Fig. 4. Expression of markers for glial cells and sensory precursors. (A-C) svp-lacZ (brown)/Repo (blue) double stainings between late stage 10 and late stage 11 (as indicated). Identified Svp-positive NBs are designated. By late stage 10, a cluster of two or three small protocerebral cells becomes Repo positive (white arrowheads in A,C; slightly different stages and focal planes). These are the first cells expressing Repo in the CNS (slightly before the longitudinal glioblast in the ventral nerve cord) (Halter et al., 1995), and might belong to the glia cluster `VPSG' described previously (Hartenstein et al., 1998). Considering their relatively small size, they could represent progeny cells of closely associated early NBs (e.g. Pcv6, Pcv7 or Pcv 9, which might act as neuroglioblasts as they are Repo negative). Slightly later, Repo is found in three further proto- and two tritocerebral cells (A). Two of the protocerebral cells (black arrows) are located ventrally in close vicinity to Pad2 (E, not in focus in A), and one more dorsally (white arrow) (possibly belonging to `VPSG' and `DPSG', respectively) (Hartenstein et al., 1998). By late stage 11, about eight additional Repo-positive cells can be detected in the protocerebrum at various positions along the DV axis (B). Considering their small size and distribution, they could be progeny cells of at least two central protocerebral NBs. Furthermore, at this stage three further Repo-expressing cells appear in the dorsal tritocerebrum at the border between the intercalary and mandibular segment (close to Td6, Td8 and Dd9; yellow arrowheads in B). The red arrowhead indicates the first deutocerebral cell expressing Repo. In two cases, glial precursors could be identified: Td7 (orange arrowhead in A,B,D) and Td4. (D) Td4 is a neuroglioblast; Ladybird early (lbe)/Repo double staining revealed a glial component (blue arrowhead) of the Lbe-positive Td4 lineage; black arrowheads mark other Lbe-positive daughters of Td4. (E) Because in all other cases it is not possible to link the Repo-labelled cells to identified precursors, their distribution relative to the NB pattern is marked by blue hatching in the semi-schematic presentation. (F,G) Atonal (blue)/A37-lacZ (brown) double staining at late stage 11. (F) In the pregnathal head A37-lacZ is found at strong levels in the ectoderm of the antennal appendage (AN), in a ventral ectodermal cell cluster near the foregut anlage (FG), and in ectodermal spots in the labral appendage (LR; violet and green arrowheads). Note that A37-lacZ is also detected at significantly lower levels in other parts of the procephalic and truncal neuroectoderm (black arrows), which is believed to be not indicative for sensory cells. Ato is co-expressed in subsets of strongly A37-lacZ positive cells (violet arrowheads); moreover, Ato is found in a dorsal cell cluster (blue arrowhead), including NB Ppd19 (E), in the primordial Bolwig organ cells of the optic anlage (OA), and in the labral appendage (black arrowheads). Dashed lines contour the outline of the mandibular, antennal and labral appendages; the dotted line contours the outline of the ventral midline (ML). (G) Close-up of the region framed in F at the level of NBs. Indicated are five A37-lacZ/Ato co-expressing antennal NBs; considering their position at the basis of the antennal appendage, Dd9, Dd11, Dd12, Dd13 are putative precursors of the Dorsal organ, the ventral Dv1 (and Dv3, which is not in focus) of the hypopharyngeal/latero-hypopharyngeal organ. Expression of A37-lacZ and Ato in stem cells is summarized in E. a, d: anterior, dorsal; AN, MD, MX: antennal, mandibular and maxillary segment, respectively; CL: clypeolabrum; FG: foregut; ML: ventral midline; OA: Bolwig organ/optic lobe anlagen.