Induction of the epibranchial placodes

The cranial sensory ganglia, in contrast to those of the trunk, have a dual embryonic origin arising from both neurogenic placodes and neural crest. N eurogenic placodes are focal thickenings of ectoderm, found exclusively in the head of vertebrate embryos. These structures can be split into two grou ps based on the positions that they occupy within the embryo, dorsolateral and epibranchial, The dorsolateral placodes develop alongside the central n ervous system, while the epibranchial placodes are located close to the top of the clefts between the branchial arches. Importantly, previous studies have shown that the neurogenic placodes form under the influence of the sur rounding cranial tissues. In this paper, we have analysed the nature of the inductive signal underlying the formation of the epibranchial placodes, We find that epibranchial placodes do not require neural crest for their indu ction, but rather that it is the pharyngeal endoderm that is the source of the inductive signal. We also find that, while cranial ectoderm is competen t to respond to this inductive signal, trunk ectoderm is not. We have furth er identified the signalling molecule Bmp7 as the mediator of this inductiv e interaction. This molecule is expressed in a manner consistent with it pl aying such a role and, when added to ectoderm explants, it will promote the formation of epibranchial neuronal cells. Moreover, the Bmp7 antagonist fo llstatin will block the ability of pharyngeal endoderm to induce placodal n euronal cells, demonstrating that Bmp7 is required for this inductive inter action. This work answers the long standing question regarding the inductio n of the epibranchial placodes, and represents the first elucidation of an inductive mechanism, and a molecular effector, underlying the formation of any primary sensory neurons in higher vertebrates.