Meeting Abstract
The presence of secondary sensory cells in tunicates was firstly signaled in 2003, when a new mechanoreceptor organ, the coronal organ, was described in the oral siphon of the ascidian Botryllus schlosseri. As tunicates are considered the sister group of vertebrates, coronal cells were immediately seen as the best candidate to address the controversial issue of hair cell evolution in chordates. Since then, the study of these cells has been active. Now, we know that the coronal organ derives from an anterior proto-placode, an ectodermal thickening expressing vertebrate placodal genes. The organ is considered a plesiomorphic feature of the taxon, as all tunicates analyzed so far possess it. Its sensory cells exhibit an apical bundle bearing cilia and microvilli (or stereovilli) and lack an axonal prolongation. Coronal cells check particle entrance into the oral siphon during filtering activity: in case of necessity, they evoke the typical “squirting reaction”, i.e. the rapid body muscle contraction that is used to eject dangerous particles from the branchial basket. Tunicate coronal cells share with vertebrate hair cells some developmental genes and neurotransmitters, the complexity of synaptic connectivity, and the susceptibility to ototoxic drugs. Some other features result specific of coronal cells, such as the absence of an ordinated pattern of stereovilli and mechanoelectrical transduction based on tip-links, or the ability to divide mitotically, rendering them a unique chordate sensory system.
