Electron microscope study of vertebrate liver innervation.

Liver fine structure was studied in various groups of vertebrates to reveal intrahepatic nerves. Nerve fibers were found in the connective tissue of the liver in all mammals, birds, and reptiles studied (Japanese monkey, crab-eating monkey, rabbit, guinea pig, rat, golden hamster, pigeon, Japanese quail, and turtle, Pseudemys scripta). Nerve fibers also made direct contact with hepatocytes in these animals except for the rat and the golden hamster. Intrahepatic nerves were rare or absent in amphibians (Rana catesbeiana and Cynops pyrrhogaster pyrrhogaster) and fishes (Anguilla japonica and Misgurnus anguillicaudatus). The livers of mammals and birds consisted of hepatic lobules and interlobular connective tissue carrying a portal triad. The liver of lower vertebrates was a simple mass of hepatic cell cords and contained relatively small amounts of connective tissue. The increased number of intrahepatic nerves appears to be correlated with the development of higher organization of liver structure and a concomitant increase in the amount of connective tissue.