Ultrastructural and developmental anatomy of the peripheral olfactory organs of Dicentrarchus labrax inhabiting Egyptian Mediterranean water.

Background: Olfaction regulates animal behavior; hence, its organs are present early in fish development. Due to their aquatic lifestyle, fish use olfaction to communicate chemical signals during gustation.

Aim: In this study, we investigated the morphogenesis and ontogeny of the European seabass's (Dicentrarchus labrax, Linnaeus, 1758) olfactory organ.

Methods: Scanning electron microscopy was used to study the development of these organs in fries, fingerlings,juveniles, and adults.

Results: Both placodes were thickened and invaginated, generating simple olfactory pits with undifferentiated cells. The non-sensory and sensory olfactory cells (OC) emerged after the first day of hatching. In addition to occasional mucous goblet cells, ciliated receptor cells are more common than microvillous receptor cells. Long, thin, disordered kinocilium and small microvilli of ciliated and microvillous supporting cells cover some of the pit floor. The SEC- lined pit bottom deepens 45 days after hatching. Fries develop tube-shaped anterior incurrents and broader posterior excurrent nostrils by the 75th day post-hatching, along with a broad epidermal nasal bridge. In this stage and after fingerling formation at 90 days post-hatching, the bottom olfactory epithelium lining each chamber forms a multilamellar rosette of lamellae that radiates equally from the median raphe. Fully mature European seabass have 38-40 lamellae. The pseudostratified epithelia surrounding each lamella are sensory and nonsensory. The first one, scattered in islets between the second, has receptor (ciliated, microvillous, and rod-tipped) and supporting (ciliated and microvillar) cells, whereas the nonsensory epithelium has ciliated and microvillar supporting cells and masses of superficial epidermal cells. Goblet mucous cells and basal stem cells on the epithelium's basal lamina are abundant in nonsensory areas but rare in sensory parts.

Conclusion: These findings underscore the functional specialization of OCs, reflecting the organ's pivotal role in chemical communication and environmental adaptation throughout the European seabass' development.