Fine structure of the liver in the larval lamprey, Petromyzon marinus L.; hepatocytes and sinusoids

The ultrastructure of hepatocytes, bile canaliculi, and hepatic sinusoids of the larval lamprey, Petromyzon marinus, was examined using thin-sectioned and freeze-fractured tissues. The liver is a "tubular gland" with hepatocytes arranged in a tubular fashion around large bile canaliculi. Hepatocytes are roughly conical in shape, with their tapered apices facing a bile canalicular lumen. They possess extensive rough and smooth endoplasmic reticulum, a well-developed Golgi complex, abundant mitochondria, and varying numbers of large secondary lysosomes. Both secondary lysosomes and the Golgi complex are concentrated in the apical or peribiliary cytoplasm, indicating a possible role in bile secretion. The apical surfaces of the hepatocytes bear numerous elongate microvilli and occasional cilia, which project into the bile canaliculi. The hepatocytes are joined, apically, by junctional complexes composed of zonulae occludentes and adhaerentes. In freeze-fracture, the zonulae occludentes are of variable apicobasal depth and consist of honeycomb-like meshworks of fibrils. Spaces of variable width frequently appear in the P-face grooves, indicating that the zonulae occludentes are "leaky." Numerous communicating (gap) junctions join the hepatocytes laterally. Varying numbers of lateral microvilli project into the intercellular spaces and, basally, the plasma membrane is deeply infolded, resulting in the formation of apparently interdigitating basal processes resting upon a thin basal lamina. Sinusoids are composed of both a heavily-fenestrated, continuous endothelium, and phagocytic reticulo-endothelial (Kupffer) cells. Depsite the difference in arrangement of their hepatocytes, the mammalian and lamprey livers show similar ultrastructural features.