Structural organization of the olfactory organ in an amphihaline migratory fish Hilsa, Tenualosa ilisha.

The histological as well as ultramicroscopic structures of olfactory system of an amphihaline migratory fish hilsa Tenualosa ilisha, were studied. The sexually matured riverine fish were collected from a common breeding habitat-the Hooghly, a tributary of river Ganga, West Bengal, India. This study revealed that the riverine hilsa has larger olfactory bulb compared to marine hilsa with the olfactory lobes well exposed through nostrils. The olfactory lamellae (OL) are 40-45 in number and posses three distinct layers of sensory cells across each lamellae, namely, outer receptor cells (RC), middle sensory cells, and inner basal cells (BC). Besides the above arrangement, the sensory part of olfactory epithelium (OE) also bears rich microvillous cells exposed to the surface of the OE. The sensory and non-sensory surfaces on OL are distinguishable, with clear dendritic cells on sensory epithelium and solitary chemosensory cells on non sensory OE. Abundance of both types of cells in the OE is an indication of its chemoattraction ability towards molecules of amino acid origin. The feature of having abundant, dense, and large dendritic knobs on the surface of OE describes resemblance to the typical morphology of the chemosensory septal organs neuron. The expression of four G protein subunits, like G alpha s/olf, G alpha q, G alpha o, and G alpha i-3 in OE indicate that its olfaction is a functional attributes of two olfactory systems, namely main olfactory system and Vomaronasal Olfactory System. Expression of ACIII and PLC beta 2 in OE further confirms two signaling pathways involved in odorant reception in hilsa. Research Highlights The olfactory bulb in the amphihaline migratory fish hilsa is big in size, with 40-45 lamellae. Its sensory areas showed multilayered cellular features with prominent sensory as well as microvillous cells, whereas non-sensory area possesses solitary chemosensory cells. The expression of four G protein subunits, G alpha s/olf, G alpha q, G alpha o, and G alpha i-3 in olfactory epithelium indicates that its olfaction is a functional attributes of two olfactory systems, namely main olfactory system and vomaronasal olfactory system.