INTRODUCTION The trigeminal ganglion, which provides sensation for much of the face, has served as a good experimental system for inves- tigating the development of peripheral ganglia because of its size and accessibility (reviewed in Davies, 1988). Trigeminal sensory neurons originate from two distinct embryonic cell populations: the neural crest and the ectodermal placodes (Yntema, 1942; Hamburger, 1961; Noden, 1978; Narayanan and Narayanan, 1980; Ayer-LeLièvre and Le Douarin, 1982; D’Amico-Martel and Noden, 1980, 1983; reviewed by Noden, 1993). The cranial neural crest population exits from the dorsal neural tube and migrates under the head ectoderm. In addition to contributing neurons and glia to cranial ganglia, neural crest cells form connective tissue and bones of the face and skull (reviewed in Le Douarin, 1982). The cranial sensory placode population undergoes an epithelial-mesenchymal transition from a thickened ectodermal epithelium; these cells then invaginate, migrate, condense and differentiate into neurons, receptors and some support cells of the peripheral nervous system (reviewed by Webb and Noden, 1993). Unlike most placodes, the trigeminal placode is not mor- phologically distinct from the surrounding ectoderm. As a con- sequence, most information about its development comes from observations of placode cells during their migration and gan- gliogenesis. The trigeminal ganglion comprises two lobes: the ophthalmic lobe and the maxillomandibular lobe. Both receive contributions from ectodermal placodes and neural crest cells. In amphibians, the two lobes remain separate or fuse secon- darily during development, suggesting that they are embryo- logically and evolutionarily distinct (Hamburger, 1961; Northcutt and Brandle, 1995). Histological analyses in the mouse and chick indicate that placode cells leave the ectoder- mal layer by breaking through the basal lamina as individuals or small clusters of cells (Hamburger, 1961; D’Amico-Martel and Noden, 1983; Nichols, 1986; reviewed by Webb and Noden, 1993). They then migrate to the distal regions of the trigeminal ganglion. Trigeminal placode cells become immunoreactive for neuronal markers and exit the cell cycle early in their development (Moody et al., 1989; D’Amico- Martel and Noden, 1980). In contrast, the neural crest component of the trigeminal ganglion only expresses neuronal markers after condensation. If placode cells fail to invaginate, they can form ectopic ganglia in the surface ectoderm (Kuratani and Hirano, 1990), suggesting that migration and interaction with neural crest cells are not necessary for their neuronal differentiation. Molecular markers of undifferentiated placodal epithelia have not previously been described (Webb and Noden, 1993), making it difficult to characterize the induction and cell speci- fication of this cranial placode. In this study, we have used the transcription factor Pax-3 and the FGF receptor FREK as 4287 Development 124, 4287-4295 (1997) Printed in Great Britain © The Company of Biologists Limited 1997 DEV4919 Cranial sensory ganglia in vertebrates develop from the ectodermal placodes, the neural crest, or both. Although much is known about the neural crest contribution to cranial ganglia, relatively little is known about how placode cells form, invaginate and migrate to their targets. Here, we identify Pax-3 as a molecular marker for placode cells that contribute to the ophthalmic branch of the trigeminal ganglion and use it, in conjunction with DiI labeling of the surface ectoderm, to analyze some of the mechanisms underlying placode development. Pax-3 expression in the ophthalmic placode is observed as early as the 4-somite stage in a narrow band of ectoderm contiguous to the midbrain neural folds. Its expression broadens to a patch of ectoderm adjacent to the midbrain and the rostral hindbrain at the 8- to 10-somite stage. Invagination of the first Pax-3-positive cells begins at the 13-somite stage. Placodal invagination continues through the 35-somite stage, by which time condensation of the trigeminal ganglion has begun. To challenge the normal tissue inter- actions leading to placode formation, we ablated the cranial neural crest cells or implanted barriers between the neural tube and the ectoderm. Our results demonstrate that, although the presence of neural crest cells is not mandatory for Pax-3 expression in the forming placode, a diffusible signal from the neuroectoderm is required for induction and/or maintenance of the ophthalmic placode. Key words: placode, cranial ganglion, neural crest, Pax-3, FREK, chick SUMMARY Neural tube-ectoderm interactions are required for trigeminal placode formation Michael R. Stark 1,2 , John Sechrist 1 , Marianne Bronner-Fraser 1, * and Christophe Marcelle 1 1 Division of Biology 139-74, California Institute of Technology, Pasadena, CA 91125, USA 2 Department of Developmental and Cell Biology, University of California, Irvine, CA 92697, USA *Author for correspondence