Contributions of heterospecific tissue recombinations to odontogenesis.

Determining the functions of cell surface and substrate adhesion molecules during heterospecific tissue recombinations constitutes a significant problem in biology. The developing tooth organ provides a fine model to pursue this problem, especially in the context of instructive epithelial-mesenchymal interactions. The interpretation of many experimental heterospecific tissue combinations is difficult because of the complexity of the embryonic systems. According to some authors, the expressed phenotype corresponds to the genotype of the epithelium; on the contrary, other studies have demonstrated the leading role of the mesenchyme. The importance of cranial neural crest in tooth morphogenesis has been established. Lizard teeth maintain a continuous morphogenetic field throughout life (polyphyodont). These tissues can be considered as biological models and are ideally suited to study neural crest cell differentiation. Oral cavity of chick embryos show a rudiment resembling the dental lamina of amphibian, reptilian and mammalian embryos, though further odontogenic interactions between the epithelial and mesenchymal tissues are not operating. Some authors have suggested that genes involved in tooth formation, which have remained silent in birds for more than 200 million years, can be activated by appropriate signals. Chick epithelium combined with mesenchyme from mouse molar tooth produced dental structures with differentiated ameloblasts depositing enamel matrix. Quail neural crest combined with lizard dental bud showed quail cells with odontoblastic processes. Combinants (quail ectoderm-lizard papilla, and quail ectoderm-rabbit embryo papilla) showed differentiated chimeric-tooth-like structures. However, controversy persists regarding the ability of avian epithelium to express the ameloblast phenotype and to secrete enamel protein.(ABSTRACT TRUNCATED AT 250 WORDS)