Fig. 5

Arm differentiation. (A-A′) oral view of arm crowns at stage 27 labeled with phallacidin to visualize F-actin in green (A) and anti-acetylated tubulin to visualize nerve tracts in red (A′). (B, C) confocal image of frontal sections of arm II (B) and arm IV (C) treated with EdU to visualize proliferating cell nuclei in cyan, merged with a DIC image of the arms in the same focal plane stacks. (B′, C′) sagittal histological sections of arms stained with toluidine blue. (B′′- C′′′) confocal image stacks of arm II (B′′- B′′′′) and arm IV (C′′- C′′′′) labeled with phallacidin to visualize F-actin in green (F′′, G′′), anti-acetylated tubulin to visualize nerve tracts in red (F′′′, G′′′) and their overlap in merged images (F′′′′, G′′′′). x marks the position of the mouth, I – V denotes the arm pairs in the order they are spatially positioned. Arms are oriented with aboral to the top and distal to the left. Dashed line in (B, B′′, C, C′′) and asterisk in (B′, C′) mark the area of the axial nerve chord. Open arrowheads point at longitudinal muscle fibers, arrows denote transverse muscle fibers, white arrowheads indicate emerging connective fibers. Dotted rectangles mark close-up shown in Additional file 4. anc, axial nerve cord; ci, cilia; ep, epithelium; mu, musculature; inmc, intramuscular nerve cord; su, sucker. Scale bars: 100 μm in (A), 10 μm in (B, C)