EARLY NEUROGENESIS OF THE DROSOPHILA BRAIN

We have studied the formation of the neuroblasts of the Drosophila bra in which segregate from the procephalic neurectoderm. The expression d omains of the segment polarity gene engrailed ten) allow one to subdiv ide the procephalic neuroectoderm into tritocerebral, deuterocerebral, and protocerebral neuromeres. Based upon the expression pattern of th e proneural gene lethal of scute (l'sc), as well as the pattern of bra in neuroblast segregation, the protocerebral and deuterocerebral neuro meres can be further subdivided into a central, anterior, and posterio r domain. A total of 75-80 neuroblasts segregate in a stereotyped patt ern from the procephalic neurectoderm of each side during stages 9-11. With respect to their position and the expression of the markers asen se (ase) and seven-up (sup), 23 small groups of one to five neuroblast s each were identified. The first eight groups (Pc1-4, Dc1-3, Dp1), co llectively called SI/II neuroblasts in analogy to the subpopulation of ventral neuroblasts which appear at the same stage), arise from the c entral domain of the protocerebral and deuterocerebral neurectoderm, r espectively. Later groups form anteriorly and posteriorly from the ear lier ones, leading to a centrifugal growth of the procephalic neurobla st population. SIII neuroblasts (Pa1-4, Pp1-2, Dp2) arise during stage 10, SIV neuroblasts (Pa5-6, Pp3-4, Da1, T1-2) during early stage 11, and SV neuroblasts (Pp5, Pdm) during late stage 11 and early stage 12. The dorsomedial domain of the procephalic neurectoderm represents a s pecial case. Unlike other procephalic neuroblasts which delaminate fro m the surface ectoderm as individual cells, cells of the dorsomedial p rotocerebral domain are internalized during stage 12 as large, coheren t clusters by a movement which can be best characterized as a combinat ion of mass-delamination and invagination. (C) 1996 Wiley-Liss, Inc.