Utilisateur:Perezoso~frwiki
Clément LAMY, PhD on Developmental biology.
Ectodermal anteroposterior patterning of the ascidian Ciona intestinalis
Ascidians are chordates which develop in a larval stage with simple embryos (few cells, fixed lineage,...). The ectodermal tissue of the ascidian Ciona intestinalis derives from the animal blastomeres. Anterior animal cells form anterior nervous system and anterior epidermis while posterior animal cells participate to the formation of the posterior nerve cord and forms posterior epidermis. Previous work indicated that, by the 8-cell stage, the anterior and posterior animal blastomeres have acquired different properties, including a differential responsiveness to inducing signals from the underlying mesendoderm (Rose, 1930 ; Hudson and Lemaire, 2001).
In vertebrates, Wnt signaling appears necessary for establishing anteroposterior patterning. Our work showed that the
expression of a secreted inhibitor of the Wnt pathway, Ci-SFRP1/5, is the earliest marker specific for the anterior ectoderm, starting just before gastrulation. We first found that this activation in the anterior ectoderm does not involve communication with other lineages. We then identified, by phylogenetic footprinting and deletion analysis, a short conserved minimal enhancer driving the onset of expression of Ci-SFRP1/5. We showed that this enhancer was a direct target of the Ci-FoxA-a gene, a FoxA2/HNF3ß orthologue expressed in anterior ectodermal and mesendodermal lineages from the 8-cell stage. Gain and loss of function experiments revealed that Ci-FoxA-a is necessary and sufficient within the ectoderm to impose an ectodermal anterior identity, and to repress the posterior programme (Lamy et al., 2006).
Thus, Ci-FoxA-a constitutes a major early zygotic anterior determinant for the ascidian ectoderm, acting autonomously in this territory, prior to the onset of vegetal inductions. Interestingly, while vertebrate FoxA2 are also involved in the regionalisation of the ectoderm, they are thought to act during gastrulation to control in the mesendoderm the expression of organiser signals. We are interested in discussing the evolution of chordate ectodermal patterning in light of our findings.