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Equipes de recherche - Biologie du Développement de Villefranche sur mer
Unité Mixte de Recherche CNRS 7009




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Développement de la polarité embryonaire

Development of embryo polarity

Maternal polarity determinants in cnidarians


It has been shown in various cnidarian species including Clytia (Phialidium) gregarium that the animal pole of the egg is fated to become the gastrulation site and later the oral (= posterior) pole of the larva.

Experiments on the hydrozoan Podocoryne carnea provided good evidence that maternal "determinants" localised in the animal hemisphere direct the development of oral fates, incuding endoderm. (Momose & Schmid, 2006). Thus vegetal halves isolated at the 8-cell stage fail to gastrulate or to show any oral-aboral polarity. Similar results have been obtained in the anthozoan Nematostella (Lee et al 2007;  Fritzenwanker et al; 2007).  In Clytia, a different result is obtained: Animal and vegetal embryo halves both develop normally (eg Freeman 1981a; 2005). This difference could reflect a broader distribution of determinants and/ or a greater regulative capacity in embryos from this species


A key role for Wnt/beta-catenin signaling




 In both the Nematostella (Wikramanayake et al; 2003) and in Clytia (Momose and Houliston, 2007Nouvelle fenêtre), the first detectable polarity in the embryo is  activation of Wnt/beta-catenin signalling on one side of the embryo.
This can be visualised as the stabilisation of the transcriptional co-regulator beta-catenin.
In Clytia, this regionalised Wnt pathway activation is driven by three Wnt pathway regulators produced from maternal mRNAs localized differentially along the animal-vegetal egg axis: two Frizzled family Wnt receptors, and one Wnt ligand (Momose and HoulistonNouvelle fenêtre, 2007  PDFNouvelle fenêtreMomose et al, 2008Nouvelle fenêtre  PDFNouvelle fenêtre).




Testing the function of each of the these three RNAs by injecting Morpholino antisense oligonucleotide into eggs before fertilisation showed that Fz1 and Wnt 3 both activate the canonical Wnt pathway and promote the development of oral fates including Brachyury expression and gastrulation. Conversely the second receptor,  Fz3, localized on the vegetal side acts to suppress Wnt pathway activation.









Our current model for polarity development in Clytia is that Wnt pathway activation is initially restricted by Fz1 acting animally and Fz3 acting vegetally, both receptors being translated from localised maternal RNAs. The ligand Wnt3 is required upstream of Fz1, but is not sufiicient to cause ectopic Wnt pathway activation.
A second 'regulatory' phase of patterning occurs once zygotic gene expression starts during the blastula stage. This involves reciprocal negative feedback between Fz1 and Fz3, and is dependent on Wnt3.




Contribution of Fz-PCP signaling to embryo polarity

Planar Cell Polarity (PCP) is the alignment of cells across plane of a tissue sheet. It is classically mediated by transmembrane complexes on opposite sides of the cell, comprising Fz+Disheveled on one side and Strabismus/VanGogh+Prickle on the other. These complexes interact extracellularly to mediate their differential localisation. This so-called “Fz-PCP pathway” (see Lapébie et al. 2011Nouvelle fenêtre) underlies many vital PCP phenomena including alignment of hairs, bristles and feathers in vertebrate epidermal and tracheal epithelia, of ommatidia in insect eyes and hair cells in the mammalian inner ear, as well as in the dynamics of mesenchymal cells sheets which drive vertebrate embryo elongation during gastrulation. In the Clytia larva each ectodermal cell bears a cilium, and PCP is responsible for their coordinated beating direction to cause directional swimming with the aboral pole in front?

In Clytia, PCP is detectable first in ectodermal cells during gastrulation as coordinated basal body positioning. At later stages more pronounced structural polarity develops around the base of each cilium in relation to cilia beating direction, including a characteristic asymmetric cortical actin organisation. We have shown that   the core PCP protein Strabismus (Stbm) plays a conserved role for tissue polarity coordination during development of the larval ectoderm (Momose et al 2012Nouvelle fenêtre).






Evelyn Houliston - 08/03/13