Fig. 2

Histological sections of the earliest stages of arm regeneration in the brittle star Amphiura filiformis. Sagittal (A and A’) and parasagittal (A”) paraffin sections at stage 1 and sagittal paraffin sections at stage 2 (B) stained with Milligan’s trichrome technique. Collagen stained cyan, all cells labeled pink/magenta. A) The wound is completely healed and re-epithelialization occurs. A’) Detail on the new thin epithelium with a recognizable cuticle (asterisk). A”) Histolysis (arrowheads) of intervertebral muscle bundles proximal to the amputation plane. B) The radial nerve, the radial water canal and the coelomic cavities start regenerating beneath the new epidermis. B’) Detail of the mesenchymal cells (arrow). B”) Detail of the mesenchymal cells at the level of the aboral dermal layer (arrow). A - aboral, ACC - aboral coelomic cavity, CT - connective tissue, D - distal, E - epidermis, ES - ectoneural sinus, intervertebral muscles - IM, N - radial nerve, O - oral, P - proximal, Po - podia, PS - parasagittal section, RWC - radial water canal, SS - sagittal section, V - vertebra