Immunocytochemical and ultrastructural changes in the caudal neurosecretory system of a seawater fish Boops boops L. (teleostei:sparidae) in relation to the osmotic stress.

A combined immunocytochemical and ultrastructural study of the caudal neurosecretory system of a seawater fish, Boops boops, maintained in a freshwater environment for different periods, was carried out. Fishes collected directly from the sea of the Straits of Messina were used as controls. The observations carried out on the controls indicate a discrete amount of sauvagine/urotensin I immunoreactive material recognizable in the neurosecretory cells, called "Dahlgren's cells", localized along both the ventral and lateral sides of the caudal spinal cord segments, and often also in the inner part of the urophysis. An accumulation of immunoreactive material is also observed in the densely packed terminal enlargements of the neurosecretory cells in the urophysis where they are associated with a rich vascular bed. On the contrary, the specimens transferred in a hypoosmotic milieu show an evident and sensible increase of neurosecretory granules mainly in the neurohemal endings of the urophysis. These findings were also confirmed by observations at ultrastructural level using the immunocolloidal gold complex method. According to the size and the electron density of their granules two neurosecretory cell types can be differentiated. Granules of both types can be found in the same neurosecretory cell. The osmotic stress affect the activity of the caudal system, thus supporting its involvement in osmoregulatory mechanisms.