(A) Example 3D reconstructions of dendrites in P25 WT (left, blue) and hevin KO (right, pink) V1. Asymmetric PSD locations (i.e., excitatory synapses) are shown in red. Scale cubes, 0.5 µm3. (B) Asymmetric PSD size was decreased in P25 hevin KO vs WT (n = 278 WT synapses, 293 KO synapses; p < 0.025, Kolmogorov–Smirnov two-sample test). (C) Left: EM examples from P25 WT and P25 hevin KO show the ‘tripartite synapse’ arrangement of postsynaptic dendritic spines (yellow), presynaptic axonal boutons (green) and glial processes (blue). Scale bar, 250 nm. Right: Quantification revealed no difference in the percentage of excitatory synapses contacted by glial processes in P25 WT vs P25 hevin KO (n = 4 dendrites per animal, 2 animals per genotype; p = 0.52, Student's t test). (D) Left: Excitatory synapses, made by axons (green) onto dendritic spines (yellow) in P25 WT, were readily seen on dendritic shafts (yellow) in the hevin KO. Scale bar, 250 nm. Right, Top: Example hevin KO dendrite with multiple excitatory shaft synapses (arrows). Scale cube, 0.5 µm3. Right, Bottom: Quantification of excitatory shaft synapse density in P25 hevin KO compared to P25 WT (n = 4 dendrites per animal, 3 animals per genotype; p < 0.01, Student's t test). (E) SMEC density was increased in P25 hevin KO compared to WT (arrows indicate excitatory PSDs on SMECs; n = 4 dendrites per animal, 3 animals per genotype; p < 0.01, one-way ANOVA with Fisher's LSD posthoc test).