Introduction

A prerequisite for studying the individualization, migration, and fate of neural crest cells is the availability of specific markers that allow this cell type to be identified throughout ontogeny. This is due to the fact that in the majority of vertebrate species the movements of embryonic cells cannot be directly followed in situ under the microscope because the embryo contains too many cells. Moreover, the migratory neural crest cells do not exhibit generally recognizable features which would allow them to be distinguished from the other embryonic cells. Teleost embryos, however, constitute a model system particularly amenable to study neural crest ontogeny since, at least in certain species, crest cells can be distinguished in living samples by direct observation. Thus, several species of fishes have been used to study neural crest development (see, e.g., Newth, 1956; Lamers et al., 1981; Langille and Hall, 1988a,b; Sadaghiani and Vielkind, 1989, 1990). In zebrafish (Brachydanio rerio), the blastomeres and their progeny are nearly transparent and the embryo has relatively few cells, making it possible to recognize them individually and to follow their migration. Moreover, in this species, crest cells have a large size and can be easily labeled, as described below, and even ablated or transplanted heterotopically (see Eisen and Weston, 1993, for a review).