Morphology of the green livers in upstream migrants of Petromyzon marinus L.

A morphological comparison was made of the green livers of male and female lampreys (Petromyzon marinus L.) collected during the upstream (prespawning) migration. Light and electron microscope histochemistry for iron, and both thin sections and freeze-fracture replicas in the electron microscope, revealed some sexual dimorphism in these livers. Ferric iron is much more abundant in the liver of females and is present in the cytoplasmic matrix, in dense bodies, and in vacuoles of hepatocytes. The numerous vacuoles of females may be the deposition site of biliverdin and other bile components that would account for the darker green coloration of the liver compared to males. Hepatocytes in females are also characterized by prominent rough endoplasmic reticulum and Golgi apparatus that reflect the involvement of the cells in vitellogenesis. The presence of numerous lipid droplets in the hepatocytes of males indicates that the liver is an important storage site for fat. The lipid droplets are associated with electron-dense deposits of unknown nature. Large gap junctions typify the parenchymal cells of both male and female livers. Perisinusoidal and sinusoidal cells are similar to those in the nonparenchymal region in other vertebrate livers, namely, endothelial and Kupffer cells, lipocytes (Ito), and some granulated cells. The relationship of lipocytes to fibrous tissue and fibrogenesis is discussed.     

Deposition of amyloid in the liver of hamsters: an enzyme-histochemical and electron-microscopical study.

In induced amyloidosis the amyloid was first deposited in the portal areas, then in the centre of the lobule and disseminated through it in the spaces of Disse. No intracellular amyloid was found in the predeposit phase. In the hepatic lobule both mononuclear phagocytes and hepatocytes showed a topographic relationship to the first deposits of amyloid. Some macrophages showed invaginations or vacuoles containing amyloid fibrils. Between the microvilli of hepatocytes, parallel amyloid fibrils occurred. Between the amyloid fibrils were 30-50 nm membrane-bounded spherical particles which may have been lipoprotein aggregates. There was no large-scale phagocytosis of amyloid during the induction period or after survival without casein treatment up to 18 weeks. Lysosomal enzyme activity was seen in the deposits of extracellular amyloid.     

Ultrastructural changes during cholestasis induced by chlorpromazine in the isolated perfused rat liver.

Addition of cholestatic doses of chlorpromazine-HC1 to the perfusate of isolated rat livers produces widespread changes in hepatocyte membrane structure. These findings include a marked increase in intrasinusoidal cytoplasmic bullae, appearance of intracellular vacuoles within hepatocytes at both sinusoidal and biliary poles, dilation of bile canaliculi and evagination of canalicular diverticuli, and the formation of myeloid bodies within hepatocytes. These findings obtained in the bile acid depleted perfused liver may result from physiochemical interactions between chlorpromazine or its metabolites and lipid-protein components of cell membranes, consistent with chlorpromazine's properties as a cationic detergent. They occur independently of the vasoconstrictive effects of chlorpromazine and suggest that chlorpromazine may produce cholestasis by altering hepatocyte membrane function.     

Fine needle aspiration biopsy of hepatocellular carcinoma. Some unusual features.

The cytologic features of fine needle aspiration smears from 28 hepatocellular carcinomas (HCC) were reviewed. All aspirations except one were guided. There were 14 well-, 11 moderately and 3 poorly differentiated HCC. The better-differentiated HCC were characterized by similarity of the tumor cells to hepatocytes (83%), cohesive cell clusters with a trabecular arrangement (72%) and presence of sinusoidal endothelial cells (66%). Other features included bile production (38%), atypical hepatocytic naked nuclei (52%), acinar formation (31%), intracytoplasmic vacuoles (14%) and abnormal vascular patterns (14%). Poorly differentiated HCC showed dyshesive pleomorphic cells. Unusual cytologic features from a well-differentiated HCC with fatty change and an HCC with a prominent acinar component are described. The identification of fatty change in dissociated well-differentiated hepatocytes or cytologic features suggestive of an adenocarcinoma do not preclude the diagnosis of HCC. The usefulness of cell blocks is emphasized.     

