A contractile acto-myosin belt promotes growth anisotropy during the early stages of pectoral fin development in zebrafish

The zebrafish pectoral fin is an in vivo model for vertebrate limb formation, well suited to investigate the integration of molecular and cellular dynamics, the results of which translate into shaping the limb bud. We used the ratio between the lengths of the anterior-posterior (AP) and dorso-ventral (DV) axes as the descriptor of how fin shape changes over time. We showed that fin shape transitions from close to hemi-spherical (ratio 1.36) to semi-ellipsoid (ratio 1.64) between 33 and 46 hours post fertilization (hpf). This shape transition coincided with the formation of a contractile actin belt at the distal rim of the fin bud along its AP axis. The actin belt emerged from a central position and expanded on both sides along the distal rim of the fin, thus marking the DV boundary between two rows of ectodermal cells. Formation of the actin belt depended on Rac protein activity, as suggested by FRET measurements using a Rac biosensor. 3D+time imaging of the developing fin in Rac-deficient embryos showed that anisotropic growth of the fin depends on the actin belt. Indeed, actin belt formation was dramatically reduced or even absent in the embryos without proper Rac activity. This correlated with isotropic growth of the fin bud from normal shape at 33 hpf to quasi hemispherical shape with AP/DV ratio of about 1 13 hours later, without affecting cell number and overall bud volume. We propose that the formation of a contractile acto-myosin belt is essential to drive the pectoral fins early anisotropic growth.