Although there has been in the past endless debate about how many segments contribute to the head and to the brain, expression of homeobox genes now confirms three embryonic brain segments only. The main issue of contention focused on the neuromeric identity of the optic lobes, which were claimed by some to have a distinct segmental origin. Developmental studies, which are summarized by Meinertzhagen and Hanson's 1993 review, showed that the inner optic lobe neuropils in the adult insect brain develop from a lateral outgrowth of the protocerebrum. George Boyan and his colleagues at the University of Munich have provided crucial evidence supporting a three-neuromere origin of the supraesophageal ganglion from studies of the segment polarity gene engrailed, which is expressed in cells (including neuroblasts) in the posterior compartment of each segment. The expression of engrailed in the first wave of neuroblast generation shows the delineation of the tritocerebrum from the first (maxillary) subesophageal neuromere, as well as the delineation between the tritocerebrum and the deuto-cerebrum and the delineation between the deutocerebrum and the protocerebrum, the last being the most rostral segmental neuromere. Crucially, the expression of engrailed shows the latter to be segmentally indistinct from the developing optic lobes.
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