Fine structure of diffused pseudobranchial neurosecretory cells associated with carotid labyrinth in an air‐breathing catfish Clarias batrachus

Abstract

Fish are known to have branchial chemoreceptors and even extrabranchial chemoreceptors to meet the challenges of aquatic environment. The pseudobranchial neurosecretory system associated with carotid labyrinth (CL) is one such example. CL – a chemosensory organ is well known in amphibians. The homologous structure also exists in fish. Clusters of neurosecretory cells, close to the CL and the first two efferent branchial arteries occur in catfish and a few other groups of teleosts. These cells belong to the pseudobranchial neurosecretory system (PNS). To reveal the ultrastructure of CL and the pseudobranchial neurosecretory cells (PNSCs), environmental scanning electron microscope (ESEM) and transmission electron microscope (TEM) investigations were made in an Asian air‐breathing catfish Clarias batrachus. Under ESEM, the PNS appeared as a mass of cells innervated by nerves and supplied by blood capillaries. The CL appeared to have a network of blood capillaries. The transmission electron microscopic investigations showed pear shaped PNSCs having different sizes of dense cored vesicles (DCVs), numerous mitochondria, nerve varicosities, indicating a secretory function of the cells. The CL shows a close association with PNSCs and smooth muscles. Although the exact function of the CL and associated PNSCs in the biology of fish is far from clear, their morphology suggests they are involved in a stress response such as to hypoxia.

The paraneuronal gill neuroendocrine system in ocellated puffer fish Leiodon cutcutia

“…Water was renewed after every 24 hr. After 10–15 days of acclimatization, the fishes were anaesthetized following a MS‐222 (Ethyl 3‐aminobenzoate methane sulphonate) protocol (Sengar et al, ) and were dissected under zoom binocular to open the palate, so as to expose the EA in the gill region and the region anterior to it (Figure ). The tissue from the region close to the first two, efferent blood vessels and the dorsal aorta connecting the two was taken out and fixed in freshly prepared Bouin's fixative overnight.…”

“…The PSNS has been located in the gill region of fishes, close to first two efferent branchial arteries (EA), the lateral dorsal aorta (LDA) connecting the two and carotid labyrinth in catfishes (Gopesh, Prakash, & Jaiswal, ; Gopesh & Srivastava, ; Gopesh & Yadav, ; Pandey, ; Sengar et al, ; Srivastava et al, ; Yadav, Sengar, Zaccone, & Gopesh, ), with the CG in clupeids like Notopterus chitala , Notopterus notopterus , (Gopesh, ; Gopesh, Sengar, & Tiwari, ; Srivastava et al, ), and with the pseudobranch in Glossogobius giuris (Gopesh, ; Srivastava et al, ), Channa punctata , Mastacembelus armatus (Devi, ; Figure ).…”