Spine enlargement induced by PSD-95 overexpression. (A) Three-dimensionally reconstructed dendritic segment of a PSD-95–transfected cell illustrating the presence of numerous large spines regularly exhibiting perforated PSDs. (B) Three-dimensionally reconstructed dendritic segment of a control cell. Note the smaller size of the spines. (C) Spine volume measured in control nontransfected (ctrl; n = 8 cells; 145 spines), EGFP-transfected (n = 4 cells; 164 spines), and PSD-95–transfected (n = 7 cells; 234 spines) pyramidal neurons. Data are mean ± SEM (error bars; *, P < 0.05). (D) Same as in C but for the total PSD area. (E) Same as in C but for the proportion of spines with complex PSDs. Bars, 1 μm.