Knockdown of tight junction protein claudin-2 prevents bile canalicular formation in WIF-B9 cells

The polarization of hepatocytes involves formation of functionally distinct sinusoidal (basolateral) and bile canalicular (apical) plasma membrane domains that are separated by tight junctions. Although various molecular mechanisms and signaling cascades including polarity complex proteins may contribute to bile canalicular formation in hepatocytes, the role of tight junction proteins in bile canalicular formation remains unclear. To investigate the role of the integral tight junction protein claudin-2 in bile canalicular formation, we depleted claudin-2 expression by siRNA in the polarized hepatic cell line WIF-B9 after treatment with or without phenobarbital. When WIF-B9 cells were treated with phenobarbital, claudin-2 expression and tight junction strands were markedly increased together with induction of canalicular formation with a biliary secretion function. Knockdown of claudin-2 prevented bile canalicular formation after treatment with or without phenobarbital. Furthermore, knockdown of claudin-2 caused a change from a hepatic polarized phenotype to a simple polarized phenotype, together with upregulation of pLKB1, pMAPK, pAkt and pp38 MAPK, but not pMLC, PTEN or cdc42, and an increase of intracellular vacuoles, which were present before bile canalicular formation. These results suggest that claudin-2 may affect not only the bile canalicular seal but also bile canalicular formation.     

Immortalized human hepatocytes as a tool for the study of hepatocytic (de-)differentiation

Primary human hepatocytes were immortalized by stable transfection with a recombinant plasmid containing the early region of simian virus (SV) 40. The cells were cultured in serum-free, hormonally defined medium during the immortalization procedure. Foci of dividing cells were seen after 3 months. Albumin- and fibrinogen-secreting cells were selected and cloned by limiting dilution to obtain homologous cell populations. The established IHH (immortalized human hepatocyte) cell lines were evaluated for their usefulness in studying the regulation of cell growth and of certain differentiated hepatocyte functions.IHH cells retain several differentiated features of normal hepatocytes. They display albumin secretion at a level comparable to cultured primary human hepatocytes (30 µg albumin/ml per day). A portion of the IHH cells are polarized, forming bile canaliculi-like vacuoles where exogeneous organic anions accumulate. The multidrug resistance (MDR) P-glycoprotein, known to be localized at the canalicular membrane, is also present in these vacuoles. The polarized features allowed the use of IHH cells for the study of localization of the newly characterized multidrug resistance protein MRP1. The homologues of MRP were found in hepatocytes, MRP1 and MRP2 (cMOAT), both functioning in ATP-dependent excretion of anionic conjugates. In differentiated hepatocytes, MRP1 expression is extremely low. In contrast, MRP1 is highly expressed in proliferating IHH cells, where it is localized in lateral membranes. A highly differentiated feature of short-term cultured primary hepatocytes which is not detectable in IHH cells is active uptake of the bile salt taurocholate. Furthermore, IHH cells secrete triglyceride (TG)-rich lipoproteins, apolipoprotein B (0.6 µg/ml per day), and apolipoprotein A-I (1 µg/ml per day). However, they secrete apoB-containing TG-rich lipoproteins mainly in the LDL density range, while short-term cultured primary hepatocytes mainly secrete TG-rich lipoproteins in the VLDL density range.In conclusion, functions that are rapidly lost in short-term hepatocyte cultures are, in general, not displayed by IHH cells. Immortalized human hepatocytes provide a valuable tool for studying the regulation of hepatocyte proliferation-related phenomena.     

Internalization of erythrocytes into liver parenchymal cells in naturally hibernating frogs (Rana esculenta L.)

Mature and intact red blood cells were found in hepatocytes of frogs during natural underground hibernation. No signs of erythrophagocytosis, e.g., separating membranes between erythrocyte and hepatocyte cytoplasm, and lysosomes, were observed. Red blood cells probably penetrated into hepatocytes by ameboid-like mechanisms, which can be deduced by cytoplasmic protrusions and invaginations. Most of the hepatocytes had large amounts of stored glycogen and few organelles, often segregated in condensed areas. The narrowed bile canalicular lumens without lysosomes and exocytotic vacuoles around them and the reduction of the Disse spaces indicate a low metabolic activity of liver during natural hibernation. The dramatic accumulation of red cells in the hepatocytes of hibernating frogs could share similarities with the phenomenon of internalization of leucocytes into epithelial cells of some vertebrate's tissues via emperipolesis, the mechanisms of which are not well understood.     

