Thermotropic lipid phase separations in human platelet and rat liver plasma membranes

Electron spin resonance (ESR) studies were conducted on human platelet plasma membranes using 5-nitroxide stearate, I(12,3). The polarity-corrected order parameter S and polarity-uncorrected order parameters S(T parallel) and S(T perpendicular) were independent of probe concentration at low I(12.3)/membrane protein ratios. At higher ratios, S and S(T perpendicular) decreased with increasing probe concentration while S(T parallel) remained unchanged. This is the result of enhanced radical interactions due to probe clustering. A lipid phase separation occurs in platelet membranes that segregates I(12,3) for temperatures less than 37 degrees C. As Arrhenius plots of platelet acid phosphatase activity exhibit a break at 35 to 36 degrees C, this enzyme activity may be influenced by the above phase separation. Similar experiments were performed on native [cholesterol/phospholipid ratio (C/P) = 0.71] and cholesterol-enriched [C/P = 0.85] rat liver plasma membranes. At 36 degrees C, cholesterol loading reduces I(12,3) flexibility and decreases the probe ratio at which radical interactions are apparent. The latter effects are attributed to the formation of cholesterol-rich lipid domains, and to the inability of I(12,3) to partition into these domains because of steric hinderance. Cholesterol enrichment increases both the high temperature onset of the phase separation occurring in liver membranes from 28 degrees to 37 degrees C and the percentage of probe-excluding, cholesterol-rich lipid domains at elevated temperatures. A model is discussed attributing the lipid phase separation in native liver plasma membranes to cholesterol-rich and -poor domains. As I(12,3) behaves similarly in cholesterol-enriched liver and human platelet plasma membranes, cholesterol-rich and -poor domains probably exist in both systems at physiologic temperatures.     

Quantitative und qualitative elektronenmikroskopische Untersuchungen zur Struktur des Leberläppchens normaler Ratten

Zusammenfassung Untersucht wurden zufällig ausgewählte Leberstückchen normaler Wistarratten. 25 lückenlose Übersichten von periportalen und zentralen Arealen des Leberläppchens, die aus 2564 elektronenmikroskopischen Einzelaufnahmen (Vergrößerung 4600 x) zusammengesetzt waren, wurden bei einer Gesamtvergrößerung von 53000 x mit Hilfe eines Liniengitters nach der Methode. von Loud quantitativ ausgewertet.Die mittlere Cytoplasmafläche eines Hepatocyten, der im Läppchenzentrum liegt, ist größer als diejenige einer in der Peripherie des Läppchens gelegenen Leberzelle, die Zahl der Anschnitte von Mitochondrien pro Cytoplasmafläche im Zentrum geringfügig größer als in der Peripherie. Die Zahl der Lysosomen pro Cytoplasmafläche ist in der Peripherie des Lobulus dreimal höher als in seinem Zentrum.Der Flächenanteil der Mitochondrienanschnitte an der Cytoplasmafläche im Zentrum beträgt 12,3%, im periportalen Bereich 19,3%. Die mittlere Fläche eines Mitochondrienanschnittes ist im periportalen Gebiet doppelt so groß wie im Zentrum, die Membranprofildichte in Mitochondrien periportaler Zellen ist um etwa ein Drittel größer.Schüsselförmige und schlegeloder hantelförmige Mitochondrien mit parallel zur Längsachse ausgerichteten inneren Membranen wurden nur im Zentrum des Leberläppchens gefunden. Dasselbe gilt für Plasmaprotusionen der Hepatocyten in den Disseschen Raum. Glykogenablagerungen sind gleichmäßig über den Lobulus verteilt, auffallend ist jedoch die ungleichmäßige Verteilung auf die einzelnen Zellen.Die quantitativen Daten werden mit histochemischen Befunden und biochemisch ermittelten Enzymaktivitäten verglichen. Die morphologischen Beobachtungen werden im Zusammenhang mit ähnlichen Befunden, die an pathologisch veränderten Lebern erhoben wurden, diskutiert.

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Structure of the hepatic lobule of the rat

Summary Small randomly chosen pieces of liver tissue of normal Wistar rats were investigated with the electron microscope. 25 survey pictures, consisting of 2564 individual micrographs, were analyzed quantitatively at a final magnification of 53,000 x by a linear scanning method as described by Loud.The average area of cytoplasm is larger in the centrolobular than in the periportal area. The number of mitochondria per unit of cytoplasmic area was roughly the same throughout a liver lobule. However, in the periportal zone the mitochondria were about two times larger than in the center and displayed a 30% higher membrane profile concentration. It was found that the total mitochondrial area per unit of cytoplasm was 12,3% in the centrolobular region as compared to 19,3% in the periportal zone. The number of lysosomes per unit of cytoplasmic area was about three times higher in the periportal than in the centrolobular zone.Cup- and dumbbell-shaped mitochondria with densely packed and longitudinally arranged internal membranes were only found in the centrolobular area. Irregular protrusions of the liver cell into the space of Disse were also found exclusively in this area. The amount of glycogen varied considerably from cell to cell but no significant difference in this respect could be seen between the two zones of the liver lobule.The quantitative findings are discussed with reference to available bio- and histochemical data. The morphological observations are compared with similar observations made on pathologically altered liver cells.

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Herrn Prof. Dr. med. Helmut Ruska zum 60. Geburtstag gewidmet.

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Mit Unterstützung durch die Deutsche Forschungsgemeinschaft.     

THE DEGENERATIVE CHANGES IN PANCREATIC ACINAR CELLS CAUSED BY DL-ETHIONINE

Degeneration of pancreatic acinar cells in rats injected with ethionine was studied by electron microscopy. The most conspicuous morphologic lesions occurred in the ergastoplasm. There was a widening of the endoplasmic reticulum, a decrease in number of membrane-associated ribosomes, and a development of fine and coarse vacuoles containing agranular disoriented membranes. Cytoplasmic ribosomes unassociated with membranes were less numerous. Nuclear changes consisted of a coarsening and clumping of the nuclear chromatin, chromatin margination, and increased osmiophilia and vacuolation of the nucleolus. Eight to ten days after the beginning of ethionine injections, changes in zymogen granules, mitochondria, and the Golgi apparatus appeared, but only after extensive damage to the acinar cell. The effects were consistent with ethionine's known interference with protein metabolism but also suggest disturbance in ribonucleic acid metabolism. The ergastoplasmic changes after ethionine were similar in some respects to the early lesions produced in liver parenchymal cells by fasting, to the changes occurring in animals on protein-free diets, or to some of the liver changes produced by azo dye carcinogens. The ribosomal and ergastoplasmic changes represent early morphologic expressions of the biochemical effect of ethionine.