Intertectal neurons connect the two hemispheres of the optic tectum. a Maximum intensity projections of an ITNGal4 transgenic larva viewed dorsally at 6 dpf in which neurons that connect the two hemispheres of the optic tectum (intertectal neurons, ITNs) are labeled (most dorsal plane through the larva is indicated by z = 0 µm). The cell bodies of ITNs are situated in two bilateral symmetric nuclei below the respective tectal lobes (ITN nuclei highlighted by dotted ellipses in lower left panel) and send their axons dorsally through the tectum crossing the tectal commissure in ladder-like trajectories (upper left panel). In addition to labeling ITNs in the mesencephalic tegmentum, Gal4 is also expressed in the pineal gland, SINs, scattered periventricular neurons (PVNs), and the spinal cord in this transgenic line. All scale bars = 50 μm. (pg: pineal gland, tc: tectal commissure, OT: optic tectum, R: rostral, C: caudal, SINs: superficial interneurons). b Schematic of the transgenic ITNGal4 line depicted in a. ITN cell bodies and axon tracts are color-coded according to the position in the dorsoventral z-plane. To increase readability only the right ITN’s connectivity with respect to the larva is shown. Scale bar = 50 μm. (MHB: midbrain-hindbrain boundary, OT: optic tectum, R: rostral, C: caudal). c Neurite tracing of multiple ITNs in a larva at 3.5 dpf. At 3.5 dpf ITN neurites start to cross the midline (e.g., the blue ITN). In addition, ITNs begin to form arbors at the boundaries between the deep layers of the neuropil and the PVN layer. Scale bar = 50 μm. (oc: mouth/oral cavity, np: tectal neuropil, SPV: stratum periventriculare). Color-coded to simplify distinction between single ITNs. d Neurite tracing of a representative ITN at 4 dpf, color-coded according to the position in the dorsoventral z-plane (most dorsal plane of the larva is indicated by z = 0 µm). ITN axons cross the midline superficially and form arborization patterns of increasing complexity in the ipsi- and contralateral neuropil structures of the optic tectum. Scale bar = 50 μm. (np: tectal neuropil, SPV: stratum periventriculare).
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