The above light sheet movie is a cropped section of a wild-type zebrafish embryo, imaged starting at 17HPF until approximately 20HPF (3hours, 20 minutes) unrestrained in an FEP tube filled with water. The Rohon-Beard cell bodies in the spinal cord often produce small apical protrusions, which seem to grow to the midline and extend and retract periodically. The purpose of these protrusions is unknown.
In embryos treated with the Kinesin Light Chain inhibitor Kinesore, apical protrusions often disobey the midline barrier and join with the contralateral central fasicle. In the above movie, a cell in the top center cluster of three cells begins to extend an apical protrusion, which takes on axon-like characteristics such as a growth cone and filopodia. The protrusion navigates to the contralateral side and appears to fasiculate. Over the course of the movie, which is nearly 17 hours long, this protrusion remains stably attached to the contralateral side.
In this movie, a zebrafish embryo treated with the Arp2/3 inhibitor CK666 at 18HPF demonstrates that even abnormally grown-out apicals can demonstrate avoidance. This embryo was imaged via LSFM as described previously. The cell marked with the asterisk extends an apical protrusion, probes the contralateral side, and retracts and collapses its growth cone.