Electron micrographs showing synapses and free spines on PC dendrites at P15. (A) CF synapse on a PC spine (Sp), identified by diffuse dark precipitates for the anterograde tracer BDA in the presynaptic terminal and an asymmetrical contact characterized by a thick postsynaptic density (arrowheads). (B) PF synapses on PC spines, identified by metal particle labeling for VGluT1 and asymmetrical contacts (arrowheads). (C) GABAergic synapse on a dendritic shaft of PC, identified by metal particle labeling for VIAAT and a symmetrical contact (arrowheads). (D) Symmetrical synapse on a right PC spine. (E) GABAergic synapse on a PC spine, identified by metal particle labeling for VIAAT and a symmetrical contact (arrowheads). (F) Serial images of a free spine, which is surrounded by Bergmann glia and lacking synaptic contact. (Scale bars, 500 nm.)