Electron microscopic localization of the anti-Id mAb labelled sites in lamina I of the rat spinal dorsal horn after an intrathecal injection of NaCl (Control rat). In (a) immunoreaction (arrowhead) was found at the interface between a glomerular C terminal (C), probably from a primary afferent, and a dendrite (D). Note the presence of synaptic differentiations (arrows) at proximity of the labelling but not at the level of the immunoreactivity. The labelling is strictly localized at the interface between the two neurites and cannot be associated with one or the other neurite. In (b), immunoreaction localized between two axons (A1 and A2). Scale bars = 500 nm.