Relocations of cell convergence sites and formation of pharyngula-like shapes in mechanically relaxed Xenopus embryos.

Influence of the relaxation of mechanical tensions upon collective cell movements, shape formation, and expression patterns of tissue-specific genes has been studied in Xenopus laevis embryos. We show that the local relaxation of tensile stresses within the suprablastoporal area (SBA) performed at the early-midgastrula stage leads to a complete arrest of normal convergent cell intercalation towards the dorsal midline. As a result, SBA either remains nondeformed or protrudes a strip of cells migrating ventralwards along one of the lateral lips of the opened blastopore. Already, few minutes later, the tissues in the ventral lip vicinity undergo abnormal transversal contraction/longitudinal extension resulting in the abnormal cell convergence toward ventral (rather than dorsal) embryo midline. Within a day, the dorsally relaxed embryos acquire pharyngula-like shapes and often possess tail-like protrusions. Their antero-posterior and dorso-ventral polarity, as well as expression patterns of pan-neural (Sox3), muscular cardiac actin, and forebrain (Otx2) genes substantially deviate from the normal ones. We suggest that normal gastrulation is permanently controlled by mechanical stresses within the blastopore circumference. The role of tissue tensions in regulating collective cell movements and creating pharyngula-like shapes are discussed.