Establishing neuronal identity in vertebrate neurogenic placodes

The trigeminal and epibranchial placodes of vertebrate embryos form differe nt types of sensory neurons. The trigeminal placodes form cutaneous sensory neurons that innervate the face and jaws, while the epibranchial placodes (geniculate, petrosal and nodose) form visceral sensory neurons that innerv ate taste buds and visceral organs. In the chick embryo, the ophthalmic tri geminal (opV) placode expresses the paired homeodomain transcription factor Pax3 from every early stages, while the epibranchial placodes express Pax2 . Here, we show that Pax3 expression in explanted opV placode ectoderm corr elates at the single cell level with neuronal specification and with commit ment to an opV fate. When opV (trigeminal) ectoderm is grafted in place of the nodose (epibranchial) placode, Pax3-expressing cells form Pax3-positive neurons on the same schedule as in the opV placode. In contrast, Pax3-nega tive cells in the grafted ectoderm are induced to express the epibranchial placode marker Pax2. and form neurons in the nodose ganglion that express t he epibranchial neuron marker Phox2a on the same schedule as host nodose ne urons. They also project neurites along central and peripheral nodose neuri te pathways and survive until well after the main period of cell death in t he nodose ganglion. The older the opV ectoderm is at the time of grafting, the more Pax3-positive cells it contains and the more committed it is to an opV fate. Our results suggest that, within the neurogenic placodes, there does not appear to be a two-step induction of 'generic' neurons followed by specification of the neuron to a particular fate. Instead, there seems to be a one-step induction in which neuronal subtype identity is coupled to ne uronal differentiation.