Fig. 5.

The axon scaffold of the midbrain during formation of the fan-shaped body in the grasshopper (a, b) and Drosophila (c–e). a. At 60% of embryogenesis, 3D confocal image following 8B7 immunolabeling (magenta) reveals axon pathways in the central brain of the grasshopper at the commencement of embryonic fan-shaped body (eFB) formation. Axons (white arrowheads) from w, x, y, z tracts (not in picture) in the pars intercerebralis (PI) are in the process of decussating from anterior commissures eAC VIII and eAC IX across the eM region to innervate the more posterior commissures ePC I–III. These decussated axons form the basis of the tract-specific columns of the FB. The general form of the eFB (white dashed parentheses) is already apparent in the midbrain. Arrow points to anterior according to the neuraxis (nA) for the grasshopper panels. b. At 70%, confocal image shows that decussated axons from the x, y, z tracts (w is not visible in this brain slice) in the PI are now more orthogonal in their projection en route from the anterior plexus (apl) to the posterior plexus (ppl) of commissures. The columns (open white arrowheads) of the FB (white dashed parentheses) are now more apparent and the anterior and posterior commissures forming apl and ppl respectively are further apart in the brain than at 60%. c. Confocal image of the central brain of Drosophila at the late third larval (L3) stage following of the x, y, z tracts by R19G02-Gal4>UAS-mcd8-GFP (magenta) originating from cell clusters in the PI (not shown) and projecting into the commissural system of the developing primordial fan-shaped body (prFB). The primordial anterior plexus (prFBapl) and projections in the isthmus tract (IT) to the primordial lateral accessory lobes (not in picture) are evident. The tracts are organized as schematically shown in panel d. Axons from lineages y and z decussate. Arrow points to posterior (P) according to the body axis and applies to all Drosophila panels. d. Schematic summarizes the projection of axons from secondary w, x, y, z cell clusters into the developing prFB of the central brain at L3. Axons are in the process of decussating from the prFBppl (red arrow) to the prFBapl (blue arrow) thereby forming the initial columnar neuroarchitecture (purple arrow) of the FB. Commissural fascicles of the posterior commissures (PCC) of the central complex and superior ellipsoid body commissure (SEC) delimit the developing prFB. Filopodia from neurons terminating within the prFB are visible. e. Confocal image following R19G02 labeling (magenta) at 24 hrs after puparium formation (P24) shows axons projecting to the FB from secondary lineages in the PI via the x, y, z tracts. Axons have begun to decussate (see above) to form the initial columnar neuroarchitecture of the FB (open white arrowheads) between the prFBppl and the prFBappl. Scale bar in c represents 15μm in a, 20μm in b, c, e