Comparative study on a unique architecture of the brook lamprey liver and that of the hagfish and banded houndshark liver.

Although the liver of the lamprey, a group of cyclostomes that diverged the earliest among vertebrates, has abundant bile ducts in the larval stage, which degenerate during metamorphosis, there is no comparative study on its architecture with other early diverged vertebrates in terms of the morphological evolution of vertebrate livers. The present study was undertaken to compare the characteristics of the brook lamprey liver with those of the hagfish and banded houndshark, which have the portal triad type liver architecture, and to discuss its evolution. Although the liver of the brook lamprey had two-cell cords of hepatocytes lined by sinusoids in the ammocoetes larval stage, intrahepatic bile ducts around portal veins penetrated into the liver parenchyma with convolution and gradual reduction in diameter. They also faced dilated sinusoids. The epithelial cells had characteristic intercellular spaces. These characteristics were distinct from those of bile ducts in the hagfish and banded houndshark livers. Although the liver architectures of the hagfish and banded houndshark were similar, the latter penetrated the intrahepatic bile ducts more deeply along the portal veins than the former, in which intrahepatic bile ducts were restricted near the hilum. After metamorphosis, bile ducts degenerated in brook lampreys. These data indicate that the liver architecture of the ammocoetes larva is unique in the parenchymal distribution of bile ducts, their sinusoidal facing, and morphology among extant vertebrates. The periportal distribution of intrahepatic biliary structures may have been established prior to the divergence of the cyclostomes and gnathostomes.