Doc1 - 276 LIVER AND PANCREAS - 7. . i ;_ Fig. 15.11 Liver...

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Unformatted text preview: 276 LIVER AND PANCREAS - 7. . i ;_ Fig. 15.11 Liver rm EM x4400rb} EMX r5 zooroppoma These mlcmgraphs demonstrate the main ultrastruemral features of the liver. Hepatocytes H are exposed on each side to the sinusoids (filled with erythrocytes E) which are lined by a discontinuous layer of sinusoid lining cells S. Them: are supported by the fine reticulin framework of the liver (see Fig. 15.10) with the space af Dim D between the lining cells and the hepatocytc surface. Via the gaps in the sinusoid lining, the space ofDisse is continuous with the stnusoid lumen, thus bathing the hepatocytt: surface with plaxma. Numerous irregular microvilli Mv extend from the hepatocyte surface into the space of Disse, greatly increasing the surface area for metabolic exchange. Between the bases aid-re microvilli are coated pits involved in mducytusls. Reflecting the extraordinary range of biosynthetic and degradative activities, the hepatecyte cytoplasm is crowded wrth organelles, particularly rough endoplasmic reticulum rER, smooth endoplasmic reticulum sER, Golgi stacks, free ribosomes, mitochondria M,1ysosomes Ly and peroxisomes. Lipid droplets L and glycogen rosettes are present in variable numbers depending on nutritional status. Bilecanaliculi BC. are seen to be formed from the plasma membranes of adjacent hepatocytes, the plasma membranes being tightly bound by junctional complexes ]: small microvilli project into the canaliculi. The subjacent cytoplasm contains a network of actin filaments, contraction of which reduces canalicular diameter, thus reducing flow rate. 277 LIVER AND PANCREAS LIVER AND PANCREAS 273 Fig. 15.4 Liver {Perfusion method x 20) This preparation shows one of the techniques used by early histologists in mapping hepatic blood flow. The hepatic portal vein (supplying the liver) has been perfused with a red dye and the hepatic vein (draining the liver) has been back- perfused with a blue dye. Thus it can be seen how a hepatic lobule is defined by a number of portal tracts peripherally (stained red) with blood draining to a single terminal hepatic venule (stained blue) at the centre. Alternatively, an hepatic acinus is centred around a portal tract with blood radiating outwards to drain into a number of terminal hepatic venules. Fig. 15.5 Liver lobule (H 633‘ E x 75} This micrograph illustrates a single human liver lobule. The irregular hexagonal boundary of the lubule is defined by portal tracts T and sparse collagcnous tissue C. Sinusoids originate at the lobulc margin and course between plates of hepatocyles to converge upon the terminal hepatic venule V. The plates of hepatocytes are usually only one cell thick and each hepatocyte is thus exposed to blood on at least two sides. The plates of hepatocytes branch and anastomose to form a three- dimensional structure like a sponge. Fig. 15.6 Liver pareri chyma (H er E x 150,‘ The arrangement of hepatocytes within the liver parenchyma can be readily seen at this magnification. The hepatocyres form fiat, anastomosing plates usually only one cell thick between which blood passes sluggishly towards the terminal hepatic venule V. The sinusoids are lined by a discontinuous layer of cells which do not rest on a basement membrane and which are separated from the hepatocytes by a narrow space (space of Dine); this drains into the lymphatics of the portal traCtSi 274: LIVER AND PANCREAS Fig. 15.7 Hepatocyres fa) H Ce E x 600 (a; PASlHaemmxyhn x 400 Hepatocytes are large polyhedral cells with large round nuclei with peripherally dispersed chromatin and prominent nucleoli. The nuclei vary greedy in size, reflecting an unusual cellular feature; more than half the hepatocytes contain twice the normal (diploid) complement of chromosomes within a single nucleus (Le. they ate tetraploid) and some contain four or even eight times this amount (polyploid). Dcoaaional binucleate cells are seen in section although up to 25 "/0 of all hepatocytes are actually binucleate. The extensive cytoplasm has a variable appearance depending on the nutritional status of the individual. When well-nourished, hepatocytes store significant quantities of glycogen and process large quantities of lipid. Both these metabolites are partially removed during routine histological preparation leaving