Ultrastructural analyses of Dscam2 single isoform mutant boutons reveal endosomal defects. (A and B) Representative electron micrographs of Dscam2A (A) and Dscam2B (B) boutons. Arrowheads indicate examples of large vesicles (red), tubular endosomes (blue), and DCVs (green). Mitochondria (M) and SSR indicated in white text. Scale bar, 500 nm. (C–H) Quantification of synaptic vesicle (SV) density (C), large vesicle density (D), tubular endosome density (E), DCV density (F), MVB density (G), and density of multilamellar foci (H; Mann-Whitney rank-sum test). (I–L) Representative electron micrographs of DCVs (green arrowheads) and endosomes (blue arrowheads) in Dscam2A (I) and Dscam2B (J). Scale bar, 200 nm. Also shown are examples of multilamellar foci (black arrowheads) in Dscam2A (K) and Dscam2B (L). Scale bar, 100 nm. (M and N) Quantification of diameter of large vesicles (M) and cross-sectional area of endosomes (N; Mann-Whitney rank-sum test). (O and P) Representative electron micrographs of Dscam2A (O) and Dscam2B (P) T-bars. Scale bar, 100 nm. (Q and R) Quantification of SV diameter (Q) and number of SVs within 200 nm of T-bar (R; unpaired Student’s t test). Data shown as mean ± SEM; n indicated in graph. *, P > 0.05; **, P < 0.01; ****, P < 0.0001 for all panels.