Fig. 3. Migration of dorsal mesenchymal cells towards the aortic wall. (A) Ultrastructure analysis of the space between the notochord and the aorta. Mesenchymal cells were widely spaced in a voluminous matrix. Cells that were located at greater distance from the vessel were polygonal (arrowheads), whereas cells closer to the aorta were more elongated (asterisks). Arrows indicate profiles of cytoplasmic extensions. (B,C) The asterisk-labeled cells in A displayed elongation in the direction of the arrows with cytoplasmic organelles distributed to the aorta-facing side of the nucleus. Scale bars: 20 µm in A; 5 µm in B,C.