Calcitonin-like immunoreactivity in the subcommissural organ–Reissner's fiber complex of some freshwater and marine teleosts

The subcommissural organ (SCO) is a highly especialised circumventricular ependymal organ covering and penetrating the posterior commissure. The secretory products of the SCO condense to form Reissner's fiber (RF). Because of its extensive secretory activity and the chemical properties of its secretion, the organ functions as similar to the neurosecretory cells. Teleosts comprised of more than 20,000 extant species that show great diversity in terms of the form, habit and habitat. Affinity of calcitonin antibodies for the SCO–RF complex was used as a histochemical tool to study the morphology of some freshwater and seawater teleosts and its potential correlate to their osmotic environment. While intense to moderate calcitonin-like immunoreactivity was seen in the cells of the SCO of majority of the freshwater species viz., common carp, catfish, eel and perch; the SCO of goldfish revealed limited immunoreactivity. Like the SCO, the RF in all species was also immunostained with antibodies against calcitonin. It appeared as a single, continuous fiber that ran from SCO into the third ventricle and extended through the aqueduct, fourth ventricle and central canal of the spinal cord. In contrast to that in the freshwater fishes, the SCO–RF complex in majority of the seawater fishes, showed no calcitonin-like immunoreactivity. The data presented in this study described the comparative histomorphology of the SCO–RF complex and suggest a possiblity that the calcitonin-like immunoreactivity in the SCO–RF complex might be a feature correlated to the osmotic environment of the fish.