Fig. 3

Appearance of the arm crown and early arm outgrowth. (A-D) overview of E. scolopes arm crown development from stages 18 to 21. Arm crowns are either labeled with anti-Histone H1 to visualize cell nuclei (A) or phallacidin to visualize F-actin (B-D), and oriented with anterior to the left and dorsal to the top. (E-E′) oral view of arm crowns at stage 21–22 labeled with phallacidin to visualize F-actin in green (E) and anti-acetylated tubulin to visualize nerve tracts in red (E′). (F, G) confocal image stacks of frontal sections of arm II (F) and arm IV (G) treated with EdU to visualize proliferating cell nuclei in cyan merged with a DIC image of the arms in the same focal plane. (F′, G′) frontal (F′) and sagittal (G′) histological sections of arms stained with toluidine blue. (F′′- G′′′) confocal image stacks of arm II (F′′- F′′′′) and arm IV (G′′- G′′′′) 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 (E′) point at the proximal part of the interbrachial ganglia’s axonal tracts joining to form the interbrachial connective. Dashed line in (G-G′′) marks the area of the axial nerve chord. ep, epithelium; mu, musculature; icm, inner cell mass. Scale bars: 50 μm in (A), 100 μm in (E)