Studies on vinblastine-induced autophagocytosis in mouse liver. III. A quantitative study

The microtubule inhibitor vinblastine (25 mg/kg, i.p.) induces autophagocytosis in mouse hepatocytes. The formation of autophagic vacuoles, their contents, and other cellular changes after vinblastine injection in hepatocytes, were studied by light and electron microscopic morphometric analysis. The volume density of autophagic vacuoles increased significantly during the experimental period (24 h). This increase was due to the significant increase in their number, which was approximately 5-fold 4 h, 12 h and 24 h after vinblastine injection. The mean volume of the autophagic vacuoles increased significantly 1 h after vinblastine injection, at which time the formation of new autophagic vacuoles was at its greatest. There was an accumulation of single membrane-limited, obviously older autophagic vacuoles in the cytoplasm. Their volume density was at its maximum 12 h after injection, suggesting a retarded turnover of autophagic vacuoles. The segregation of cytoplasmic components into autophagic vacuoles may not be selective after vinblastine injection. The injurious effects of vinblastine were evident both in light and electron microscopic studies. In the parenchymal cells the Golgi cisternae were dilated and disorganized and the volume density of the Golgi apparatus was significantly decreased 12 h after vinblastine injection. The volume density of lysosomes was increased during the 12 h after vinblastine injection. Vesicles containing very low density lipoprotein particles accumulated in the cytoplasm so that their volume density was significantly increased during the entire experimental period. Vinblastine apparently interfered with the transport and secretion of the very low density lipoproteins from the parenchymal cells.     

Unbuffered osmium staining of cell organelles: alterations induced by cell injury

We have studied the localization of osmium reduction products to investigate the functional state of organelles as well as organelle interrelationships during cell injury. In normal hepatocytes osmium deposits of variable intensity are seen in nuclear envelope, endoplasmic reticulum. Golgi cisternae and vesicles and lysosomes. Buffering of osmium with s- collidine (pH 7.4) prevents the deposition of osmium. Reversible (30 min) and irreversible (60 min) ischemia without reflow causes no change in the pattern of osmium deposition. Irreversible ischemia followed by reflow causes decreased staining of endoplasmic reticulum (ER) and redistribution of the osmium deposits through the cytoplasm. Reversibly injured pancreatic acinar cells in cultured explants manifest a similar loss of osmium staining in the endoplasmic reticulum cisternae. The administration of antimicrotubule drugs induces an accentuation of osmium staining in localized cisternal elements of hepatocytes. These heavily stained cisternae appear to give rise to the bounding membranes of drug-induced autophagic vacuoles. Cytoplasmic organelles sequestered inside the autophagic vacuoles acquire intense staining when they begin to undergo degradation. In homogenized liver tissue all the subcellular organelles show osmium deposits. The deposits are preferentially localized along the organelle membranes. In particular the dense deposits in the ER lumen are not seen in the subcellular fractions. Phospholipase A2 (3 units/mg protein) enhances the deposition of osmium in the lumen of microsomal vesicles, whereas the presence of detergent has no such effect. Addition of EDTA to the homogenizing medium enhances the ultrastructural preservation of the subcellular fractions but has little effect on the deposition of osmium. OsO4 deposition occurs at acid pH and the intensity and pattern of the stain can be modified in vivo and in vitro. Osmium tetroxide deposition is induced at sites of membrane transformation (autophagic vacuoles) and degradation (lysosomes). Calcium influx and phospholipase activation (ischemia, tissue homogenization, phospholipase addition) enhance osmium deposition and/or influence the localization of the staining pattern.     

Microtubule disruption interferes with the structural and functional integrity of the apical pole in primary cultures of rat hepatocytes.

The effect of microtubule disruption on the development and maintenance of cell polarity was studied in rat hepatocytes cultured as primary monolayers in the presence of colchicine or nocodazole. Addition of colchicine immediately after plating did not inhibit the generation of bile canaliculi (the apical pole) after 1 day of culture, as judged by electron microscopic examination, and did not allow penetration of Ruthenium Red through the tight junctions. However, the bile canaliculi developed in the presence of colchicine or nocodazole were not fully normal since they were not able to concentrate fluorescein diacetate in their lumina, and did not enrich with proteins of the apical plasma membrane domain, as control cells did. When the drugs were added after 1 or 2 days of culture, the new bile canaliculi appeared to be unaffected when examined by electron microscopy, but many of them did not concentrate fluorescein and were not enriched with apical membrane proteins within 4 to 24 h after drug addition. Whenever the drugs were added, the proteins that would normally concentrate on the membrane of the bile canaliculi accumulated intracellularly in endocytic vesicles after 2 to 4 h of drug treatment, and in vacuoles resembling lysosomes when the drugs were maintained for 24 h or more. These results show that microtubule disruption does not inhibit the structural reconstitution of bile canaliculi, but impairs their normal function and the transport of proteins of the apical plasma membrane domain.     

