Fig. 7. The regulation of angioblast migration in the trunk by sema3a1, vegf and neuropilin 1. (Left) A side view of the trunk showing the expression of sema3a1, vegf and neuropilin 1 with the posterior-most somite showing the earliest pattern of somitic sema3a1 expression, and each successive anterior somite showing a slightly more mature pattern. (Right) Transverse section showing the expression of sema3a1, vegf and neuropilin 1 in a segment corresponding to the middle segments shown in the side view. Initially expression of vegf by medial somitic cells attracts neuropilin 1-positive angioblasts to migrate dorsally (arrows), whereas expression of sema3a1 in the posterior somitic cells channels these angioblasts to the anterior regions of each somite. As the angioblasts migrate dorsally to their position just ventral to the notochord, sema3a1 is changing so that it is no longer expressed by the horizontal myoseptal cells at the level of the notochord but is expressed by the ventral third of the somite. The new pattern of Sema3a1 is proposed to restrict the angioblasts to their final destination. Overlapping expression of Vegf and Sema3a1 may generate functional gradients of attraction by Vegf and repulsion by Sema3a1

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