The shark’s solution to a fatty liver

Abstract

The liver performs an array of essential metabolic functions in vertebrates, including storage of energy in the form of lipids and glycogen. In humans, a chronic build-up of excess fat in liver cells results in a pathological condition called hepatic steatosis (fatty liver disease). Hepatic steatosis has become one of the most prevalent liver diseases in the developed world, and leads to severe injury and death. However, elasmobranch fishes have livers filled with low density oils to maintain buoyancy, and unlike mammals, experience no apparent pathological consequences of such high hepatic lipid concentrations. Aside from its hydrostatic function, little published information is available detailing the structure of the elasmobranch liver, especially in terms of lipid storage. Elasmobranchs may serve as natural models of the fatty liver condition. How do sharks handle large lipid stores? This study aimed to describe the parenchymal structure and ultrastructure of the elasmobranch liver, in comparison to those of the mammal and teleost. Numerous methods of staining and fixation were employed for light and electron microscopic visualisation of tissues. The parenchymal arrangement of shark liver was random, compared to those of rat and fish. Shark liver tissue also contained enormous amounts of fat, stored within hepatocytes in large lipid vacuoles. This mode of lipid storage bore resemblance to lipid deposition seen in fatty liver disease in other animal groups, but without evidence of an inflammatory response or any damage to components of liver cells. Pigment granules, identified as melanins, were abundantly present in elasmobranch liver samples, but absent in mammals and fish. Further research is needed to determine the mechanism by which sharks maintain healthy liver function in the presence of excessive fats, and to develop better techniques for fixation and study of oily tissues. This research built heavily upon the existing knowledge of the elasmobranch liver structure, creating a sturdy foundation for future study of this physiologically peculiar organ. Keywords: fatty liver, hepatic steatosis, elasmobranch, shark, liver, hepatocyte, melanin