Mechanisms of cranial placode assembly

International audienceCranial placodes are transient ectodermal structures contributing to the paired sensory organs and ganglia of the vertebrate head. Placode progenitors are initially spread and intermixed within a continuous embryonic territory surrounding the anterior neural plate, the so-called pan-placodal region, which progressively breaks into distinct and compact placodal structures. The mechanisms driving the formation of these discrete placodes from the initial scattered distribution of their progenitors are poorly understood, and the implication of cell fate changes, local sorting out or massive cell movements is still a matter of debate. Here, we discuss different models that could account for placode assembly and review recent studies unraveling novel cellular and molecular aspects of this key event in the construction of the vertebrate head

Cranial placodes: Models for exploring the multi-facets of cell adhesion in epithelial rearrangement, collective migration and neuronal movements

AbstractKey to morphogenesis is the orchestration of cell movements in the embryo, which requires fine-tuned adhesive interactions between cells and their close environment. The neural crest paradigm has provided important insights into how adhesion dynamics control epithelium-to-mesenchyme transition and mesenchymal cell migration. Much less is known about cranial placodes, patches of ectodermal cells that generate essential parts of vertebrate sensory organs and ganglia. In this review, we summarise the known functions of adhesion molecules in cranial placode morphogenesis, and discuss potential novel implications of adhesive interactions in this crucial developmental process. The great repertoire of placodal cell behaviours offers new avenues for exploring the multiple roles of adhesion complexes in epithelial remodelling, collective migration and neuronal movements

La croissance axonale par étirement : un processus universel encore peu exploré

International audienceThe growth of axons is a key step in neuronal circuit assembly. The axon starts elongating with the migration of its growth cone in response to molecular signals present in the surrounding embryonic tissues. Following the formation of a synapse between the axon and the target cell, the distance which separates the cell body from the synapse continues to increase to accommodate the growth of the organism. This second phase of elongation, which is universal and crucial since it contributes to an important proportion of the final axon size, has been historically referred to as “stretch-induced axon growth”. It is indeed likely to result from a mechanical tension generated by the growth of the body, but the underlying mechanisms remain poorly characterized. This article reviews the experimental studies of this process, mainly analysed on cultured neurons so far. The recent development of in vivo imaging techniques and tools to probe and perturb mechanical forces within embryos will shed new light on this universal mode of axonal growth. This knowledge may inspire the design of novel tissue engineering strategies dedicated to brain and spinal cord repair.La croissance des axones est une étape clef dans l'assemblage des circuits neuronaux. L'axone commence par grandir grâce à la migration de son cône de croissance en réponse à des signaux moléculaires présents dans les tissus de l'embryon. Après la formation d'un contact synaptique entre l'axone et la cellule-cible, la distance qui sépare le corps cellulaire de la synapse continue d'augmenter au même rythme que la croissance de l'organisme. Cette seconde phase d'élongation, universelle et cruciale puisqu'elle est responsable d'une part importante de la taille finale des axones, a été historiquement qualifiée de croissance « par étirement ». Il est en effet vraisemblable d'imaginer qu'elle résulte d'une tension mécanique générée par la croissance des tissus de l'organisme, mais les mécanismes sous-jacents restent encore mal caractérisés. Dans cette revue, nous présentons les études expérimentales de ce processus, analysé principalement sur des neurones en culture. Le développement récent de techniques d'imagerie in vivo et d'outils de mesure et de perturbation des forces mécaniques au sein des embryons permettra de mieux comprendre ce mode universel de croissance axonale. Ces connaissances pourraient être exploitées pour développer de nouvelles stratégies d'ingénierie tissulaire dédiées à la réparation des lésions du système nerveux