Cajal-Retzius cells and GABA-positive neurons are transient synaptic targets of entorhinal and commissural axons.A, B, Electron micrographs show biocytin-labeled entorhinal axon terminals (AT) at P0 in asymmetric synaptic contact (arrows) with the dendrites of Cajal-Retzius cells immunostained for calretinin. Biocytin-positive boutons are identified by the homogenous, electron-dense reaction product (DAB-nickel), whereas calretinin immunostaining shows a granular, less dense precipitate (DAB).C, D, Biocytin-positive commissural axon terminals (AT) at P2 (D) and P5 (C) establish asymmetric synaptic contacts (large arrows) with GABA-positive dendrites identified by the presence of gold particles (small arrows). Scale bar, 0.4 μm.