Differentiation of fly visual interneurons after laser ablation of their central targets early in development

To test the differentiation of visual interneurons that had their targets removed before axogenesis, we ablated neuronal precursors in brains of first instar fly larvae by using a laser microsurgery unit. We describe ablations that resulted in the elimination of the third neuropil region (the lobula complex) of the optic lobes. Neural differentiation in the more peripheral second and first neuropil regions (the medulla and the lamina) was thus studied in the absence of the lobula complex. It was found that the medulla neuropil differentiated with normal columnar and layered organization. The neuropil, however, folded along its central surface. The only connection between the medulla and more central neuropil (the midbrain) was via a bundle of axons (the Cuccati bundle) present also in the normal optic lobes. Some types of neurons that normally connect the medulla and the lobula complex could be identified. These appeared to end in a disorganized neuropil mass in the center of the folded medulla. The differentiation of the lamina neuropil also appeared normal in flies with the lobula complex eliminated and the medulla folded. Also, in optic lobes where the medulla was severely disorganized and/or reduced due to laser ablations, the lamina neuropil appeared more or less normal. The results suggest that lamina and medulla nerve cells can differentiate and develop normal neuropil patterns in absence of their appropriate targets.