Spatial organization of the epithelium and the role of neural crest cells in the initiation of the mammalian tooth germ.

Teeth develop from composite organ rudiments that are formed through the interaction of oral epithelium and mesenchyme of the first branchial arch; cells of the former differentiate into enamel-secreting ameloblasts whereas those of the latter differentiate into dentine-secreting odontoblasts. Experimental analysis of odontogenic tissue interactions in mammalian embryos has focused on the late developmental stages of morphogenesis and cytodifferentiation; little is known about initial pattern-forming events, during which presumptive tooth-forming cells are specified and the sites of tooth initiation become established. It requires to be shown, for example, whether the mesenchymal cells of mammalian teeth are derived, like those of amphibians, from the cranial neural crest, and if so, whether these form a specified subpopulation in the neural folds. Alternatively, are they specified after migration into the mandibular arch, possibly by interaction with the oral epithelium? The developmental potentials of mouse embryo premigratory cranial neural crest cells (CNC - explanted from the caudal mesencephalic and rostral metencephalic neural folds) have been studied in intraocular homograft recombinations with various regions of embryonic surface ectoderm. Cartilage, bone and neural tissue developed in all combinations of CNC and epithelium. Teeth formed in combinations of CNC with mandibular arch epithelium but not in combinations of CNC with limb bud epithelium. Teeth also formed in combinations of mandibular arch epithelium with neural crest explanted from the trunk level. These results indicate that mammalian neural crest has an odontogenic potential but that this is not restricted to the crest of presumptive tooth-forming levels. Normal migration appears not to be a prerequisite for expression of odontogenic potential but this does require an interaction with region-specific epithelium. It is reasonable to infer that during normal development the neural crest that enters the mandibular arch is odontogenically unspecified before or during migration and that the oral epithelium is the earliest known site of tooth pattern.