Development of the Medaka Liver: An Ultrastructural and Morphometric Analysis.

Abstract

The ultrastructure of the liver of medaka (Oryzias latipes) was analyzed at particular stages of development to establish a base-line for investigations concerning carcinogenicity of hepatic tissue. Specifically, hepatic cell types were characterized during the first, sixth, and thirteenth weeks of development. Ten fish from each stage were randomly selected and sacrificed; seven were processed for transmission electron microscopy and three for light microscopy.

Cell types observed in medaka liver included hepatocytes, sinusoidal endothelial lining cells, fat storing cells of Ito, bile preductular lining cells, and columnar epithelial cells lining bile ducts. Ultrastructural changes associated with normal liver development were evident in hepatocytes. Increases in lipid content, cytoskeletal components, organelles asociated with secretion, and large decreases in glycogen became evident as liver development progressed. Sinusoidal lining cells were characterized by fenestrated cytoplasmic extensions at all stages of development. Bile preductular lining cells contributed few microvilli to bile ductules and displayed significant increases in the concentration of microfilaments with age. Microvilli were present on the luminal surfaces of columnar epithelial cells of the bile duct. Ito cells, in most instances, were laden with lipid droplets and microfilaments regardless of age. Statistical analysis of morphometric data indicated no significant difference in nuclear to cytoplasm ratios of hepatocytes during development. The number of bile preductular lining cells progressively increased as the liver developed. The volume of lipid droplets increased significantly with age.