PLACODAL ORIGIN OF BRN-3-EXPRESSING CRANIAL SENSORY NEURONS

The Brn-3 class of POU-domain transcription factors includes three gen es in mammals which have key roles in the development of specific grou ps of sensory neurons. Here, we have identified three avian genes whic h correspond to the murine genes Brn-3.0, Bm-3.1, and Bm-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 placod al and migrating precursors of the trigeminal ganglion. By stage 19, B rn-3.0 protein is detectable in the trigeminal and vestibulocochlear g anglia with Bm-3.0-specific antisera, and Brn-3 message expression has extended to the dorsal root ganglia. At later stages, when condensati on of the trigeminal ganglion is complete, Brn-3.0-immunoreactive neur ons are concentrated in the portion of the ganglion distal to the brai n stem. To examine the developmental origin of the Brn-3 expressing ce lls, we combined lipophilic dye (DiI) labeling with in situ hybridizat ion. 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 Brn-3-expressing neurons in the trigeminal ganglia are derive d from the sensory placodes and not from the neural crest, and thus, t hat Brn-3 is an early marker of the placode-derived sensory neural lin eage, (C) 1998 John Wiley & Sons, Inc.