Development of the olfactory system in turbot (Psetta maxima L.) M.J. Dolda ´ n, P. Cid, L. Mantilla, E. de Miguel Villegas * Laboratory of Cell Biology, Department of Functional Biology, University of Vigo, 36200 Vigo, Spain 1. Introduction The olfactory system of vertebrates distinguishes a vast array of odorous molecules that are involved in behaviors central to survival. In fishes, odorants are usually small molecules dissolved in the surrounding water, which convey information about feeding, migration and mating. The olfactory organ of teleosts contains three different types of olfactory receptor cells (ORCs) (ciliated, microvillous and crypt cells) as well as supporting and proliferative basal cells (Hansen and Zeiske, 1998; Hansen and Finger, 2000; Laberge and Hara, 2001; Hansen and Zielinski, 2005; Hamdani and Døving, 2006, 2007; Castro et al., 2008). Axons from ORCs join the olfactory nerve and project to the olfactory bulb (OB), the primary neural processing center of odorant information in the central nervous system. The organization of the OB in teleosts includes characteristic layers containing specific bulbar cell types - mitral, granular and ruffed cells- (Kosaka and Hama, 1979, 1980; Alonso et al., 1987; Fuller and Byrd, 2005), and superficial spherical structures, termed glomeruli in which olfactory axons contact with bulbar neurons (Baier and Korsching, 1994; Byrd and Brunjes, 1995). In teleosts, as well as in other vertebrates, the olfactory organ differentiates from the olfactory placode, which further forms an olfactory pit and will give rise to all the cells forming the sensory epithelium. The OB develops as an evagination from the forebrain. Neuronal identity is achieved in the developing OB following cell division in proliferative zones and subsequent migration of postmitotic cells. Different aspects of the developing olfactory systems have been studied in teleosts. Most of these studies refer to the emergence of the placode and histogenesis of the olfactory epithelium (Zielinski and Hara, 1988; Hansen and Zeiske, 1993; Porteros et al., 1997; Whitlock and Westerfield, 2000; Yamamoto et al., 2004a,b; Hansen and Zielinski, 2005; Castro et al., 2008). Data relative to the establishment and differentiation of the olfactory projection (Wilson et al., 1990; Porteros et al., 1997; Dynes and Ngai, 1998; Li et al., 2005) as well as changes of proliferative zones (Wullimann and Puelles, 1999; Wullimann and Knipp, 2000) in the developing olfactory bulb have also been reported. The causal interrelationship between peripheral and central olfactory struc- tures in teleosts during normal development has not been fully established. Turbots (Psetta maxima L.) belong to flatfishes (order Pleur- onectiformes), which include advanced teleost species with larvae that undergo a striking metamorphosis that transforms the previously symmetric body (larval pelagic period) into flattened Journal of Chemical Neuroanatomy 41 (2011) 148–157 ARTICLE INFO Article history: Received 23 December 2010 Received in revised form 18 January 2011 Accepted 18 January 2011 Available online 1 February 2011 Keywords: Olfactory receptors Asymmetry Olfactory bulb Teleost Morphogenesis Immunohistochemistry ABSTRACT We have examined the histogenesis of the olfactory system during turbot development using histological and immunohistochemical methods. Proliferating cell nuclear antigen (PCNA) immunohis- tochemistry was used to detect dividing cells, whereas calretinin (CR) immunohistochemistry was used to distinguish some neuronal components of the olfactory system. Around hatching, the olfactory placode of embryos transforms into an olfactory pit, which enlarges progressively during development. In metamorphic turbots, the right olfactory organ moves to the tip of the head. Each olfactory chamber opens to the external medium by two nostrils and accessory nasal sacs develop during metamorphosis. The order of birth of olfactory receptor cells in the sensory epithelium follows the pattern of most teleosts: ciliated cells differentiate prior to microvillous cells in turbot larvae, and crypt cells are generated during metamorphosis. Axons of olfactory sensory neurons reach the rostral forebrain by hatching, and calretinin-immunoreactive (CR-ir) glomerular fields were apparent during the subsequent larval development. During metamorphosis olfactory bulbs become strongly distorted by head torsion and glomeruli acquire asymmetric organization. The spatio-temporal course of proliferation in the olfactory system reveals changes in the distribution of dividing cells in the sensory epithelium throughout the developmental period investigated. In the olfactory bulb, proliferative activity becomes restricted to the ventral periventricular zone in turbot larvae, as well as in metamorphic specimens. ß 2011 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +34 986 812388; fax: +34 986 812556. E-mail address: villegas@uvigo.es (E. de Miguel Villegas). Contents lists available at ScienceDirect Journal of Chemical Neuroanatomy journal homepage: www.elsevier.com/locate/jchemneu 0891-0618/$ – see front matter ß 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jchemneu.2011.01.003