Anaesthetized animals were perfused with a buffered solution of glutaraldehyde (2.5%) and paraformaldehyde (2%), and then vibratome sectioned at 80 µm thickness cut through the mouse cortex, close to the site of injection. Selected sections were then further fixed with 1.5% potassium ferrocyanide and 1% osmium tetroxide, followed by 1% osmium alone, and then uranyl acetate. After dehydration in a series of ascending concentrations of ethanol, they were embedded in durcupan resin and hardened at 65 °C for 24 h. Thin sections at 50 nm thickness were cut through the region of interest, stained with lead citrate and uranyl acetate, and imaged in a transmission electron microscope at 80 kV (Tecnai Spirit, FEI Company). To quantify microtubule number and density, 30 axonal cross-sections per sample were randomly analyzed in the mouse cortex, using the TrakEM2 plugin (Fiji)44. For each axon, the number of microtubules was counted and divided by the measured cross-section area to determine microtubule density.