Immunocytochemical localization of delta 3, delta 2-enoyl-CoA isomerase in rat liver. The effects of di-(2-ethylhexyl)phthalate, a peroxisome proliferator

Immunocytochemical localization of delta 3, delta 2-enoyl-CoA isomerase (isomerase) was investigated in rat liver. Livers of di-(2-ethylhexyl)phthalate (DEHP)-treated or untreated rats were perfusion-fixed and embedded in Epon or Lowicryl K4M. By light microscopy, reaction deposits for the enzyme were present in the cytoplasmic granules of hepatocytes and interlobular bile duct epithelium. Weak staining was noted in sinus-lining cells. After administration of DEHP, the granular staining of the hepatocytes was markedly enhanced, whereas the staining reaction of the sinus-lining cells decreased. The isomerase staining pattern was quite similar to that of long-chain acyl-CoA dehydrogenase (a mitochondrial marker), but different from that of catalase (a peroxisomal marker). Under electron microscopy, gold particles for isomerase were seen to be confined mainly to mitochondria of the hepatocytes, the bile duct epithelial cells and sinus-lining cells. Peroxisomes were weakly labeled. After DEHP administration, the peroxisomes were markedly induced, but the mitochondria were not. Quantitative analysis showed that the induction of the peroxisomal isomerase was only 2-fold whereas the mitochondrial isomerase was enhanced about 5-fold, 40 times as high as the peroxisomal enzyme. The results show that the mitochondria are the main intracellular site for isomerase and the peroxisomes a minor site. The mitochondrial isomerase of the rat liver is markedly induced by peroxisome proliferators, DEHP and clofibrate.     

Ultrastructural Observations on the Infection of Rat Liver by Plasmodium berghei Sporozoites In Vivo1

The invasion of liver parenchymal cells by sporozoites of Plasmodium berghei Vincke & Lips, 1948, was studied in vivo using transmission electron microscopy. Livers of Brown Norway rats were examined 30 and 60 min after intraportal injection of 15 million sporozoites each. Sporozoites found after incorporation into vacuoles in hepatocytes were often located near a bile canaliculus at the lateral cell surface, surrounded by hepatocyte lysosomal structures; however, degradation of sporozoites caused by lysosomal digestion inside hepatocytes was never observed. Due to the crescent shape of sporozoites, serial sections were necessary to demonstrate the actual process of invasion of the hepatocyte. The hepatocyte's plasmalemma appeared to invaginate due to the sporozoite's action, thereby creating a parasitophorous vacuole. It was suggested that the sporozoite actively penetrated the hepatocyte; however, no visible depletion of rhoptries and micronemes was observed.     

Ultrastructural observations on the infection of rat liver by Plasmodium berghei sporozoites in vivo

The invasion of liver parenchymal cells by sporozoites of Plasmodium berghei Vincke & Lips, 1948, was studied in vivo using transmission electron microscopy. Livers of Brown Norway rats were examined 30 and 60 min after intraportal injection of 15 million sporozoites each. Sporozoites found after incorporation into vacuoles in hepatocytes were often located near a bile canaliculus at the lateral cell surface, surrounded by hepatocyte lysosomal structures; however, degradation of sporozoites caused by lysosomal digestion inside hepatocytes was never observed. Due to the crescent shape of sporozoites, serial sections were necessary to demonstrate the actual process of invasion of the hepatocyte. The hepatocyte's plasmalemma appeared to invaginate due to the sporozoite's action, thereby creating a parasitophorous vacuole. It was suggested that the sporozoite actively penetrated the hepatocyte; however, no visible depletion of rhoptries and micronemes was observed.     

