 2002 Apr 30;99(9):6340-5. doi: 10.1073/pnas.052151299. Epub 2002 Apr 23.
Along unmyelinated central axons, synapses occur at focal swellings called axonal varicosities (boutons). The mechanisms regulating how frequently synapses and varicosities occur along axons remain poorly understood. Here, to investigate varicosity distribution patterns and the extent to which they may be conserved across different axons, we analyzed varicosity numbers and positions along fluorescently labeled axon branches in hippocampal area CA1 (CA3-to-CA1 "Schaffer collateral" axons) and five other synaptic regions of rat hippocampus and cerebellum. Varicosity spacing varied by region; e.g., 3.7 +/- 0.6 microm (mean +/- SD) for CA3-to-CA1 axons and 5.2 +/- 1.0 microm for cerebellar parallel fibers. Surprisingly, when 56 axons from these different regions were pooled into a single heterogeneous group, a general relationship emerged: the spacing variability (SD) was a constant fraction of the mean spacing, suggesting that varicosities along different axons are distributed in a fundamentally similar, scaled manner. Varicosity spacing was neither regular nor random but followed a pattern consistent with random synaptic distributions and the occurrence of multiple-synapse boutons. A quantitative model reproduced the salient features of the data and distinguished between two proposed mechanisms relating axonal morphogenesis and synaptogenesis.
在无髓鞘的中枢轴突上，突触出现在称为轴突膨体（终扣）的局灶性肿胀处。关于轴突上突触和膨体出现频率的调节机制，目前仍知之甚少。在这里，为了研究膨体的分布模式以及它们在不同轴突间的保守程度，我们分析了大鼠海马体CA1区（CA3到CA1的“谢弗侧支”轴突）以及海马体和小脑的其他五个突触区域中荧光标记轴突分支上的膨体数量和位置。膨体间距因区域而异；例如，CA3到CA1轴突的间距为3.7±0.6微米（平均值±标准差），小脑平行纤维的间距为5.2±1.0微米。令人惊讶的是，当将来自这些不同区域的56条轴突合并为一个单一的异质组时，出现了一种普遍关系：间距变异性（标准差）是平均间距的一个恒定比例，这表明不同轴突上的膨体以基本相似的比例方式分布。膨体间距既不是规则的也不是随机的，而是遵循一种与随机突触分布和多突触终扣的出现相一致的模式。一个定量模型再现了数据的显著特征，并区分了两种关于轴突形态发生和突触发生的机制。
