PSD-95–overexpressing pyramidal neurons in hippocampal slice cultures. (A) Low magnification confocal view of a CA1 pyramidal cell 2 d after transfection with a PSD-95–EGFP construct. (B) Higher magnification of a dendritic segment illustrating the characteristic punctate aspect of PSD staining obtained after PSD-95–EGFP overexpression. (C) Low magnification electron microscopic view of a section through a dendrite (dark) of a transfected neuron upon revelation of EGFP immunostaining. (D) Higher magnification serial sections of a dendritic spine from an EGFP-immunostained neuron (the arrow points to a dendrite). Note the presence of a clearly visible PSD (arrowheads). (E) Contour representation of the multiple sections obtained from a three-dimensionally reconstructed dendritic segment of a PSD-95–EGFP-transfected cell. Note the presence of many large dendritic spines. (F) Three-dimensionally reconstructed dendritic segments (dark contours) from nontransfected neurons located in the vicinity of the PSD-95–transfected cell (light gray contours). Bars: (A) 10 μm; (B, E, and F) 5 μm; (C) 1 μm; (D) 0.5 μm.