The role of actin cytoskeleton during early stages of pectoral fin shape establishment

Elena Kardash
BioEmergences lab at CNRS, Gif-Sur-Yvette


The mechanism of a three-dimensional shape establishment during vertebrate organogenesis is not fully understood. We used pectoral fin growth in zebrafish as a model for vertebrate limb formation to study the molecular and cellular dynamics leading to the emerging 3D shape. At the early stages of development between 28 and 48 hours post fertilisation (hpf), the fin consists of two cell types: the mesodermal inner mass and an ectodermal outer layer. We show, that at about 36 hpf, fin structure exhibits the simple symmetry of a semi ellipsoid, which later elongates along its anterior-posterior (AP) axis and shortens along its dorso-ventral (DV) axis. We use the ratio between AP and DV axes a readout for fin shape changes to show that AP/DV increases from 1.33 at 32 hpf and to 1.67 at 44 hpf. This anisotropic shape change coincides with an accumulation of a dense actin structure, which we named “actin belt” in the two stripes of ectodermal cells at the most distal rim of the fin bud corresponding to its DV boundary. Using FRET imaging and morpholino-injected embryos, we have shown that Rac activity is essential for actin belt formation and anisotropic shape changes. When actin belt was not present in the morphant embryos with compromised Rac ativity, fin shape retrograded to a shorter semi-ellipsoid geometry as observed at 32 hpf, while its volume and cell number was not affected. Our results suggest a novel mechanism in which an acto-myosin contractile belt promotes fin shape transition from a shorter to a longer semi-ellipsoid.