Placodal origin of Brn-3-expressing cranial sensory neurons.

The Brn-3 class of POU-domain transcription factors includes three genes in mammals which have key roles in the development of specific groups of sensory neurons. Here, we have identified three avian genes which correspond to the murine genes Brn-3.0, Brn-3.1, and Brn-3.2. Using an in situ hybridization probe generic for this gene class, the earliest detectable expression of Brn-3 in the chick is at stage 15, in placodal and migrating precursors of the trigeminal ganglion. By stage 19, Brn-3.0 protein is detectable in the trigeminal and vestibulocochlear ganglia with Brn-3.0-specific antisera, and Brn-3 message expression has extended to the dorsal root ganglia. At later stages, when condensation of the trigeminal ganglion is complete, Brn-3.0-immunoreactive neurons are concentrated in the portion of the ganglion distal to the brain stem. To examine the developmental origin of the Brn-3 expressing cells, we combined lipophilic dye (DiI) labeling with in situ hybridization. DiI labeling of the placodal surface ectoderm and of premigratory neural crest cells in the neural tube reveals that all, or nearly all, of the Brn3-expressing neurons in the trigeminal ganglia are derived from the sensory placodes and not from the neural crest, and thus, that Brn-3 is an early marker of the placode-derived sensory neural lineage.