Hair cells in ascidians and the evolution of lateral line placodes

The vertebrate hair cells are ciliary highly differentiated mechanoreceptors whose name derives from the peculiar microvilli, called stereovilli, that protrude into the fluid-filled cavities of the inner ear or lateral line organs. They differ from the primary sensory cells found in most invertebrates in that they are axonless, thus being secondary sensory cells that synapse with the dendrites of neurons whose cell bodies are located in the central nervous system (CNS). Although their morphology varies in different vertebrate species, hair cells typically have a single eccentric cilium and a collar of stereovilli graded in length from one side to the other. Hair cells derive from placodes of the acustico-lateralis system that, together with the other neurogenic placodes, are generally believed to originate ontogenetically from a wide panplacodal field and are usually considered exclusive to craniates. However, recent molecular and morphological data suggest that cell populations with the properties of neurogenic placodes are also present in cephalochordates and tunicates. Extending our study now to eight species belonging to the three orders of ascidians, we find that all possess coronal organs with hair cells. All molecular and morphological data are consistent with the idea that the ancestor of chordates possessed the ability to differentiate hair cells and that the latter derive embryologically from an area having the characteristics of a neurogenic placode. We propose that the ascidian embryonic stomodeum contains a population of cells homologous to the vertebrate acustico-lateralis placodes.