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The Central Complex

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From front to back in the brain (i.e along the y axis), the central body complex comprises the ellipsoid body, the superior arch and fan shaped body above the paired noduli, and the protocerebral bridge.

In Drosophila, the ellipsoid body (e. b.) is an almost circular neuropil lying anterior to the fan-shaped body. It consists of ring-like terminals of neurons arising in the lateral protocerebrum. The e.b. is further subdivided into 16 radial "segments" that recieve terminals of neurons that originate in the protocerebral bridge. Dendrites in the ellipsoid body are shared between it and strata of the fan-shaped body lying immediately posterior.

The fan-shaped body (f.b.) is a many layered neuropil whose architecture is dominated by stave like arborizations of terminals and dendrites linking the f.b. to lateral protocerebral regions. Its cellular elements are also derived from the protocerebral bridge via a complex system of crossed axons. Subtle details of the f.b.'s internal organization are revealed by cobalt fills and immunocytology.

The superior arch (s.ar.) is the most dorsal layer of neuropil surmounting the f.b.  Neurons in the s.ar. are generally oriented tangentially and their branches do not usually show the stave like appearance typical of the f.b.  In Bodian sections the s.ar. appears generally paler. It also receives inputs from the protocerebral bridge.

The noduli (no.) are two ball-like neuropils that receive connections from protocerebral bridge neurons that, en route, provide collaterals to the staves of the f.b.  The no. comprise concentric layers and are divided into a dorsal and ventral component. These features can be seen in cobalt and in immunostains of homologous regions in larger flies.

In frontal section, the protocerebral bridge (p.b.) looks like a pair of outstretched wings. The p.b. connects the two dorsal lobes of the protocerebrum. It comprises a system of commissural fibres that are intersected by a palisade of extremely small dendritic trees. These, in turn, provide axons that project into the f.b. where they branch, and from where they project further into the e.b. and/or no.

The components described above comprise the central body complex . They have been seen in all insect species examined. The fan-shaped body and protocerebral bridge have also been observed in isopod crustaceans. The round dough-nut shape of the ellipsoid body in flies is not typical of all insects. In most other groups the ellipsoid body is an arched neuropil. In flies the arch is bent to such a degree that its two lateral edges are fused at the ventral mid-line.


Reference:

Hanesch, U., K.-F. Fischbach, and M. Heisenberg (1989) Neuronal architecture of the central complex in Drosophila melanogaster. Cell Tiss. Res. 257:343-366.

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