Ultrastructural characterization of the olfactory organ in glass eels,Anguilla anguilla(Osteichthyes, Anguilliformes)

Olfactory Rod Cells: A Rare Cell Type in the Larval Zebrafish Olfactory Epithelium With a Large Actin-Rich Apical Projection

We report the presence of a rare cell type, the olfactory rod cell, in the developing zebrafish olfactory epithelium. These cells each bear a single actin-rich rod-like apical projection extending 5–10 μm from the epithelial surface. Live imaging with a ubiquitous Lifeact-RFP label indicates that the olfactory rods can oscillate. Olfactory rods arise within a few hours of the olfactory pit opening, increase in numbers and size during larval stages, and can develop in the absence of olfactory cilia. Olfactory rod cells differ in morphology from the known classes of olfactory sensory neuron, but express reporters driven by neuronal promoters. A sub-population of olfactory rod cells expresses a Lifeact-mRFPruby transgene driven by the sox10 promoter. Mosaic expression of this transgene reveals that olfactory rod cells have rounded cell bodies located apically in the olfactory epithelium and have no detectable axon. We offer speculation on the possible function of these cells in the Discussion.

A comprehensive hypothesis on the migration of European glass eels (Anguilla anguilla)

The European eel (Anguilla anguilla) is a catadromous fish that spawns in the Sargasso Sea. As larvae, eels cross the Atlantic Ocean and reach the continental slope of Europe, where they metamorphose into post-larval glass eels. These reach the continent, where some enter fresh water, some remain in marine waters, and others move between fresh and marine waters. After 5-25 years, as adult silver eels, they migrate back from fresh water to the Sargasso Sea to spawn and die. The glass eel stage is a critical step during which the eels cross the continental shelf and recruit to estuaries, where they facultatively transition to fresh water. Extensive research has been conducted to understand the behavioural mechanisms and environmental cues that aid and guide glass eels' migration. Glass eels follow odours and salinity gradients, they avoid light, and they change orientation and depth according to the tides. Recent work revealed that European glass eels also use Earth's magnetic field and lunar cues to orient. However, while we understand many aspects of their orientation behaviour, a unifying theory describing how glass eels migrate from the continental slope to fresh water is lacking. The goal of this review is to develop a comprehensive hypothesis on the migration of European glass eels, integrating previous knowledge on their orientation behaviour with recent findings on magnetic and celestial orientation. This review follows the journey of a hypothetical glass eel, describing the nature and the role of orientation cues involved at each step. I propose that, although glass eels have the sensory capacity to use multiple cues at any given time, their migration is based on a hierarchical succession of orientation mechanisms dictated by the physical properties of the environments that they occupy: (i) lunar and magnetic cues in pelagic water; (ii) chemical and magnetic cues in coastal areas; and (iii) odours, salinity, water current and magnetic cues in estuaries.

Ultrastructural study of the olfactory organ in adult and developing European sea bass,Dicentrarchus labrax

The olfactory organ of the European sea bass (Dicentrarchus labrax) in adults and during development has been studied by light microscopy and by transmission and scanning electron microscopy. This organ includes two cavities, each extended by an accessory sac and opening to the outside through two nostrils. It contains a rosette consisting of about forty lamellae. The olfactory epithelium is characterized by the presence of two types of receptor cells, ciliated or with microvilli, and numerous ciliated nonsensory cells. Rod cells, essentially found in the altered epithelia of farmed bass, and rodlet cells are also observed. The olfactory organ forms very early in the developmental process. Two olfactory pits holding both types of sensory receptors appear 24 h before hatching. The ciliated nonsensory cells only appear at the end of the endotrophic period, shortly before the mouth opens. Although it is rather unspectacular during the larval stage, the development of the olfactory organ is characterized at the start of the juvenile stage by three important events: the formation of the nostrils, the hollowing of the accessory sacs, and the development of the rosette. This is created by raising the floor of the cavity and forming successive folds, which are the lamellae where the sensory epithelium is found.