Building the central complex in Drosophila: the generation and development of distinct neural subsets.

The distinctive midline neuropil, the central complex (CX), is one of the most prominent features of the insect brain. We investigated the development of the four CX structures and several sets of CX neurons in the Drosophila brain using immunostaining for two cell adhesion molecules, DN-cadherin and Echinoid, and a set of seven enhancer trap lines. Our results showed that the CX is first identifiable in the third instar larva and that it elaborates over the first 48 hours of metamorphosis. The first identifiable structures to appear in their immature form are the protocerebral bridge and fan-shaped body, which are present in the brain of the third instar larva, followed by the noduli (from P12h), and finally the ellipsoid body (from P24h). We observed that neurons are added incrementally to the developing CX structure, with sets of small-field neurons projecting to the CX prior to the large-field neurons. The small-field neurons first project to the developing fan-shaped body, before arborizing or extending to the other structures. We found evidence to suggest that small-field neurons exist in sets of 16 and that they originate from eight common clusters of perikarya in the cortex, suggesting a common origin. We also identified a novel set of pontine neurons that connect contralateral segments in the fan-shaped body.