Early neurogenesis of the Drosophila brain.

We have studied the formation of the neuroblasts of the Drosophila brain which segregate from the procephalic neurectoderm. The expression domains of the segment polarity gene engrailed (en) allow one to subdivide the procephalic neuroectoderm into tritocerebral, deuterocerebral, and protocerebral neuromeres. Based upon the expression pattern of the proneural gene lethal of scute (l'sc), as well as the pattern of brain neuroblast segregation, the protocerebral and deuterocerebral neuromeres can be further subdivided into a central, anterior, and posterior domain. A total of 75-80 neuroblasts segregate in a stereotyped pattern from the procephalic neurectoderm of each side during stages 9-11. With respect to their position and the expression of the markers asense (ase) and seven-up (svp), 23 small groups of one to five neuroblasts each were identified. The first eight groups (Pc1-4, Dc1-3, Dp1), collectively called SI/II neuroblasts in analogy to the subpopulation of ventral neuroblasts which appear at the same stage), arise from the central domain of the protocerebral and deuterocerebral neurectoderm, respectively. Later groups form anteriorly and posteriorly from the earlier ones, leading to a centrifugal growth of the procephalic neuroblast 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 special case. Unlike other procephalic neuroblasts which delaminate from the surface ectoderm as individual cells, cells of the dorsomedial protocerebral domain are internalized during stage 12 as large, coherent clusters by a movement which can be best characterized as a combination of mass-delamination and invagination.