Vincent Fleury, SLT, 10-01-2007 à 17:27 : Bon évidemment, mon point de vue a légèrement évolué depuis le premier papier: pour faire un peu plaisir aux biologistes, j’ai laissé le pôle attracteur. Je crois qu’il n’est même pas nécessaire. Le mouvement est générique.
Une fois que c’est parti, le pôle est réduit en bouillie par le mouvement lui-même. De toute façon on ne peut pas sans sortir sans de l’advection diffusion, et quand vous voyez les moulinets que fait la blastula, ça laisse songeur sur le rôle des molécules.
Determination of embryonic polarity in a regulative system: evidence for endogenous inhibitors acting sequentially during primitive streak formation in the chick embryo
Federica Bertocchini, Isaac Skromne, Lewis Wolpert and Claudio D. Stern
Development 131, 3381-3390 doi:10.1242/dev.01178
Avian embryos have a remarkable capacity to regulate: when a pre-primitive streak stage embryo is cut into fragments, each fragment can spontaneously initiate formation of a complete embryonic axis. We investigate the signalling pathways that initiate primitive streak formation and the mechanisms that ensure that only a single axis normally forms. As reported previously, an ectopic primitive streak can be induced by misexpression of Vg1 in the marginal zone. We now show that Vg1 induces an inhibitor that travels across the embryo (3 mm distance) in less than 6 hours. We provide evidence that this inhibitor acts early in the cascade of events downstream of Vg1. We also show that FGF signalling is required for primitive streak formation, in cooperation with Nodal and Chordin. We suggest that three sequential inhibitory steps ensure that a single axis develops in the normal embryo: an early inhibitor that spreads throughout the embryo (which can be induced by Vg1), a second inhibition by Cerberus from the underlying hypoblast, and finally a late inhibition from Lefty emitted by the primitive streak itself.
Fig. 7. Cell interactions during primitive streak formation. The diagrams show four successive stages in the development of the primitive streak, illustrating the sequential signalling steps and the three proposed inhibitory activities. At stage X, chick Wnt8C is expressed all around the marginal zone andchick Vg1 in its posterior part (PMZ; red), while FGF is expressed in Koller’s sickle (lilac) and in the islands of hypoblast (not shown). Vg1 induces an inhibitor (shown as a yellow gradient) that travels through the embryo. At stage XII, the hypoblast (blue) starts to form a layer and secretes the Nodal antagonist Cerberus, while the combined action of Vg1 and Wnt8C induce expression of Nodal in the adjacent epiblast (hatched pattern). As the primitive streak (hatched) starts to form at stage 2 (under the influence of Nodal and Chordin, with additional input from FGF secreted from the adjacent tissues), it expresses Chordin and Nodal. At stage 3 (mid-primitive streak stage) the streak (black) expresses the inhibitor Lefty (green) which further blocks Nodal signalling. Extra-embryonic tissues are shown in white. Note the three sequential inhibitions: an early inhibitor induced by Vg1, followed by Cerberus emitted by the hypoblast and finally Lefty from the primitive streak itself.