Pharyngeal arch patterning in the absence of neural crest

Pharyngeal arches are a prominent and critical feature of the developing ve rtebrate head. They constitute a series of bulges within which musculature and skeletal elements form; importantly, these tissues derive from differen t embryonic cell types [1], Numerous studies have emphasised the role of th e cranial neural crest, from which the skeletal components derive, in patte rning the pharyngeal arches [2-4]. It has never been clear, however, whethe r all arch patterning is completely dependent on this cell type. Here, we s how that pharyngeal arch formation is not coupled to the process of crest m igration and, furthermore, that pharyngeal arches form, are regionalised an d have a sense of identity even in the absence of the neural crest. Thus, v ertebrate head morphogenesis can now be seen to be a more complex process t han was previously believed and must result from an integration of both neu ral-crest-dependent and -independent patterning mechanisms. Our results als o reflect the fact that the evolutionary origin of pharyngeal segmentation predates that of the neural crest, which is an exclusively vertebrate chara cteristic. (C) 1999 Elsevier Science Ltd. All rights reserved.