Discoidal cells

FIGURE 8.3 ABIS. The image of the forewing shows due to illumination from below nearly transparent cells enclosed by the venation. R, Ci, Cj, C3, B, D, and Dj depict the radial cell, up to three cubital cells, brachial cell and two discoidal cells, respectively. 

Submembranous microtubules are often present in parallel bundles beneath the plasma membrane in the cells of higher plants, particularly during cell wall formation (Hardham and Gimning, 1978). Circular submembranous bundles of microtubules are a feature of bird erythrocytes and mammalian blood platelets, where they maintain the discoid shape of these structures (Dustin, 1980). 

DL-CFU Dendritic cell/Langerhans cell colony forming DLE Discoid lupus erythematosus DMARD Disease-modifying antirheumatic drug... 

PLATELETS. Platelets are nonnucleated discoid or elliptical cells that originate from the fragmentation of giant polyploid megakaryocytes located in the bone marrow. The average diameter of the platelet is 1.5 pm. Each platelet is surrounded by a trilaminar membrane, and its cytoplasm contains a dense body (delta granule), a surface-connected canalicular system,... 

Very low density lipoprotein is not the only lipoprotein to be secreted by the liver. HDL is released into the blood as a nascent (immature) discoid particle. As the HDL circulates within the circulation, it matures by exchanging apoproteins and lipid components with other lipoproteins and cells. Mature spherical HDL is... 

Figure 11.15 The reaction catalysed by lecithin cholesterol acyltransferase (LCAT). LinoLeate is transferred from a phospholipid in the blood to cholesterol to form cholesteryl linoleate, catalysed by LCAT. The cholesterol ester forms the core of HDL, which transfers cholesterol to the liver. Discoidal HDL (i.e. HDL3) is secreted by the liver and collects cholesterol from the peripheral tissues, especially endothellial cells (see Figure 22.10). Cholesterol is then esterified with lin-oleic acid and HDL changes its structure (HDL2) to a more stable form as shown in the lower part of the figure. R is linoleate.

Fig. 5.2.1 The major metabolic pathways of the lipoprotein metabolism are shown. Chylomicrons (Chylo) are secreted from the intestine and are metabolized by lipoprotein lipase (LPL) before the remnants are taken up by the liver. The liver secretes very-low-density lipoproteins (VLDL) to distribute lipids to the periphery. These VLDL are hydrolyzed by LPL and hepatic lipase (HL) to result in intermediate-density lipoproteins (IDL) and low-density lipoproteins (LDL), respectively, which then is cleared from the blood by the LDL receptor (LDLR). The liver and the intestine secrete apolipoprotein AI, which forms pre-jS-high-density lipoproteins (pre-jl-HDL) in blood. These pre-/ -HDL accept phospholipids and cholesterol from hepatic and peripheral cells through the activity of the ATP binding cassette transporter Al. Subsequent cholesterol esterification by lecithinxholesterol acyltransferase (LCAT) and transfer of phospholipids by phospholipid transfer protein (PLTP) transform the nascent discoidal high-density lipoproteins (HDL disc) into a spherical particle and increase the size to HDL2. For the elimination of cholesterol from HDL, two possible pathways exist (1) direct hepatic uptake of lipids through scavenger receptor B1 (SR-BI) and HL, and (2) cholesteryl ester transfer protein (CfiTP)-mediated transfer of cholesterol-esters from HDL2 to chylomicrons, and VLDL and hepatic uptake of the lipids via the LDLR pathway...

Equation 8.7 [6] was obtained to correlate the experimental data on membrane plasmapheresis, which is the MF of blood to separate the blood cells from the plasma. The filtrate flux is affected by the blood velocity along the membrane. Since, in plasmapheresis, all of the protein molecules and other solutes will pass into the filtrate, the concentration polarization of protein molecules is inconceivable. In fact, the hydraulic pressure difference in plasmapheresis is smaller than that in the UF of plasma. Thus, the concentration polarization of red blood cells was assumed in deriving Equation 8.7. The shape of the red blood cell is approximately discoid, with a concave area at the central portion, the cells being approximately 1-2.5 pm thick and 7-8.5 pm in diameter. Thus, a value of r (= 0.000257 cm), the radius of the sphere with a volume equal to that of a red blood cell, was used in Equation 8.7. 

II Usually with numerous discoid chromatophores motile cells radial, usually with two furrows harboring one transverse and one... 

