Fine structure of the liver in the larval lamprey, Petromyzon marinus L.; bile ducts and gall bladder.

The purpose of the present investigation was to describe ultrastructural characteristics of hepatic bile ducts and the gall bladder in larvae of the sea lamprey, Petromyzon marinus, using freeze--fracture replicas as well as ultrathin sections. Comparison of these structures with those of other vertebrates was necessary to provide a basis in future studies for characterization of biliary degeneration during metamorphosis. The bile ducts were composed of a simple cuboidal to columnar epithelium with the cells separated by wide lateral intercellular spaces and containing a prominent brush border. Vacuole-like intracytoplasmic cisternae formed a peripheral network within the cells and were confluent with intercellular spaces at the site of numerous pores in the lateral plasma membranes. The cells were joined apically by well developed zonulae occludentes surmounting zonulae adhaerentes. The zonulae occludentes, as observed in freeze--fracture replicas, appeared as a honeycomb-like meshwork. Frequent gaps in P-face ridges suggested a 'leaky' epithelium. The cytoplasm of bile duct cells contained few organelles except for large numbers of mitochondria; many microfilaments were present. The ultrastructural features of those cells reflected an epithelium specialized for absorption and transport and they were similar to cells of the bile ducts in other vertebrates. The general organization of epithelium in the gall bladder resembled that of bile ducts, but intercellular spaces were narrower, peripheral pores and cisternae were absent laterally, and the brush border was less extensively developed at the apical surface. The cytoplasm also contained large pools of glycogen and numerous microfilaments were situated in the apical ectoplasm. The overall appearance of the gall bladder of lampreys suggested that the epithelium was less specialized compared to the water transporting organs of other vertebrate species.