Building the antennal lobe: engrailed expression reveals a contribution from protocerebral neuroblasts in the grasshopper Schistocerca gregaria

The expression pattern of the engrailed protein was studied in neuroblasts which delaminate at the border of the protocerebrum and antennal lobe of th e deutocerebrum in the early embryonic brain of the grasshopper. The antenn al lobe is a complex structure comprising both glomerular and non-glomerula r components, a cellular organization which distinguishes it from the stria te-like neuropil comprising the remainder of the deutocerebrum. Early in em bryogenesis engrailed expression in the protocerebrum is restricted to a co mpact block of neuroblasts located at its interface with the antennal lobe. Subsequently engrailed expression in these cells disappears in a stepwise manner from anterior to posterior so that by 37% of embryogenesis only a si ngle row of three engrailed positive neuroblasts and their progeny remains. Contemporaneously engrailed expression reappears in a group of more anteri or progeny deriving from neuroblasts which are no longer immunoreactive. Th e three remaining engrailed positive neuroblasts then become separated from their non-immunoreactive neighbours by an invagination of the perineurium called the lateral cleft and come to lie completely within the developing a ntennal lobe. These cells then direct columns of immunoreactive progeny cen trifugally towards the centre of the lobe. Such a protocerebral contributio n to the antennal lobe suggests that the evolution and ontogeny of this bra in region need to be reconsidered. (C) 2001 Elsevier Science Ltd. All right s reserved.