Morphological changes in the liver of the sea lamprey, Petromyzon marinus L., during metamorphosis. II. Canalicular degeneration and transformation of the hepatocytes.

Degeneration of all bile canaliculi takes place in the liver of the sea lamprey, Petromyzon marinus, during metamorphosis. Disintegration of microvilli is observed during earlier stages, and membranous debris ultimately accumulates within the canalicular lumina. Complete occlusion of the lumina and disorganization of junctional complexes is followed by a complete loss of the exocrine biliary pole of hepatocytes and a reorganization of these cells into solid cords. An increase in the size and number of acid phosphatase-containing cytoplasmic bodies coincides with the events of canalicular degeneration. These secondary lysosomes apparently participate in some manner in the isolation and disposal of iron and other bile constituents which no longer can be excreted in bile canaliculi. The loss of the exocrine biliary pole of hepatocytes is concomitant with vascular disturbances in the form of disordered arrangements of sinusoidal endothelial cells and an increase in the population of activated Kupffer cells involved in erythrophagocytosis. The significance of the shift in functional organization of the liver in adult lampreys is discussed in relation to physiological changes in this organism and to human hepatic cholestasis, for which this organism is a potentially valuable experimental model.