 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.^[ref]

A 2017 study in Drosophila melanogaster combined optical imaging with serial-section electron microscopy. Reconstructions from seventy-five $60\mathrm{nm}$ sections covered $75\times 60=4500\mathrm{nm}=4.5\mu \mathrm{m}$ of axon. Comparing sections revealed tubular branches and continuity that one cross-section alone could not establish.^[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.^[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.^[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.