Fig. 4

Arm elongation. (A-A′) oral view of arm crowns at stage 23 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 stacks 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. (B′, C′) frontal (B′) and sagittal (C′) 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. White arrowheads in (A′) point at the axonal tracts of the interbrachial ganglia extending into individual arm buds. Dashed line in (B-B′′, C-C′′) marks the area of the axial nerve chord. Open arrowheads point at longitudinal muscle fibers, arrows denote transverse muscle fibers. ci, cilia; ep, epithelium; mu, musculature; icm, inner cell mass; su, sucker. Scale bars: 100 μm in (A), 10 μm in (B)