irregular unstained areas within the cytoplasm. The cytoplasm is otherwise strongly cosinophilic due to numerous mitochondria with a fine basophilic granularin due to extensive free ribosomes and rough ER. Fine brown granules of the ‘wear and tear’ pigment lipofuscin (see Fig. 1.3) are present in variable amounts, increasing with age. All these features are seen in micrograph (a). Sinusoid lining cells S are readily distinguishable from hepatooytes by their flattened condensed nuclei and attenuated poorly stained cytoplasm. Mcrograph (’3) demonstrates the presence within the hepatocytes of glycflgen granules which, being polysaccharide, are PAS positive, i.e. stain magenta; the nuclei are counterstained blue. Fig. 15.8 Sinusoid lining cells {Peri‘r Prussian blue X 48) The nature and function of the sinusoid lining cells has been in dispute ever since the liver was first studied. At last a proportion ofthese celis can be shown to be highly phagocytic when ‘fed’ either artificially or under pathological conditions with appropriate particulate matter. The animal used for this preparation was iniecwd intravenously with a particulate iron-sugar compound which, with this staining method, is demonstrated as a dark deposit within the sinusoid lining cells. The sinusoid lining cells include at least three cell types. The majority are endothelial cells E with flat darkly stained nuclei and attenuated cytoplasm Scattered among the endothelial cells are large plump phagqutil: cells with ovoid nuclei. Known as Kapfier cell: K, these cells form part of the monocyte-maetophage defence system (see Cha. 3 and 4) and with the spleen, participate in the removal of spent erythrocytes and other particulate debris from the circulation. Occasional fibroblasts with the capacity to store lipid are found in the space ofDisse between the sinusoid lining cells and the hepatocytes. Fig. 15.10 I Bile is ay-nthea system of mini network withii have no discte fine channelsf bepatocytes; t1 15.11. Bile canalict canals ofHerit portal tracts. The hepatic: rough endoplasmic nucleus Figure '2‘; A" electmn m. reticulum , It . ‘ of part of a liver cell seen in section. Examples or most om. intracellular I:ompaq-1,-,-.e,..Ls are ind. a f .7 fl _ ., lw ‘ ~ ' - - _ ' .7 7 (Courtesy of Daniel Sir-rend) peroxisome mitochondrron ocalization of both the ER and the n intact microtubule array; if the th a drug, the Golgi appara- d the ER network collapses the cytoskeleton. Thel for instance. depends on a of the organelles with Golgi apparatus. microtubules are exp tus fragments and dispers toward the cell center (discusse erimentally depolymcrized wi es throughout the cell, an d in Chapter 16), s of Membrane-enclosed The Topological Relationship ed in Terms of Their Organelles Can Be interpret Evolutionary Origins nd the relationships between the compartments of the cell, it is der how they might have evolved. The precursors of the first thought to have been simple organisms that resembled bac- lly have a plasma membrane but no internal membranes. The plasma membrane in such cells therefore provides all membrane-depen- dent functions, including the pumping of ions, ATP synthesis, protein secretion, and lipid synthesis, Typical present-day eucaryotic cells are 10730 times larger in linear dimension and icon-10,000 times greater in volume than a typical bac- terium such as E. wit, The profusion of internal membranes can be seen in part as an adaptation to this increase in size: the eucaryotic cell has a much smaller ratio of surface area to volume, and its area of plasma membrane is presumably too small to sustain the many vital functions forwhich membranes are required. The extensive internal membrane systems of a eucaryotic cell alleviate this ccompanied the Specializas akoid To understa helpful to consi eucaryotic cells are teria, which genera imbalance. The evolution of internal membranes evidently a pie, the generation of thyl [ion of membrane function, Consider, for exam 662 Chapter l1 :INTRACELLUlAR COMPARTMENTS AND l‘ROTElN SORT‘NG ...
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This note was uploaded on 10/13/2009 for the course YIPH ln;ljhdfi taught by Professor Wanger during the Spring '09 term at Heriot-Watt.

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Doc1 - 276 LIVER AND PANCREAS - 7. . i ;_ Fig. 15.11 Liver...

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