Formation and accumulation of lipolysosomes in developing chick hepatocytes

What's in a name?-"Lipolysosome": ultrastructural features of a lipid-containing organelle

The prevalence of fatty liver is rising not only in adults but also in children and adolescents. The authors describe the ultrastructure of 12 biopsies from 10 males and 2 females aged 7-18 years. All subjects had fatty liver by ultrasonography and were overweight or obese according to BMI classification. They all had elevated aminotransferases and/or lipid/cholesterol levels, ultimately confirmed by biopsy. Steatosis was mild in 2, moderate in 3, and severe in 7 cases. Nonalcoholic steatohepatitis was diagnosed in 7 and nonalcoholic fatty liver disease in 5 patients. Lipolysosomes, identified in all 12 biopsies, were defined as fat droplets surrounded by a trilaminar membrane and lipofuscin-like deposits within or adjacent to the enveloping membrane. The lysosome marker CD68 revealed lysosomal activity in all lipolysosomes identified by electron microscopy. The ultrastructural features, here illustrated in diverse human biopsies, enabled lipolysosome classification in 3 types: monolocular (type I), multilocular (type II), and giant multilocular (type III). Type II, previously described in some conditions with abnormal lipid metabolism, was found in all biopsies, though with variable frequency. Type III was observed only in severe steatosis and associated with prominent connective tissue and conspicuous lipofuscin deposits. These new findings demonstrate the presence of lipolysosomes in a variety of fatty livers, in conditions hitherto unknown, in relation to the severity of steatosis, fibrogenic process, autophagy, lipolysis, and lipofuscin formation.

Uptake behavior of embryonic chick liver cells

The capacity of selective uptake by liver cells, focusing particularly on the parenchymal and perisinusoidal stellate cells during chick liver development (8-18 days of incubation), was ultrastructurally examined after injection of 240-nm-diameter lecithin (phosphatidylcholine) -coated or noncoated beads into the extraembryonic circulation. Cytoplasmic projections of both cells as well as extrasinusoidal macrophages reached into the sinusoid-like vascular spaces. The primitive perisinusoidal stellate cells were identified by immunocytochemistry as being rich in desmin-positive cytoplasmic intermediate filaments. The cells demonstrated selective uptake of noncoated beads by means of their cytoplasmic projections. These findings were significant in the early period of incubation, indicating that the phagocytic activity is a characteristic and transient phenomenon of developmental differentiation. Large numbers of coated and a few noncoated beads penetrated into the perivascular spaces. The parenchymal cells incorporated only the coated beads that passed through the endothelial lining, suggesting that these cells express selective but limited phagocytic capacity against large "foreign" substances even long before their maturation. The cell projections were not engaged in uptake function. Extrasinusoidal macrophages, Kupffer cells, and intraluminal primitive macrophages all took up both beads; however, lecithin coating of the beads clearly suppressed their uptake function. These data suggest that the uptake function of large "foreign" substances appears to be intrinsic to liver cells and lecithin coating would be useful for delivering large substances to parenchymal cells.

Differential incorporation of docosahexaenoic and arachidonic acids by the yolk sac membrane of the avian embryo

During avian development, lipoproteins derived from yolk lipid are assembled in the yolk sac membrane (YSM) for secretion into the embryonic circulation. To investigate how yolk polyunsaturated fatty acids, essential for the development of certain tissues, are distributed among the lipid classes of the lipoproteins, pieces of YSM were incubated in vitro with [14C]arachidonic and [14C]docosahexaenoic acids (DHA). There was a marked difference in the partitioning of these two precursors among the lipid classes of the tissue. Of the radioactivity incorporated into total lipid from [14C]-arachidonic acid during 1 h of incubation, 67.3% was esterified as phospholipid and 29.5% as triacylglycerol. In contrast, only 14.6% of the label incorporated from [14C]-DHA was esterified as phospholipid, whereas 73.2% was recovered in triacylglycerol. This pattern of differential partitioning was observed at all time points and across a 20-fold range of fatty acid concentrations. There was no evidence for conversion of the radioactive arachidonic and DHAs to other fatty acids prior to incorporation into tissue lipids. It is suggested that the selective incorporation of yolk-derived DHA into the triacylglycerol of secreted lipoproteins represents part of a mechanism for directing this polyunsaturate to particular embryonic tissues.

