Active reinforcement of externally imposed folding in amphibians embryonic tissues

► Epithelial fold initiated by external compression force is actively reinforced. ► Fold reinforcement is caused by constriction and engulfment of apical cell surfaces. ► Endocytosis and actomyosin contraction play the main role in apical constriction. ► Folding reaction of Xenopus embryonic tissues exhibits antero-posterior gradient. ► Range of mechanical work during fold reinforcement is evaluated.