Fig. 7. Expression of proneural genes in the procephalic neuroectoderm and brain NBs. Semi-schematic presentation of the expression patterns of achaete (ac), scute (sc), lethal of scute (l'sc) and atonal (ato) (from stage 8 to 11) at the level of the procephalic ectoderm (A,C,E,G; positions of underlying NBs are marked by broken lines) and at the level of individual brain NBs (B,D,F,H) (see key for neuroblasts, NB and neuroectoderm, NE; high (+) and low () levels of expression are indicated by colour intensities). In the peripheral ectoderm (A,C,E,G) single cells are outlined when expression domains comprise less than eight cells. Additionally, the expression of engrailed (en) in the peripheral ectoderm is highlighted. Orientation as in Fig. 2. By stage 7/8, L'sc protein is detected in a large domain covering the central neuroectoderm (encompassing mitotic domain B and 9) from where first proto- and deutocerebral NBs develop (A,B). By stage 10, L'sc expression has expanded into other regions of the procephalic neuroectoderm (E), but by late stage 11 becomes confined to several smaller ectodermal domains, and especially to the dorsomedial neuroectoderm from where anteriormost protocerebral NBs emerge (G,H). Of all investigated proneural genes only L'sc is found in the intercalary NE (E,G) and about seven deriving tritocerebral NBs (F,H). Ac expression is detected by stage 8 in a small dorsal ocular and antennal group of neuroectodermal cells (from which Pcd21 and Dd8 derive). Furthermore, Ac is expressed in a large domain of the central procephalic neuroectoderm where four or five NBs, which co-express L'sc derive from (A,B). During stage 9 Ac is found in several large domains at different sites of the procephalic neuroectoderm (C), from which, by stage 10, about nine additional, weakly Ac-positive NBs emerge (D,F). During stage 10, Ac expression decreases in the peripheral ectoderm and is, by stage 11, confined to two most dorsal NBs (Pcd16 and Pcd19; E-H). sc is not expressed before stage 10, when it is found in about six small patches in the antennal and preantennal ectoderm, which (except for a patch in the clypeolabral ectoderm) give rise to single, or small groups of, NBs. Furthermore, it is strongly expressed in a domain of the anteriormost procephalic neuroectoderm and a corresponding group of about seven protocerebral NBs (E-H). Ato protein is expressed in a cluster of about six ocular ectodermal cells (A) that presumably represent the `proneural cluster' from which Ppd19 derives (D). Dv3 expresses Ato only after formation (compare B with D) in contrast to the adjacent Ato-positive Dv1 (H), which develops from the Ato-expressing proneural cluster in the ventral antennal ectoderm (C,E). By stage 11 Ato is additionally expressed in primordial cells of the optic anlagen (OA in G). a, d: anterior, dorsal; as, is: antennal and intercalary en stripe, respectively; hs: en head spot; AN, MD, MX: antennal, mandibular and maxillary segment, respectively; CL: clypeolabrum; FG: foregut; ML: ventral midline; OA: Bolwig organ/optic lobe anlagen.