Bile canalicular barrier function and expression of tight-junctional molecules in rat hepatocytes during common bile duct ligation

Tight junctions of hepatocytes form the intercellular barrier between the blood circulation and bile flow. We focused on early stages of common bile duct ligation to observe changes in tight junctions without the irreversible changes seen after lengthy ligation. Common bile ducts of 12-week-old male rats were ligated for 6 h because, at this time point, no histological changes were observed. Serum bilirubin and bile acid levels began to increase 3 h after ligation and were restored to the control level immediately after surgical removal of the ligation. To examine the barrier of hapatocytes, horseradish peroxidase was injected via the femoral vein, and bile was collected for the first 10 min. A four-fold elevation of the secretion and concentration was observed in the bile of ligated rats compared with that of control animals. We next examined lanthanum permeability by perfusion fixation of the liver. At 6 h after ligation, both dilation of the bile canaliculi and partial loss of microvilli were commonly observed. There were dense deposits of lanthanum in almost all bile canaliculi of ligated rats. In control animals, neither dilation of the bile canaliculi nor loss of microvilli was detected, and only 44% of bile canaliculi exhibited deposits. An apparent increase of occludin mRNA expression was detected in livers after 6 h ligation, whereas the expression of claudin-1, -2, and -3 was not influenced by ligation. These results indicate that regulation of occludin gene expression is different from that of claudin-1, -2, and -3. The early phase of bile stasis employed in this study is thought to be an indispensable approach for understanding the precise regulation of tight junctions.     

Some observations on the morphological evidence for mechanism of the bile secretion

The morphological evidence of the intracellular route of bile secretion was investigated in the liver of goldfish (Carassius auratus) as revealed by electron microscopy. Smooth surfaced tubules or cisterns within or adjacent to the Golgi apparatus showed linear saccular forms and contained sparse particulate or cloudy materials of low electron density. The isolated vacuoles were restrictedly found between the Golgi apparatus and the intracellular bile canaliculus or hepatocytic side at the zone of transition. These vacuoles showed no reaction for acid phosphatase activity, and contained only a few cloudy materials similar to those found in the saccular tubules and within the bile canaliculus. Some of these vacuoles fused with the luminal cytolemmas of the bile canaliculus. Bases on these findings, it was assumed that these vacuoles are structures participating in transport and secretion of bile constituents and derive from the linearly sacculated tubules or cisterns in the Golgi zone. Duct cells showed no morphological evidence to suggest bile secretion.     

Terminal deoxynucleotidyl transferase in AKR leukemic cells and lack of relation of enzyme activity to cell cycle phase.

Cholate and taurocholate uptakes were studied in presence of albumin using isolated rat hepatocytes. Albumin decreased nonspecific binding of both bile acids and inhibited cholate uptake noncompetitively and taurocholate uptake competitively. Although different bile acids except dehydrocholate inhibited both cholate and taurocholate uptake, their relative inhibitory potency was not the same for both bile acids. Uptake of both bile acids was characterized by a saturable as well as an unsaturable process both in presence and in absence of albumin. The results suggest that both bile acids may be transported by more than one carrier and taurocholate is transported more efficiently than cholate by hepatocytes.     

Intramembrane particles and filipin labelling on the membranes of autophagic vacuoles and lysosomes in mouse liver

Summary Morphologically detectable protein (intramembrane particles) and cholesterol (filipin labelling) in the membranes of autophagic vacuoles and lysosomes were studied in mouse hepatocytes using thin-section and freeze-fracture electron microscopy. Both isolated autophagic vacuoles and lysosomes, and intact tissue blocks were used due to the facts (i) that lysosomes are difficult to recognize in freeze-fracture replicas of intact hepatocytes, and (i) that filipin penetration into the tissue blocks is unsatisfactory. Intramembrane particle density was low in the membranes of early autophagic vacuoles (defined as round-shaped vacuoles in which an inner membrane parallel with the outer limiting membrane was clearly visible). The lysosomal membranes contained considerably more intramembrane particles. Particle-rich lysosomes or other vesicles were observed to fuse with the early autophagic vacuoles. The membranes of nascent autophagic vacuoles with morphologically intact contents were usually not labelled by filipin, whereas the membranes of all other autophagic vacuoles and lysosomes were heavily labelled. The increased cholesterol in the membranes of slightly older autophagic vacuoles is presumably derived from cholesterol-rich lysosomes or other vesicles fusing with the vacuoles and from the degrading organelles inside the autophagic vacuoles.     

