Vol. 4, Issue 4, Part D (2018)

Abstract
Nano-sized silica particles (SiO₂NPs) that are largely released into the aquatic environment are known to associate with naturally occurring sediments or colloids to find their way into cellular compartments, thereby causing harmful effects on aquatic organisms. Among several parameters used to assess the adverse toxic effects of nanoparticles on aquatic organisms, histology is the direct and sensitive tool to determine the tissue damage. In the present study SiO₂NPs at sublethal concentration i.e., 12 mg/ L were exposed to the freshwater fish, Oreochromis mossambicus for 96 h and 60 days. After the exposure period, histological alterations in the gill, liver and brain tissues were examined and compared with that of control tissues. Treatment withdrawal was also carried out for 60 days in order to determine if the exposed nanoparticles undergo recovery after certain period of time. Nanoparticles treatment showed gross damage in gill, liver and brain tissues and the extent of severity of tissue damages were time-dependent. Gill tissue showed pathological changes such as upliftment of gill epithelium along with mucous deposition, damage and vacuolization in gill arches, aneurysm, absence and curling of secondary lamella. Major hepatic lesions includes loss of normal tissue architecture, segmentation of hepatocytes, vacuolization, spindle shaped nucleus and necrosis. Brain tissue of the treated fish showed degeneration of nerve cells, formation of vacuolization, cerebral edema, necrosis of neurofibrillar region and lesion in choroid plexus. The morphological changes caused by nanoparticles remained unchanged after the treatment withdrawal period suggesting irreparable architectural damage. The study benefits to contribute for environmental screening and monitoring programs to restrict the production and use of nanoparticles that naturally cause environmental variations.