Spatial restriction of actomyosin-based tissue invagination to form a long and thin epithelial tube. (A) Molecular components of the inhibitory pathway. (B) Formation of a long and thin epithelial tube when localized, inductive signaling for apical constriction (green line) is combined with an inhibitory threshold (red line) across the developing salivary gland of a wild-type embryo. (C) Formation of a shorter and wider epithelial tube when localized, inductive signaling for apical constriction (green line) has abnormally broad effects in an arc mutant embryo lacking the inhibitory threshold.