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Inductive patterning of the embryonic brain in Drosophila

MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK

View larger version (41K): [in a new window]   Fig. 1. Drosophila embryonic brain anatomy. (A) DIC lateral view of an ES15 whole wild-type embryo stained with BP102, an antibody that recognizes CNS axons. The brain, which forms around the foregut, is outlined by the box. (B) Schematic representations of the brain and adjacent tissues at ES12 and late ES13. Mesoderm is in green, foregut ectoderm is in red, neuroblasts are in blue and neurons are in purple. (C) Dorsal view of the embryonic brain. b1 is the only segment visible in this view, and shows distinct separation into left and right hemispheres. (D) Lateral view of the embryonic brain, with b1-S3 visible. In this and the following figure panel, the outline of the foregut is shown by broken lines. Schemes represent a simplified arrangement of brain structures. CC, corpora cardiaca; CEC, circumesophageal connective; D/T, deutocerebral/tritocerebral neuron cluster; FC, frontal commissure; FG, frontal ganglion; PBC, preoral brain commissure; VNC, ventral nerve cord.

View larger version (76K): [in a new window]   Fig. 2. Ablation of the foregut results in brain patterning defects. For fluorescent images, staining is as follows: -Fasciclin II (which labels a subset of axons) in red; -Elav (which labels neuronal nuclei) in blue; and -Repo (which labels peripheral glial nuclei) in green. All embryos at ES15. (A) Dorsal view of embryo with laser-ablated foregut, showing a missing frontal commissure and ganglion, a fusion of brain hemispheres at the midline (arrowhead), and abnormal defasciculation in the preoral brain commissure in b1 (arrow). (B) Lateral view of an embryo with a laser-ablated foregut. (C,D) Dorsal and lateral views, respectively, of forkhead (fkh) loss-of-function mutant embryos. Brain defects resemble those seen in laser-ablated embryos (arrows and arrowhead as in A,B). (E) Expression pattern of Forkhead in b1 at ES13, as visualized using anti-Forkhead antibody staining (brown). Forkhead appears to be expressed in the central and lateral regions of the b1 hemispheres (arrowhead), but not in the dorsomedial edges of b1 up to ES13. The staining at the midline seen in this image is from Forkhead expression in the foregut (arrow), which lies in a focal plane immediately adjacent to the b1 midline.

View larger version (128K): [in a new window]   Fig. 3. Brain defects resulting from removal of mesoderm. Staining as described for Fig. 2. All embryos at ES15. (A,B) Dorsal and lateral views, respectively, of tinman (tin) mutant embryos. b1 shows a thinning of the preoral brain commissure (arrow) and abnormal closeness of the brain hemispheres (arrowhead). (C,D) Dorsal and lateral views, respectively, of embryos in which the mesoderm was ablated using a pan-mesodermal GAL4 driver to express Ricin toxin. These embryos also show a thinning of the preoral brain commissure (arrow) and a fusion of the brain hemispheres at the dorsal midline (arrowhead). In addition, these embryos lack the frontal commissure and ganglion.

View larger version (58K): [in a new window]   Fig. 4. Changes in apoptosis associated with brain patterning defects. All embryos are at late ES13, and are view dorsally at the level of the preoral brain commissure. Acridine Orange staining for apoptotic cells (green) is superimposed onto a DIC image taken simultaneously on a confocal microscope. The outline of the brain is indicated by white broken lines for reference. (A) The wild-type brain shows groups of apoptotic cells at the dorsomedial edges of the brain hemispheres (arrows). (B) In fkh loss-of-function embryos, there is a reduction in the number of apoptotic cells at the dorsal midline (arrow). (C) A similar reduction in the number of apoptotic cells is seen in tin loss-of-function embryos.

View larger version (78K): [in a new window]   Fig. 5. Involvement of Hedgehog signaling in patterning the brain. All embryos at stage 15 unless otherwise noted. (A,B,I) -Fasciclin II is in red, -Elav is in blue and -Repo is in green. (C-G) -GFP is in green and BP102 is in red. (A,B) Dorsal and lateral views, respectively, of a hedgehog (hh) null mutant embryo. (A) b1 with the phenotype of a missing frontal commissure, fused hemispheres (arrowhead), and a abnormally defasciculated preoral brain commissure (arrow). (B) The axon tracts in b2-S3 are thinned or broken, and there is a decrease in the number of glia and a disruption in the pattern of neurons in this region. (C,D) Lateral and frontal views, respectively, of ES13 hedgehog-GAL4, UAS-tau-GFP embryos, showing expression of hedgehog in a segmentally repeated pattern in b1-S3 (arrowhead), and in the foregut adjacent to b1 (arrow). (E-G) Lateral, frontal and dorsal views, respectively, of ES13 patched-GAL4, UAS-tau-GFP embryos. patched (ptc) appears to be upregulated in a segmental pattern complimentary to that of hedgehog in the brain (arrowhead), and in a group of brain cells surrounding the foregut (arrow). (H) Late ES13 hedgehog null embryo strained with Acridine Orange for apoptosis. There is a reduction in the number of apoptotic cells at the dorsal midline in these embryos (arrow) when compared with wild type. (I) Dorsal view of embryo in which expression of UAS-patchedloop2 was driven using the nervous system specific line 1407-GAL4. The size of the brain in these embryos is decreased when compared with wild type, but there does not appear to be excess cells at the dorsal midline of b1. Note that the frontal commissure is present.

View larger version (64K): [in a new window]   Fig. 6. Influence of the foregut and mesoderm on the formation of neural precursors. All embryos viewed laterally. (A-C) ES10 wild type, forkhead loss-of-function and tinman loss-of-function embryos, respectively. The pattern of neuroblasts in b1 was visualized using anti-Hunchback antibody staining (brown) and did not show obvious differences between these groups. (D-F) ES11 wild-type, forkhead loss-of-function, and tinman loss-of-function embryos, respectively. GMCs in the brain were stained using anti-Prospero antibody (brown). (E) In the absence of Forkhead function, the number of GMCs in b1 appeared to be reduced (arrow). (F) A similar reduction in GMCs was not seen in Tinman null embryos. (H) ES11 Hedgehog null embryo, showing a reduction in the number of GMCs in b1 similar to that seen in Forkhead null embryos (arrow).