A cellular basis for high levels of vitamin A in livers of polar bears (Ursus maritimus): the Ito cell

Abstract

Numerous large perisinusoidal cells containing prominent cytoplasmic lipid droplets were observed in the livers of adult and subadult polar bears (Ursus maritimus). The cells were identified as Ito cells by histological, histochemical, and ultrastructural criteria. The ratio of Ito cell to hepatocyte numbers in three bears was 1:5.6, much higher than that reported in humans. Total liver retinol and retinol palmitate in three livers correlated well with mean Ito cell area (r = 0.98). We postulate that the large number and size of Ito cells in polar bear livers is an adaptation to high dietary levels of vitamin A.