= Axonal endoplasmic reticulum
<Axonal endoplasmic reticulum> is the predominantly smooth tubular <endoplasmic reticulum> network extending through an <axon>. It connects the organization of a membrane compartment to the long geometry of a <neuron>. Network amount, tubule diameter, number of tubules per cross-section and continuity along the axon are distinct structural properties.<Modeling of axonal endoplasmic reticulum network by spastic paraplegia proteins by Belgin Yalçın and others>[Abstract; Figure 7, axonal ER structure and quantification]{ref}
A 2017 study in <Drosophila melanogaster> combined optical imaging with <serial-section electron microscopy>. Reconstructions from seventy-five $60\,\mathrm{nm}$ sections covered $75\times60=4500\,\mathrm{nm}=4.5\,\mu\mathrm m$ of axon. Comparing sections revealed tubular branches and continuity that one cross-section alone could not establish.<Modeling of axonal endoplasmic reticulum network by spastic paraplegia proteins by Belgin Yalçın and others>[Figure 7F and K, serial-section reconstructions; Results, axonal ER ultrastructure]{ref}
The investigated membrane-shaping-protein mutants showed enlarged tubules and a less extensive network. Measurements of gaps separately considered gap length, number per length and the proportion of axonal length affected.<Modeling of axonal endoplasmic reticulum network by spastic paraplegia proteins by Belgin Yalçın and others>[Figure 7B–O and associated Results]{ref}
An invented comparison explains why those readouts differ. Two $10\,\mu\mathrm m$ axon segments each contain one detected gap, but their gap lengths are $0.2\,\mu\mathrm m$ and $2\,\mu\mathrm m$. Both gap-count densities are $0.1\,\mu\mathrm m^{-1}$, while their affected-length fractions are two percent and twenty percent. Equal counts can conceal a tenfold difference in the extent affected.
The observation criterion matters too. The study's serial-imaging definition required absent tubule signals in at least three successive sections.<Modeling of axonal endoplasmic reticulum network by spastic paraplegia proteins by Belgin Yalçın and others>[Video 9 legend and Axonal EM analyses]{ref} Such an operational criterion connects a structural claim to what the images can resolve; it does not make every dark or faint region in a single image a demonstrated break in a three-dimensional network.
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