Unlike fatty acids, cholesterol is not degraded to yield energy. Instead excess cholesterol is removed from tissues by HDL for delivery to the liver from which it is excreted in the form of bile salts into the intestine. The transfer of cholesterol from extrahepatic tissues to the liver is called reverse cholesterol transport. When HDL is secreted into the plasma from the liver, it has a discoidal shape and is almost devoid of cholesteryl ester. These newly formed HDL particles are good acceptors for cholesterol in the plasma membranes of cells and are converted into spherical particles by the accumulation of cholesteryl ester. The cholesteryl ester is derived from a reaction between cholesterol and phosphatidylcholine on the surface of the HDL particle catalyzed by lecithimcholesterol acyltransferase (LCAT) (fig. 20.17). LCAT is associated with FIDL in plasma and is activated by apoprotein A-I, a component of HDL (see table 20.3). Associated with the LCAT-HDL complex is cholesteryl ester transfer protein, which catalyzes the transfer of cholesteryl esters from HDL to VLDL or LDL. In the steady state, cholesteryl esters that are synthesized by LCAT are transferred to LDL and VLDL and are catabolized as noted earlier. The HDL particles themselves turn over, but how they are degraded is not firmly established. 

The most dramatic illustration of a mass-specific illusion is the comparative heat dissipation of the human erythrocyte and platelet. In mammals, both of these cell types are anucleate and discoid in shape, but the longest dimension of the former is four times that of the latter. Yet heat production of a human erythrocyte was shown to be 10 fW, a sixth that of a human platelet (61 fW see Table 1). The relatively high metabolic activity of platelets is probably due to the need to maintain a considerable phosphagen (phosphocreatine) pool for actomyosin contraction at stimulation and clot retraction. Phosphocreatine is synthesized from creatine using ATP and acts as a demand on the ATP cycle to drive the coupled catabolic half-cycle. On the other hand, ATP requirements of the erythrocyte are relatively small, being mostly confined to active transport of ions at the plasma membrane. 

The nascent HDL particles change shape and composition as they acquire additional free cholesterol by passive cellular diffusion of free cholesterol from cell membranes or from other plasma lipoproteins. HDL surface-localized LCAT progressively converts the free cholesterol on the surface of the particles to cholesterol ester, which occupies the core of the lipoprotein particle. This process converts the shape of the HDL particles from discoidal to spherical. The lipid unloading of HDL in the liver follows at least two pathways. In the first route, the cholesterol ester transfer protein (CETP) mediates cholesterol ester transfer from HDL to VLDL and LDL in exchange for triglyceride LDL in turn are taken up by the liver via the LDL receptor. In the second route, HDL binds to the scavenger receptor Bl, and cholesterol esters are selectively taken into the liver cells without internalization of HDL proteins (Fig. 15-2). 

Epithelial bullae can be found in some cases of disciform keratitis, as can a Wessley ring, which is composed of immune cells surrounding the discoid edema.A mild to moderate uveitis with keratic precipitates is usually present, although it may not be visible due to corneal edema. Secondary glaucoma can also develop, primarily the result of intraocular inflammation (trabeculitis). 

Figure 3. Sdiematic representation of discoid platelets Features of a discoid platelet EC, exterior coat CM, cell membrane CS, channels of the surface connected canalicular system SMF, submembrane filaments Sol-gel zone contains actinmiaDfilanients,microtubule(Ml), and glycogen (Gly), fiamed elements a-granuies(G), dense bodies (DB) and mitodiondria(NQ DTS and are part of the membrane system (Courte James G.

HDLs are secreted in nascent form by hepatocytes and en-terocytes (Figure 20-7). Loss of surface components, including phospholipids, free cholesterol, and protein from chylomicrons and VLDL as they are acted on by lipoprotein lipase, may also contribute to formation of HDL in plasma. Discoidal, nascent HDL is converted to spherical, mature HDL by acquiring free cholesterol from cell membranes or other lipoproteins. This function of HDL in peripheral cholesterol removal may underlie the strong inverse relationship between plasma HDL levels and incidence of coronary heart disease. After esterification of HDL surface cholesterol by LCAT, which is activated by apo A-I, HDL sequesters the cholesteryl ester in its hydrophobic core. This action increases the gradient of free cholesterol between the cellular plasma membrane and HDL particles. Cholesteryl esters are also transferred from HDL to VLDL and LDL via apo D, the cholesteryl ester transfer protein (Figure 20-8). 

FIGURE 101-5. Elongated sickle and normal discoid shaped red blood cells. 

Blood platelets circulate in the vascular system as discrete discoid-shaped cells approximately 2-3 urn in diameter. Although anucleated, platelets contain mitochondria, glycogen particles... 

FIGURE 5-3 Variation in biomembranes in different cell types, (a) A smooth, flexible membrane covers the surface of the discoid erythrocyte cell, (b) Tufts of cilia (Cl) project from the ependymal cells that line the brain ventricles, (c) Many nerve axons are enveloped in a myelin sheath composed of multiple layers of modified plasma membrane. The individual myelin layers can be seen in this electron micrograph of a cross section of an axon (AX). The myelin sheath is formed by an adjacent supportive (glial) cell (SC). [Parts (a) and (b) from R. G. Kessel and R. H. Kardon, 1979, Tissues and Organs A Text-Atlas oT Scanning Electron Microscopy, W. H. Freeman and Company. Part (c) from P C. Cross and K. L. Mercer, 1993, Cell and Tissue Ultrastructure A Functional Perspective, W. FI. Freeman and Company, p. 137]... 

---