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Variability of hair cells in the coronal organ of ascidians (Chordata, Tunicata)

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Abstract:

The tunicate ascidians are nonvertebrate chordates that possess mechanoreceptor cells in the coronal organ in the oral siphon, which monitor the incoming water flow. Like vertebrate hair cells, the mechanoreceptor-coronal cells are secondary sensory (axonless) cells accompanied by supporting cells and they exhibit morphological diversities of apical specialisations: they are multiciliate in ascidians of the order Enterogona, whereas they are more complex and possess one or two cilia accompanied by stereovilli, also graded in length, in ascidians of the order Pleurogona. In morphology, embryonic origin, and arrangement, coronal sensory cells closely resemble vertebrate hair cells. We describe here the coronal organs of five ascidians (Pyura haustor (Stimpson, 1864), Pyura stolonifera (Heller, 1878), Styela gibbsii (Stimpson, 1864), Styela montereyensis (Dall, 1872), and Polyandrocarpa zorritensis (Van Name, 1931)), belonging to Pleurogona, also comprising species of one family (Pyuridae), not yet considered, and thus completing our overview of the order. Each species possesses at least two kinds of secondary sensory cells, some of them characterized by stereovilli graded in length. In some species, the coronal sensory cells exhibit secretory activity; in P. haustor, a mitotic sensory cell has also been found. We compare the coronal organ in both ascidians and with other chordate sensory organs formed of secondary sensory cells, and discuss their possible homologies.

Les ascidies (tuniciers) sont des chordés non vertébrés qui possèdent des cellules mécanoréceptrices dans l’organe coronal de leur siphon oral, qui contrôlent l’influx d’eau. Comme les cellules ciliées des vertébrés, les cellules mécanoréceptrices-coronales sont des cellules sensorielles secondaires (sans axone) accompagnées de cellules de soutien et elles possèdent une diversité morphologique de spécialisations apicales : elles sont pluriciliées chez les ascidies de l’ordre Enterogona, alors qu’elles sont plus complexes, avec un ou deux cils accompagnés de stéréovillosités, arrangées par taille, chez les ascidies de l’ordre Pleurogona. Par leur morphologie, leur origine embryonnaire et leur arrangement, les cellules sensorielles coronales ressemblent de près aux cellules ciliées des vertébrés. Nous décrivons ici l’organe coronal de cinq ascidies (Pyura haustor (Stimpson, 1864), Pyura stolonifera (Heller, 1878), Styela gibbsii (Stimpson, 1864), Styela montereyensis (Dall, 1872) et Polyandrocarpa zorritensis (Van Name, 1931)) appartenant aux Pleurogona, y compris d’espèces d’une famille (Pyuridae) non encore étudiée, ce qui complète notre présentation synoptique de l’ordre. Chaque espèce possède au moins deux types de cellules sensorielles secondaires, certaines d’entre elles caractérisées par des stéréovillosités arrangées par taille. Chez certaines espèces, les cellules sensorielles coronales ont une activité sécrétrice; P. haustor possède aussi un type de cellule sensorielle mitotique. Nous comparons les organes coronaux tant chez les ascidies qu’avec les autres organes sensoriels de chordés formés de cellules sensorielles secondaires et nous discutons des homologies possibles.

Document Type: Research Article

Publication date: June 1, 2010

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