Light and electron microscope observations on pathological changes in the gills of the marine fish Lycodes lavalaei Vladykov and Tremblay associated with the proliferation of an unidentified cell

Abstract. Pale and swollen gills were observed in 23 of 240 Laval's eelpouts, Lycodes lavalaei Vladykov and Tremblay (Perciformes), taken from several locations off the coasts of Newfoundland and southern Labrador. Light microscopic study of histological sections revealed clusters of unidentified cells ('X-cells') in the basal region of secondary gill lamellae and extending along the latter, between the epithelium and lamellar capillaries. In the electron microscope two types of X-cells were enclosed in a network of cytoplasmic processes from dense 'envelope' cells. Both types of X-cells were bound by a densely-coated limiting membrane and had nuclei with large, dense nucleoli, numerous mitochondria with short vesicular cristae, lipid inclusions, abundant smooth endoplasmic reticulum and small dense inclusions. The Type II X-cells were larger, contained a much greater concentration of lipid and small dense inclusions, and an additional inclusion with a dense core surrounded by a light halo. The X-cells are compared with strikingly similar cells described from epidermal and pseudobranch tumours of marine fish. Although transformation of a host cell by virus or possibly pollution has been suggested, neither the nature nor the origin of the X-cells can be determined.