Ultrastructural study on olfactory sensory neuron in an air-breathing teleost, anabas testudineus (bloch, 1792)

Olfaction represents a crucial chemosensory modality in vertebrates, mediated by specialized olfactory sensory receptor neurons. Unlike terrestrial vertebrates, the olfactory organs of fish remain in close contact with the aquatic environment, enabling them to sense waterborne chemical cues and trigger specific behaviors. This chemosensory system comprises nostrils, nasal cavities housing sensory lamellae, olfactory nerves, bulbs and brain. Anabas testudineus (Bloch, 1792), an air-breathing teleost (Anabantiformes: Anabantidae; IUCN Red List: Least Concern) that periodically interacts with an air-water interface, representing its dual-mode lifestyle. This study aims to explore the ultrastructure of olfactory sensory receptor neurons (OSRNs) of A. testudineus using electron microscopy. The olfactory organs of A. testudineus were dissected, the rosettes were examined under a scanning electron microscope (SEM) and transmission electron microscope (TEM) after fixation in 2.5% glutaraldehyde and Karnovsky’s fixative, respectively, and subsequent histological processing. In A. testudineus, the oval-shaped olfactory rosette is distinguished into prominent sensory and non-sensory epithelium. The olfactory neuroepithelium is a ciliated and pseudostratified structure with ciliated supporting cells, bipolar OSRNs, basal cells, etc. A highly developed sensory epithelium equipped with abundant ciliated olfactory sensory neurons (CiOSNs), which appear dominant, and microvillous olfactory sensory neurons (MiOSNs), distributed within dense aggregations of ciliated non-sensory cells, reflecting the specific adaptive nature of the species concerned.