Fig. 5. Procephalic mitotic domains and the origin of brain NBs. (A) Projection of the spatial arrangement of mitotic domains (Foe, 1989) onto a schematic flat preparation at stage 8 [anterior (a) is towards the top, dorsal (d) is towards the left] based on 4D microscopic data (shown in C) as well as anti-alpha-Tubulin antibody staining (see Fig. 8). Different colours mark the mitotic domains 1, 2, 5, 9, 20 and B. The earliest set of NBs derives from mitotic domains B and 9. (B) Schematic flat preparation at late stage 11. Mitotic domains 1, 2, 5, 9 and B (and perhaps 20) contribute NBs to the embryonic brain (see also Table 1). Coloured hatched lines mark subpopulations of NBs that derive from the respective domains (compare A). (C) 4D microscopic analysis. Left panel depicts Nomarski pictures taken from an in vivo timelapse sequence of the same embryo between stages 6 and 10 (st6, stage 6; est8, early stage 8; est9, early stage 9; st10, stage 10; lateral views of the procephalic ectoderm in the same focal plane; anterior is towards the left, dorsal is towards the top; red, stippled lines mark the cephalic furrow). Right panel demonstrates computer models of the pictures on the left side (see Materials and Methods); each ectodermal cell is represented by a dot, the centre of which corresponds to the centre of the nucleus. According to their typical mitotic behaviour (e.g. time point of mitosis; orientation of mitotic spindle), ectodermal cells can be grouped into distinct mitotic domains (Foe, 1989). Colour code indicates cells belonging to the same mitotic domain. The movements and fate of each blastodermal cell can be traced through ongoing development (here shown until stage 10). Posterior black dots at stage 6 indicate cells that subsequently move into the cephalic furrow and thus out of the focal planes (not shown in following stages). Ventral dark grey dots indicate cells belonging to other mitotic domains that were not traced because they move out of focal planes. Note the slight rotation of the head ectoderm: dorsal cells move towards posterior positions and posterior cells towards more ventral positions. Importantly, despite these movements and the delamination of NBs, the relative positions among mitotic domains and among cells within each domain do not change in principal. a, d: anterior, dorsal; AN, MD: antennal and mandibular segment, respectively; FG: foregut; OA: Bolwig organ/optic lobe anlagen.