(A) Sample serial EM sections and representative 3D reconstructed dendrites illustrate the distribution of endosomal compartments from control and LTP conditions. Dendrites are yellow, synapses are red, and color-coded arrows point to endosome-containing spines. The color-coded key in the lower left corner indicates amorphous vesicles (AV), recycling complexes (RC), coated pits (CP), coated vesicles (CV), large vesicles (LV), sorting complexes (SC), small vesicles (SV) and degradative structures (DEG); these abbreviations apply also to the graphs. Vesicles are represented as 100 nm spheres (AV, CP, CV, LV, and SV). The other structures (RC, SC, DEG) are reconstructed in 3D to scale. (B) Endosomal structures in dendritic shafts (#/µm) with relative distributions to aspiny and spiny segments in control (CON) and LTP conditions. Overall, shaft endosomes (hnANOVA F(1,293)=0.93104, p=0.33539), degradative structures (hnANOVA F(1,293)=0.47789, p=0.48993) or constructive endosomal compartments (Constr. = all minus degradative; hnANOVA F(1,293)=0.62167, p=0.43107) did not differ between LTP and control conditions or segment locations. Recycling complexes (RC) were greater in the LTP than control dendritic shafts (hnANOVA F(1,293)=6.4920, p=0.01135, η2 = 0.022), but no significant differences occurred in the other categories: amorphous vesicles (hnANOVA F(1,293)=1.5092, p=0.22025); small vesicles (hnANOVA F(1, 293)=1.1699, p=28031); coated pits, coated vesicles, and large vesicles (hnANOVA F(1,293)=0.89152, p=0.34584); and sorting complexes (hnANOVA F(1,293)=0.45286, p=0.50151). (For control (CON) n = 151 aspiny + spiny segments and for LTP n = 158 aspiny + spiny segments.) (C) More dendritic spines contained endosomes along the dendrites in the LTP than the control condition (ANOVA F(1,12)=18.047, p=0.00113, η2 = 0.60), an effect that was carried by spines with PSD areas less than 0.05 µm2 (ANOVA F(1,12)=23.642, p=0.00039, η2 = 0.66) but not in spines with PSD area >0.05 µm2 (ANOVA F(1,12)=0.84714, p=0.37550). (D) Stability in percentage of spines containing endosomes following TBS (ANOVA F(1,12)=.72158, p=0.41225). (E) Among spines with PSD area less than 0.05 µm2, the increase in occupancy of endosomes was due to more with coated pits, coated vesicles, and large vesicles (ANOVA F(1,12)=4.94433, p=0.046140, η2 = 0.29), recycling complexes (ANOVA F(1,12)=11.009, p=0.00613, η2 = 0.48), and more with small vesicles (ANOVA F(1,12)=5.2575, p=0.04072, η2 = 0.30). No significant changes in spine occupancy occurred for amorphous vesicles (ANOVA F(1,12)=1, p=0.33705), sorting complexes (ANOVA F(1,12)=1, p=0.33705), or degradative structures (ANOVA F(1,12)=0.46689, p=0.5074). Bar graphs show mean ± S.E.M. (For C–E), Control (CON, n = 8 full dendrite reconstructions) and LTP (n = 8 full dendrite reconstructions).