Plasma Membrane Of Microvilli Research Articles

Electron microscopical observations on the brush border of proximal tubule cells of mammalian kidney.

The ultrastructure of the plasma membrane and the core of microvilli of proximal tubule cells has been investigated by electron microscopy using sectioned and negatively stained material. By the technique of negative staining, a particulated coat is disclosed on the outside of the plasma membrane of microvilli of brush borders isolated from rat, rabbit and ox. This coat is composed of 30 to 60 A particles and is 150 to 300 A thick and appears to be a distinguishing feature for the luminal plasma membrane (brush border) of proximal tubule cells. The plasma membrane of the basal part of tubule cells is found to be smooth. By thin sectioning, an axial bundle of 50 to 70 A diameter filaments regularly arranged in an “1+6 configuration”, one axially located filament being surrounded by a ring of six, is disclosed. The distance from the ring of filaments to the inner surface of the plasma membrane is 250–300 A, the diameter of the ring 300 A and the center-to-center distance between filaments 120 A. Negative staining also discloses 60 A filaments in microvilli of isolated brush borders. Broken off, single microvilli (fingerstalls) are observed with thin filaments projecting from their broken ends. Filaments up to 1 μ in length are seen. At high magnification, the filaments appear beaded and show strong resemblance with actin filaments isolated from skeletal muscle. Based on present evidence, it is postulated that microvilli constituting renal brush borders possess contractile properties, which may play a role in the absorption process operating at the luminal part of the cells.

An electron microscopic study of vitellogenesis and egg membrane formation in Lytta nuttalli Say (ColeopterarMeloidae)

Vitellogenesis and egg membrane formation in the terminal ovarian follicles of Lytta nuttatii were investigated by electron microscopy. Three kinds of yolk globules are produced. They apparently are composed predominantly of carbohydrates, lipids, and proteins, respectively. The "carbohydrate" and "lipid" yolk are assembled in the ooplasm, the former by rough endoplasmic reticulum and the latter by Golgi complexes. Their production begins early in oogenesis. "Proteid" yolk formation begins somewhat later. The "proteid" yolk globules evidently are formed from exfraovarian materials that pass through large spaces that develop between the follicular epithelial cells, then through the oocyte plasma membrane by pinocytosis. Fairly late in development, glycogen granules appear in the inner ooplasm. In the nearly fully grown follicle, the "membranous system" of the vitelline membrane is elaborated. It probably is formed largely from an electron-dense material of undetermined origin that accumulates outside the bases of the oocyte plasma membrane microvilli. Immediately after completion of the vitelline membrane, the chorion is laid down, presumably from dense globules of material produced by Golgi complexes in the follicle cells.

Ultrastructure of the ileum of the European corn borer, Ostrinia nubilalis

The ileal epithelial cells of the diapause European corn borer were found to have elongated microvilli on their apical and lateral surfaces. Fine filaments were seen extending from tips of microvilli into the intestinal lumen. Fibrous material was present in intercellular spaces, and a basement membrane of uniform thickness persisted. Numerous mitochondria occupied microvilli, cytoplasmic regions surrounding nuclei, and basal parts of the cell. Changes in mitochondrial morphology accompanied changes in the autofluorescent cycle. Progressive development of dense granules, formed from whorled basal parts of microvilli and trapped cellular organelles, was followed during the 8-hr autofluorescent period. Increased autofluorescence correlated directly with development of aggregates of whorls and formation of dense granules. Discharge of granule contents apparently occurred into the haemolymph. Microtubules were commonly seen in cytoplasmic regions of the cell particularly surrounding aggregations of whorls. The presence of smooth endoplasmic reticulum rose as autofluorescence increased as did also the number of ribosomes seen free in the cytoplasm. On favourable sections small plaques were visible on outer plasma membranes of microvilli. Plaques could likewise be seen on membranes within whorls and on disintegrating membranes in dense bodies. The morphology of these cells strengthens the supposition that they are active in absorption and secretion or in ion-exchange processes.

Lipid composition of the isolated rat intestinal microvillus membrane.

1. Rat intestinal microvillus plasma membranes were prepared from previously isolated brush borders and the lipid composition was analysed. 2. The molar ratio of cholesterol to phospholipid was greatest in the membranes and closely resembled that reported for myelin. 3. Unesterified cholesterol was the major neutral lipid. However, 30% of the neutral lipid fraction was accounted for by glycerides and fatty acid. 4. Five phospholipid components were identified and measured, including phosphatidylethanolamine, phosphatidylcholine, phosphatidylserine, sphingomyelin and lysophosphatidylcholine. Though phosphatidylethanolamine was the chief phospholipid, no plasmalogen was detected. 5. In contrast with other plasma membranes in the rat, the polar lipids of the microvillus membrane were rich in glycolipid. The cholesterol:polar lipid (phospholipid+glycolipid) ratio was about 1:3 for the microvillus membrane. Published data suggest that this ratio resembles that of the liver plasma membrane more closely than myelin or the erythrocyte membrane. 6. The fatty acid composition of membrane lipids was altered markedly by a single feeding of safflower oil. Membrane polar lipids did not contain significantly more saturated fatty acids than cellular polar lipids. Differences in the proportion of some fatty acids in membrane and cellular glycerides were noted. These differences may reflect the presence of specific membrane glycerides.

Rat intestinal microvillus membranes. Purification and biochemical characterization.

1. A technique is described for the removal of subcellular contaminants from intact rat intestinal brush borders, and for the subsequent separation of a microvillus membrane fraction from a fibrillar residue. 2. Increments in invertase activity, microscopic homogeneity and low nucleic acid content indicate that the microvillus plasma membrane has been extensively purified. Multiple membrane preparations have been shown to be highly reproducible with respect to their invertase specific activity, cholesterol content and phospholipid content. Alkaline phosphatase, leucine aminopeptidase, Mg(2+)- and Ca(2+)-dependent adenosine triphosphatase and seven separate disaccharidases were shown to be predominantly confined to the membrane fraction. 3. The fibrillar fraction has been shown to contain approximately 30% of the total protein of purified brush borders, plus most of the residual nucleic acid contaminant. No evidence was found for the localization of any specific enzyme in this fraction.

Ultrastructural changes in the microvillus plasma membrane during lipid absorption and the form of absorbed lipid: An in Vitro study at 37°C

The absorption in vitro at 37°C of a micellar solution composed of fatty acid, monoglyceride and bile salt into rings of hamster small intestine was studied with special reference to the initial entry of lipid into the epithelial cells. During the incubation of small intestinal rings in a micellar solution at 37°C, the brush border plasma membrane changes from an asymmetric triple-layered structure to a symmetrical triple-layered structure. Finely dispersed material is found attached to the surfaces of the plasma membrane. Intracellularly the absorption process appears to proceed as in vivo, for globular fat is first found within smooth-surfaced apical vesicles below the terminal web region of the cells. The membrane of these vesicles is not of the plasma membrane type. Indentations between microvilli as well as small vesicles within the terminal web region each bounded by a membrane of the plasma membrane type are found with about the same frequency in both absorbing and nonabsorbing cells. The ultrastructural changes in the peripheral layer of the brush border plasma membrane are interpreted as representing the incorporation of micellar material into the surface of the membrane. This would be the first step in the absorption of digested lipid by the intestinal epithelial cells. The small 50 Å particles found over sections which are stained with lead citrate appear to be lead precipitates.