Netrin is present in tectal neurons and is identified at synaptic sites in the stage 44–45 Xenopus optic tectum. A, B, Localization of netrin immunoreactivity in the tectal midbrain. B, Schematic diagram of a stage 44–45 Xenopus tectal midbrain (horizontal view). Neuronal precursors, born in rows adjacent to the ventricle in the right and left sides of the optic tectum, follow a lateral and rostral migratory path while extending a primary dendrite that will eventually terminate in the tectal neuropil and begin to elaborate a dendritic arbor. In this diagram, the boxed region corresponds to the micrograph shown in A. A, This horizontal view of the optic tectum demonstrates that a large, evenly distributed subset of cells is immunopositive for netrin-1. Punctate immunostaining is also observed throughout the tectal neuropil (n). C–F, Confocal micrographs of a horizontal section through the stage 44–45 Xenopus optic tectum coimmunostained with antibodies to netrin (green immunofluorescence) and the dendritic marker MAP2 (red immunofluorescence). C, Note that MAP2 immunostaining identifies optic tectal neuron dendritic processes projecting to and branching in the tectal neuropil (n). D–F, The high-magnification confocal image reveals that netrin immunoreactivity is localized to cell bodies (asterisk) and proximal dendrites (arrow) of neurons within the medial portion of the optic tectum that are also immunoreactive for MAP2. G, Confocal micrograph of the tectal neuropil in stage 44–45 Xenopus optic tectum coimmunostained with antibodies to netrin (red immunofluorescence) and the presynaptic marker SNAP-25 (green immunofluorescence). Note the punctate distribution of netrin (red) and SNAP-25 (green) in the tectal neuropil; the netrin-immunoreactive puncta are in direct apposition to SNAP-25-labeled presynaptic sites. H, Confocal micrograph of stage 44–45 Xenopus optic tectum coimmunostained with antibodies to netrin (red immunofluorescence) and the postsynaptic marker, PSD-95 (green immunofluorescence). Netrin-immunoreactive puncta in the tectal neuropil colabel with PSD-95-positive postsynaptic densities. Scale bars: A, 50 μm; C, 100 μm; D–F, 10 μm; G, H, 5 μm.