Acta Zoologica. 2018;1–7. wileyonlinelibrary.com/journal/azo | 1 © 2018 The Royal Swedish Academy of Sciences Accepted: 12 December 2017 DOI: 10.1111/azo.12242 ORIGINAL ARTICLE First demonstration of the neuroepithelial cells and their chemical code in the accessory respiratory organ and the gill of the sharptooth catfish, Clarias gariepinus: A preliminary study Giacomo Zaccone 1 | John Maina 2 | Antonino Germanà 3 | Giuseppe Montalbano 3 | Gioele Capillo 4 | Luisa Aragona 4 | Michal J. Kuciel 5 | Eugenia Rita Lauriano 4 | José M. Icardo 6 1 Department of Biomedical and Dental Sciences and Morphofunctional Imaging, Section S.A.S.T.A.S., University of Messina, Messina, Italy 2 Department of Zoology, University of Johannesburg, Johannesburg, South Africa 3 Department of Veterinary Sciences, University of Messina, Messina, Italy 4 Department of Chemical, Biological, Pharmaceutical, and Environmental Sciences, University of Messina, Messina, Italy 5 Poison Information Centre and Laboratory Analysis, Department of Toxicology and Environmental Disease, Faculty of Medicine, Jagiellonian University, Krakow, Poland 6 Department of Anatomy and Cell Biology Polıgono de Cazona, Faculty of Medicine, University of Cantabria, Santander, Spain Correspondence Giacomo Zaccone, University of Messina, Department of Biomedical and Dental Sciences, and Morphofunctional Images, Section SASTAS, Polo Universitario dell ‘Annunziata, Messina, Italy. Email: gzaccone@gmail.com Abstract Available studies that have examined O 2 sensing in fish have indicated that oxygen- sensitive neuroepithelial cells (NECs) are O 2 sensors in the gills and initiate cardi- orespiratory reflexes in aquatic vertebrates. This is the first study describing the occurrence of NECs in accessory respiratory organs in the air-breathing catfish Clarias gariepinus. Immunocytochemical stainings with specific neuronal markers such as nNOS, VAchT, 5-HT and TH have been shown to be very useful for location and distribution of these cells in the gill fans and suprabranchial chamber that take origin from the transformation of the gill tissue. But the response of these putative O 2 chemoreceptors, their role in the respiratory reflexes and their innervation await investigation. KEYWORDS accessory respiratory organ, Clarias gariepinus, neuroepithelial cells, neurotransmitter substances, nitric oxide 1 | INTRODUCTION Peripheral chemoreceptors associated with oxygen sensing are present in all the vertebrates studied to date. Oxygen- sensing cells have been comprehensively studied in fish where these cells are called neuroepithelial cells (NECs) that are found dispersed throughout the gill arches of fishes and the lung of air-breathing fishes (Jonz, Buck, Perry, Schwerte, & Zaccone, 2016; Porteus, Wright, & Milsom, 2014; Porteus et al., 2015; Zaccone, Fasulo, & Ainis,1995; Zaccone, Mauceri, & Fasulo,2006; Zaccone, Mauceri, Giannetto, Parrino, & Fasulo,2007; Zaccone et al.,2003,2017). O 2 -sensing cells were conserved among vertebrates with their location, stimulus modalities and plasticity in their reflex