Abstract
Mercury (Hg) is a global pollutant that may exert its toxic effects on living organisms and is found in both aquatic and terrestrial ecosystems in three chemical forms; elemental, organic, and inorganic. The inorganic form (iHg) tends to predominantly accumulate in aquatic environments. The gill apparatus is a very dynamic organ that plays a fundamental role in gas exchange, osmoregulation, acid-base regulation, detoxification, and excretion, and the gills are the primary route of waterborne iHg entrance in fish. In the present work we investigated the morphofunctional and ultrastructural effects in Danio rerio gills after 96 h exposure to two low HgCl2 concentrations (7.7 and 38.5 µg/L). Our results clearly demonstrated that a short-term exposure to low concentrations of mercury chloride resulted in gill morphology alterations and in the modifications of both Na+/K+-ATPase and metallothioneins (MTs) expression pattern. The main morphological effects recorded in this work were represented by hyperplasia and ectopia of chloride cells (CCs), lamellar fusion, increased mucous secretion, alteration of pavement cells (PVCs), detachment of the secondary epithelium, pillar cell degeneration, degeneration, and apoptosis. Trough immunohistochemistry and real-time PCR analysis also showed a dose-related modulation of Na+/K+-ATPase and MTs.
Highlights
Mercury (Hg) is a highly toxic heavy metal that has no known vital or beneficial function for living organisms [1]
The most common ultrastructural changes observed are represented by hyperplasia and ectopia of chloride cells (CCs), lamellar fusion, increased mucous secretion, ultrastructural alteration of pavement cells (PVCs), detachment of the secondary epithelium, pillar cells degeneration, degeneration, and apoptosis
The teleost gill epithelium is characterized by the presence of chloride cells (CCs); these are large roundish cells typically distributed in the trailing edge of the filament epithelium and at the junctions between the filament and lamellae
Summary
Mercury (Hg) is a highly toxic heavy metal that has no known vital or beneficial function for living organisms [1]. As suggested by Poopal and colleagues [29], since the modulation of Na+/K+-ATPase activity precedes the appearance of gross osmoregulatory dysfunction, this enzyme can represent an early warning of pollution [29,30,31] These findings lead to the use of the Na+/K+-ATPase pump as a powerful biomarker of osmotic stress after exposure to iHg. Metallothionein (MTs) are low molecular weight proteins with a strong affinity for several metals that are typically involved in the regulation of essential metals (such as copper/Cu and zinc/Zn) and in the cellular detoxification of non-essential metals such as Hg, cadmium (Cd), and silver (Ag) [32]. To our knowledge this is the only report on the histological, ultrastructural, and functional effects induced by iHg on D. rerio gills
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