Fig. 4. Single-cell labelings of wild-type and mutant RP2, aCC and pCC neurons. RP2, aCC and pCC neurons (green) were anterogradely labeled (using Lucifer Yellow) in late stage 16 wild-type (A,E,H) and RP2 mutant (B-D,F,G,I,J) embryos. The neuropile was visualized with anti-HRP antibodies and is shown in blue. (A) In the wild type, the RP2 cell body is normally located medially on the anterior part of the anterior commissure. The RP2 axon exits the CNS via the pISN (arrow). Dendritic arbors (arrowhead) emerge from the proximal axon, mainly anteriorly, but frequently also, although to a lesser extent, posteriorly (not shown here). (B-D) Three of the most frequent morphological classes of mutant RP2 neurons exhibiting (B) relatively normal morphology with axon (arrow) exiting the CNS via the pISN and with dendritic arbors (arrowheads); (C) contralateral axonal projection; (D) anterior axonal (arrow) and dendritic (arrowhead) projections. (E) Axons (arrow) of wild-type aCC neurons exit via the aISN. Dendrites (arrowhead) extend from the proximal axon mostly anteriorly as well as contralaterally through the posterior commissure. (F,G) Two examples of mutant aCC neurons: axons fail to exit the CNS; the neuron in F still reflects the normal bipolar geometry of aCC. (H) Wild-type pCC neurons extend their axons (arrow) anteriorly for many segments along a medial fascicle. (I,J) Most mutant pCC neurons are relatively wild-type in appearance (I), although a fraction shows midline crossing in the next anterior commissure (J, arrow). All images are projections of confocal z-stacks. Anterior is towards the left. Triangles indicate the ventral midline, `AC' the anterior and `PC' the posterior commissure. Numbers indicate the fraction of labeled cells in the morphological class represented by the images. Scale bar: 10 µm in A-C,E,F; 16 µm in D,G-J.

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