Morphological characteristics of lipid accumulation in liver-constituting cells of acid lipase deficiency rats (Wolman's disease model rats)

Lysosomal acid lipase is a hydrolase essential for the intracellular degradation of cholesteryl esters and triglycerides. In the laboratory, rats with congenital deficiency of lysosomal acid lipase and marked accumulation of cholesteryl ester, cholesterol free and triglyceride in livers (Wolman's disease rat or Yoshida rat) that corresponded to human Wolman's disease were found and maintained. The morphological characteristics of accumulated lipids in the livers of affected rats were examined also. Many small lipid droplets and lipid crystals were found in the cytoplasms of hepatocytes and ED1-positive and ED2-positive foamy Kupffer's cells, respectively. Electron microscopically, many electron-lucent lipid droplets with limiting membrane were found in hepatocytes. Foamy Kupffer's cells had many multivesicular bodies with limiting membrane, which contained crivilinear bodies, lipid droplets and crystal clefts. At areas of aggregation of foamy Kupffer's cells forming islets, there were many desmin-positive Ito cells. Small lipid droplets with limiting membrane were also found in the cytoplasm of Ito cells and endothelial cells. These findings, which were obtained by morphological methods, indicated that triglyceride and both cholesteryl ester and free cholesterol accumulated in lipolysosomes mainly in hepatocytes and Kupffer's cells, respectively, and suggest that lysosomal acid lipase could participate in dissolution of the membrane.

Biogenesis and function of lipolysosomes in developing chick hepatocytes

Proliferation of lipolysosomes is one of the characteristic aspects of embryonic chick hepatocytes. Formation of lipolysosomes is observed in the well-developed trans-Golgi network, with the highest frequency occurring from 11 to 14 days of incubation. The lipolysosomes usually contain a small and electron-dense lipid inclusion; however, during development, they gradually enlarge with an accompanying reduction in the electron density of the inclusion. Lipolysosomes isolated from neonatal chick liver homogenates were mainly composed of esterified cholesterol and showed considerably high activity of lysosomal enzymes. Moreover, the lipolysosome fraction is clearly shown to be a function of intralysosomal lipolysis via acid lipase. This accumulation of esterified cholesterol within lipolysosomes might be attributed to an excessive uptake and conversion of plasma lipoproteins to lipolysosomes. This concept is supported by the appearance of an abundance of coated pits and both "early" and "late" endosomes. The major components of plasma lipoprotein are low density lipoprotein (LDL) and high density lipoprotein (HDL), the cholesterol-rich lipoproteins, whose cholesterol content increases during the last week of incubation when the lipolysosomes quickly enlarge. Plasma lipoprotein particles are produced in the yolk sac epithelium from yolk very low density lipoprotein (VLDL) and transferred via the vitelline circulation to the chick liver. After hatching, when the supply of nutrients from the yolk sac is terminated, lipolysosomes immediately decrease in size and number. The cholesterol and fatty acids released are useful as an energy source and lipid metabolism in general, especially after hatching. Food intake induces the use of and accelerates the disappearance of lipolysosomes. Instead of lipolysosomes, lipid droplets appear and increase in number and size with concomitant increases of triglyceride concentrations in the liver homogenates, suggesting that lipogenesis has begun in the chick hepatocyte.

Ultrastructural aspects of human gallbladder epithelial cells in cholelithiasis: production of anionic mucus

The surface epithelium of 28 gallbladders removed during elective cholecystectomies and pathology collection was studied ultrastructurally. Focusing on 10 of the 28 cases that were diagnosed as cholecystitis, we found that the epithelium displayed numerous apical mucous granules and bulging apical apices. Mucous granule changes included 1) hyperproduction of secretory granules of neutral type containing an electron-dense proteinaceous spherule, similar to that described in other mucus-producing glands of the digestive system, and 2) production of anionic, osmiophilic secretory mucus. Other alterations of the surface epithelial cells included the production of bizarre surface appendages resembling primitive cilia without axoneme and epithelial excrescences.

Participation of endodermal epithelial cells on the synthesis of plasma LDL and HDL in the chick yolk sac

We ultrastructually examined the chick yolk sac endodermal epithelium and evaluated our findings in combination with the biochemical analysis of serum and yolk lipoproteins. Twenty-five to 30 nm-sized particles were demonstrated to be a principal element of the extracellular yolk mass and these were determined to be yolk very low density lipoprotein (VLDL). The particles were shown to be taken up by the epithelial cells via coated pits and engulfed by plasma membrane invaginations together with yolk subdroplets, another element of the yolk mass. Through apical vacuoles, the two yolk elements were incorporated into yolk drops, which were identified to be one of the lysosomal structures by a cytochemical procedure using acid phosphatase (AcP)ase activity. During the last week of incubation, which is the final third of the incubation period, the digestion seemed to progress rapidly in the yolk drops, which came to resemble lipolysosomes; lipoprotein production became active as expressed by an enlarged Golgi apparatus. The newly produced lipoprotein particles were electron-lucent and irregular in size (50-120 nm). They were sequestered in secretory vacuoles and secreted from the vascular surface of the epithelial cells. Finally, the particles were thought to be taken into the vitelline circulation as plasma lipoproteins. The major component of lipoprotein in serum was determined to be low density lipoprotein (LDL) and high density lipoprotein (HDL), while cholesterol content was found to increase during incubation. We concluded that endodermal epithelial cells participate the synthesis of plasma LDL and HDL. For this synthesis the cells probably apply lipids and apo-protein generated from yolk VLDL degradation.