Specification of pharyngeal endoderm is dependent on early signals from axial mesoderm

The development of taste buds is an autonomous property of the pharyngeal e ndoderm, and this inherent capacity is acquired by the time gastrulation is complete. These results are surprising, given the general view that taste bud development is nerve dependent, and occurs at the end of embryogenesis. The pharyngeal endoderm sits at the dorsal lip of the blastopore at the on set of gastrulation, and because this taste bud-bearing endoderm is specifi ed to make taste buds by the end of gastrulation, signals that this tissue encounters during gastrulation might be responsible for its specification. To test this idea, tissue contacts during gastrulation were manipulated sys tematically in axolotl embryos, and the subsequent ability of the pharyngea l endoderm to generate taste buds was assessed. Disruption of both putative planar and vertical signals from neurectoderm failed to prevent the differ entiation of taste buds in endoderm. However, manipulations of contact betw een presumptive pharyngeal endoderm and axial mesoderm during gastrulation indicate that signals from axial mesoderm (the notochord and prechordal mes oderm) specify the pharyngeal endoderm, conferring upon the endoderm the ab ility to autonomously differentiate taste buds. These findings further emph asize that despite the late differentiation of taste buds, the tissue-intri nsic mechanisms that generate these chemoreceptive organs are set in motion very early in embryonic development.