Behaviour of phospholipase modified-HDL towards cultured hepatocytes. II. Increased cell cholesterol storage and bile acid synthesis

Human total HDL (hydrated density 1.070-1.210), HDL2 (1.070-1.125), HDL3 (1.125-1.210) or HDL separated by heparin affinity chromatography were treated with or without purified phospholipase A2 from Crotalus adamanteus. Control and treated HDL were reisolated and were then incubated with cultured hepatocytes. 1. Mass measurements evidenced a time-dependent cholesterol enrichment in hepatocytes cultured in the absence of lipoproteins. Addition of HDL2 still enhanced by 25% the cell cholesterol content and down-regulated endogenous sterol synthesis in similar proportions. Conversely, HDL3 slightly decreased the amount of free cholesterol in hepatocytes (-12%). 2. Incubations with phospholipase A2-treated HDL resulted in a 35%-50% increase of both the cellular cholesterol esterification and the cholesterylester accumulation, when compared to cells cultured in the presence of control-HDL. This effect was observed with HDL2, HDL3 and combining the data with all subfractions. 3. Cultured hepatocytes secreted cholic and beta-muricholic acids as major bile acids and HDL2 showed a tendency to stimulate their secretion. Phospholipase treatment of HDL again induced an increased production by hepatocytes of those two bile acids. Thus, whereas HDL2 and HDL3 display different behaviours with respect to cell cholesterol content, neosynthesis and bile acid secretion, their modifications by phospholipases always orientate the cell sterol metabolism in the same direction: increased cholesterylester accumulation and bile acid production.     

Rates of RNA degradation in isolated rat hepatocytes. Effects of amino acids and inhibitors of lysosomal function.

1. RNA degradation in isolated rat hepatocytes was measured as the release of radioactive cytidine from fed rats previously labeled in vivo for 60 h with [6-14C]orotic acid. Rates were determined from the linear accumulation of [14C]cytidine between 30 and 120 min of incubation in the presence of 0.5 mM unlabeled cytidine to suppress reutilization. 2. In the absence of amino acids, rates of RNA degradation in isolated hepatocytes averaged 3.97%/h. A complete mixture of amino acids added at 10-20 times normal plasma concentration inhibited RNA degradation by 65-70%. However, at physiological concentrations of amino acids, RNA degradation in isolated rat hepatocytes was less responsive as compared to perfused rat livers. 3. Numerous and large autophagic vacuoles at various stages of digestion were identified throughout the cytoplasm of isolated hepatocytes after 2 h of incubation in the absence of amino acids. The addition of amino acids at 20 times normal plasma concentration abolished almost completely the appearance of autophagic vacuoles. Furthermore, prophylamine, which accumulates in lysosomes, suppressed RNA degradation by 65% and the inhibitor of autophagic vacuole formation, 3-methyladenine, inhibited 70-80% of the degradation. Taken together, these results strongly suggest a contribution of the lysosomal system in the increase of RNA degradation rates in isolated rat hepatocytes.     

Morphological changes in the liver of the sea lamprey, Petromyzon marinus L., during metamorphosis: I. Atresia of the bile ducts

The bile ducts in the liver of larval sea lamprey, Petromyzon marinus, undergo programmed degeneration during metamorphosis. The degenerative process is most dramatic in the middle metamorphic stages (3-5), and is asynchronous, occurring more rapidly in small peripheral biliary components than in larger, medial ducts. All classes of bile ducts within the biliary tree exhibit similar histological changes during regression. The initial evidence of degeneration in the epithelium is a folding of the basal lamina, and this is accompanied by cell shrinkage and disruption of cell order. "Shedding" of microvilli and cytoplasmic constituents then takes place at the apical surface resulting in the accumulation of periodic acid-Schiff positive membranous debris in the Lumen. The appearance of "hyalin bodies" in the lumen coincides with the depletion of intermediate-sized filaments from the cytoplasmic matrix. Numerous, large dense bodies, myelin figures, and autophagic vacuoles are consistently observed in necrotic cells. Following cytolysis, bile duct remnants become ensheathed within regions of fibrosis. Ultimately, these fibrous regions are replaced with cords of hepatocytes. By stage 7, all bile ducts have disappeared. The events of biliary atresia in lampreys are comparable to tissue regression which is associated with normal development and pathological conditions in other vertebrates but are particularly reminiscent of those in human biliary atresia. The unique ability of the adult lamprey to service without bile ducts enhances the value of this organism as an experimental model for studying human biliary atresia.