Fig. 2. The expression patterns of sema3a1/sema3a2 and neuropilin 1 in the trunk region correlate with the migration pathway of angioblasts. Expression of sema3a1 (B,C,G,H), sema3a2 (A), their receptor subunit neuropilin 1 (D-F,I-K), and co-expression of neuropilin 1 and fli1 (L-N) were assayed by in situ hybridization. Unless otherwise noted, embryos are oriented with rostral towards the left and dorsal upwards. (A) At 17 hpf, sema3a2 is expressed by the posterior part of newly segmented somites as seen in a dorsal view. (B) sema3a1 is also initially expressed by the posterior regions of each somite, as seen in a lateral view of a 20 hpf embryo. (C) Subsequently, sema3a1 expression changes so that it is expressed by the dorsal and ventral region of each somite, but not the horizontal myoseptal region as seen at 23 hpf. (D) At 18 hpf neuropilin 1 is expressed by gut endoderm ventral to the somites and by a strip of cells that appear to be extending dorsally (arrow). These strips of cells were adjacent to the anterior regions of the somites as judged by comparison with the borders of the somites that are out of the plane of focus. neuropilin 1 is also expressed by subsets of cells within the spinal cord dorsal to the notochord. (E) By 22 hpf many more strips of neuropilin 1expressing cells were evident, with the anterior strips generally extending more dorsally compared with more posterior strips (arrows). (F) The neuropilin 1-expressing strips are missing in 22 hpf hsp70:^gfpsema3a1^myc transgenic embryos following ubiquitous induction of sema3a1 at 15 hpf, whereas expression in the ventral spinal cord is unperturbed. (G) Transverse section of the trunk of a 23 hpf wild-type embryo showing expression of sema3a1 in the dorsal and ventral but not horizontal myoseptal region of the somites. (H) In the U-type mutant, you-too (yot), sema3a1 is expressed throughout the entire somite as seen in a transverse section. (I) Transverse section of the trunk of an 18 hpf wild-type embryo showing expression of neuropilin 1 in the strip of cells that appears to be extending along the medial surface of the somites (arrow). neuropilin 1 is also expressed by hypochord cells just ventral to the notochord. (J) At 22 hpf, cells expressing neuropilin 1 (arrow) are seen extending to the dorsal aortic region beneath the hypochord. (K) Transverse section of a 22 hpf hsp70:^gfpsema3a1^myc transgenic embryo heat induced at 15 hpf showing that the neuropilin 1-expressing cells normally found between the notochord and the yolk tube are missing. (L-N) Transverse sections of a 22 hpf wild-type embryo double-labeled for neuropilin 1 (green, L) and the endothelial marker fli1 (red, M) showing that many of the neuropilin 1-positive cells (arrow) located between the notochord and yolk tube express fli1 (red and green merged, N). Scale bars: 20 µm.

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