Cytodifferentiation of hair cells during the development of a basal chordate.

Tunicates are unique animals for studying the origin and evolution of vertebrates because they are considered vertebrates' closest living relatives and share the vertebrate body plan and many specific features. Both possess neural placodes, transient thickenings of the cranial ectoderm that give rise to various types of sensory cells, including axonless secondary mechanoreceptors. In vertebrates, these are represented by the hair cells of the inner ear and the lateral line, which have an apical apparatus typically bearing cilia and stereovilli. In tunicates, they are found in the coronal organ, which is a mechanoreceptor located at the base of the oral siphon along the border of the velum and tentacles and is formed of cells bearing a row of cilia and short microvilli. The coronal organ represents the best candidate homolog for the vertebrate lateral line. To further understand the evolution of secondary sensory cells, we analysed the development and cytodifferentiation of coronal cells in the tunicate ascidian Ciona intestinalis for the first time. Here, coronal sensory cells can be identified as early as larval metamorphosis, before tentacles form, as cells with short cilia and microvilli. Sensory cells gradually differentiate, acquiring hair cell features with microvilli containing actin and myosin VIIa; in the meantime, the associated supporting cells develop. The coronal organ grows throughout the animal's lifespan, accompanying the growth of the tentacle crown. Anti-phospho Histone H3 immunostaining indicates that both hair cells and supporting cells can proliferate. This finding contributes to the understanding of the evolution of secondary sensory cells, suggesting that both ancestral cell types were able to proliferate and that this property was progressively restricted to supporting cells in vertebrates and definitively lost in mammals.