Chordate mechanoreceptors and origin of vertebrate hair cells

Urochordates and Cephalochordates are simple non-vertebrate chordates, the study of which, based on comparison of homologous structures, may contribute toward understanding the origin of vertebrates and the characteristics of the common ancestor. A recently debated issue regards the presence, in non-vertebrate chordates, of cell populations homologous with cranial placodes and the neural crest, considered evolutionary innovations in vertebrates. Data are accumulating to demonstrate that both the genetic machinery involved in placode/neural crest differentiation and ectodermal homologues are found in non-vertebrate chordates. In the oral region of urochordate ascidians, we have found the coronal organ, perhaps a counterpart of the vertebrate lateral line, composed of mechanoreceptors which share, with vertebrate hair cells, a number of features supporting their possible common derivation from an ancestral type of sensory cell. These features regard comparative analysis of position, cytological characteristics (mechanoreception ability, row alignment, presence of cilia, presence of afferent and efferent synapses) and developmental gene expression. All these aspects indicate that cell populations with some characteristics of acoustico-lateral placodes also occur in non-vertebrate chordates and, in particular, that the rudiment of the oral siphon has the properties of a neurogenic placode. By extension, the last common chordate ancestor also possessed neurogenic placodes. We discuss the possibility that the hair cell is an old acquisition, the origin of which may be traced back to the chordate ancestor and which evolved independently in the different chordate lines.