General and variable features of varicosity spacing along unmyelinated axons in the hippocampus and cerebellum

Gordon M G Shepherd*, Morten Raastad, Per Andersen

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

83 Scopus citations


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 μm (mean ± SD) for CA3-to-CA1 axons and 5.2 ± 1.0 μm 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.

Original languageEnglish (US)
Pages (from-to)6340-6345
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number9
StatePublished - Apr 30 2002

ASJC Scopus subject areas

  • General


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