Epidermal morphology of the Antarctic notothenioid fish Trematomus bernacchii, emphasizing the secretion of glycoproteins/glycolipids

Trematomus bernacchii Boulenger, 1902 is a notothenioid fish that occurs in southern oceans around the continent of Antarctica. With a body salt content lower than that of the surrounding saltwater and having to tolerate temperatures down to about -2°C in its environment, the fish must avoid the formation of ice in and on its body. Although antifreeze glycoproteins are known to be produced by the fish, the first external barrier against ice crystal formation is the skin. However, epidermal cells have not been scrutinized as possible producers of antifreeze substances. We therefore studied epidermal cells of the head of T. bernacchii by light and electron microscopy as well as histochemistry, with emphasis on their possible glycoprotein/glycolipid content. We show that among the proteins of the head epidermis, glycoproteins/glycolipids are likely secreted, as indicated by the periodic acid-Schiff-Alcian blue-positive mucus, and Blue Nile lipid histochemical staining. These substances are produced by sparse goblet or mucous cells seen in light and transmission electron micrographs. Mucous cells are more frequently seen in small pocket regions of the epidermis, located at the tips of the scales. Our study suggests that from the epidermal pockets, mucus with possible antifreeze glycoprotein content may be distributed over the epidermal surface where it forms a slimy coat. The study supports the hypothesis of glycoprotein/glycolipid production and suggests that antifreeze glycoproteins may be part of this secretion as the first external barrier against the formation of ice crystals on and in the body of the fish.