Fenestral Pore Research Articles

A Na,K-ATPase-Fodrin-Actin Membrane Cytoskeleton Complex is Required for Endothelial Fenestra Biogenesis.

Fenestrae are transcellular plasma membrane pores that mediate blood–tissue exchange in specialised vascular endothelia. The composition and biogenesis of the fenestra remain enigmatic. We isolated and characterised the protein composition of large patches of fenestrated plasma membrane, termed sieve plates. Loss-of-function experiments demonstrated that two components of the sieve plate, moesin and annexin II, were positive and negative regulators of fenestra formation, respectively. Biochemical analyses showed that moesin is involved in the formation of an actin–fodrin submembrane cytoskeleton that was essential for fenestra formation. The link between the fodrin cytoskeleton and the plasma membrane involved the fenestral pore protein PV-1 and Na,K-ATPase, which is a key regulator of signalling during fenestra formation both in vitro and in vivo. These findings provide a conceptual framework for fenestra biogenesis, linking the dynamic changes in plasma membrane remodelling to the formation of a submembrane cytoskeletal signalling complex.

Computational study of LDL mass transport in the artery wall

A two-dimensional (2D) numerical simulation of convective–diffusive transport of LDL in the artery wall, coupled with the wall shear stress gradient (WSSG)-dependent LDL consumption of smooth muscle cells (SMCs) is presented. SMCs are modeled as an array of solid cylindrical pillars embedded in a continuous porous media which represents the interstitial proteoglycan and collagen fiber matrix. The internal elastic lamina (IEL), which separates the artery media from the intima, is modeled as an impermeable barrier to both water and LDL except for the fenestral pores that are assumed to be uniformly distributed over the IEL. The predictions demonstrate a range of interesting features of LDL transport and uptake in the media. For cells immediately below the fenestral pores, LDL uptake of SMCs is highly dependent on WSSG. Moreover, the rate of LDL consumption by SMCs is also affected by the diameter of the fenestral pore. This will be helpful in understanding the involvement of transmural transport processes in the initiation and development of atherosclerosis.

An in vitro assay reveals a role for the diaphragm protein PV-1 in endothelial fenestra morphogenesis.

Fenestrae are small pores in the endothelium of renal glomerular, gastrointestinal, and endocrine gland capillaries and are involved in the bidirectional exchange of molecules between blood and tissues. Although decades of studies have characterized fenestrae at the ultrastructural level, little is known on the mechanisms by which fenestrae form. We present the development of an in vitro assay in which rapid and abundant fenestra induction enables a detailed study of their biogenesis. Through the use of agents that stabilize or disassemble actin microfilaments, we show that actin microfilament remodeling is part of fenestra biogenesis in this model. Furthermore, by using a loss-of-function approach, we show that the diaphragm protein PV-1 is necessary for fenestral pore architecture and the ordered arrangement of fenestrae in sieve plates. Together, these data provide insight into the cell biology of fenestra formation and open up the future study of the fenestra to a combined morphological and biochemical analysis.

Flow through internal elastic lamina affects shear stress on smooth muscle cells (3D simulations).

We describe a three-dimensional numerical simulation of interstitial flow through the medial layer of an artery accounting for the complex entrance condition associated with fenestral pores in the internal elastic lamina (IEL) to investigate the fluid mechanical environment around the smooth muscle cells (SMCs) right beneath the IEL. The IEL was modeled as an impermeable barrier to water flow except for the fenestral pores, which were assumed to be uniformly distributed over the IEL. The medial layer was modeled as a heterogeneous medium composed of a periodic array of cylindrical SMCs embedded in a continuous porous medium representing the interstitial proteoglycan and collagen matrix. Depending on the distance between the IEL bottom surface and the upstream end of the proximal layer of SMCs, the local shear stress on SMCs right beneath the fenestral pore could be more than 10 times higher than that on the cells far removed from the IEL under the conditions that the fenestral pore diameter and area fraction of pores were kept constant at 1.4 microm and 0.05, respectively. Thus these proximal SMCs may experience shear stress levels that are even higher than endothelial cells exposed to normal blood flow (order of 10 dyn/cm(2)). Furthermore, entrance flow through fenestral pores alters considerably the interstitial flow field in the medial layer over a spatial length scale of the order of the fenestral pore diameter. Thus the spatial gradient of shear stress on the most superficial SMC is noticeably higher than computed for endothelial cell surfaces.

Fenestral pore size in the internal elastic lamina affects transmural flow distribution in the artery wall.

Interstitial flow through the subendothelial intima and media of an artery wall was simulated numerically to investigate the water flow distribution through fenestral pores which affects the wall shear stress on smooth muscle cells right beneath the internal elastic lamina (IEL). A two-dimensional analysis using the Brinkman model of porous media flow was performed. It was observed that the hydraulic permeability of the intimal layer should be much greater than that of the media in order to predict a reasonable magnitude for the pressure drop across the subendothelial intima and IEL (about 23 mostly at a 70 mm Hg luminal pressure). When Ki was set equal to the value in the media, this pressure drop was unrealistically high. Furthermore, the higher value of Ki produced a nearly uniform distribution of water flow through a simple array of fenestral pores all having the same diameters (1.2 microm), whereas when Ki was set at the value in the media, the flow distribution through fenestral pores was highly nonuniform and nonphysiologic. A deformable intima model predicted a nonuniform flow distribution at high pressure (180 mm Hg). Damage to the IEL was simulated by introducing a large fenestral pore (up to 17.8 microm) into the array. A dramatic increase in flow through the large pore was observed implying an altered fluid mechanical environment on the smooth muscle cells near the large pore which has implications for intimal hyperplasia and atherosclerosis. The model also predicted that the fluid shear stress on the bottom surface of an endothelial cell is on the order of 10 dyne/cm2, a level which can affect cell function.

Interstitial flow through the internal elastic lamina affects shear stress on arterial smooth muscle cells.

Interstitial flow through the tunica media of an artery wall in the presence of the internal elastic lamina (IEL), which separates it from the subendothelial intima, has been studied numerically. A two-dimensional analysis applying the Brinkman model as the governing equation for the porous media flow field was performed. In the numerical simulation, the IEL was modeled as an impermeable barrier to water flux, except for the fenestral pores, which were uniformly distributed over the IEL. The tunica media was modeled as a heterogeneous medium composed of a periodic array of cylindrical smooth muscle cells (SMCs) embedded in a fiber matrix simulating the interstitial proteoglycan and collagen fibers. A series of calculations was conducted by varying the physical parameters describing the problem: the area fraction of the fenestral pore (0. 001-0.036), the diameter of the fenestral pore (0.4-4.0 microm), and the distance between the IEL and the nearest SMC (0.2-0.8 microm). The results indicate that the value of the average shear stress around the circumference of the SMC in the immediate vicinity of the fenestral pore could be as much as 100 times greater than that around an SMC in the fully developed interstitial flow region away from the IEL. These high shear stresses can affect SMC physiological function.

A Predictive Model for Reservoir Distribution in the Permian (Leonardian) Clear Fork and Glorieta Formations, Robertson Field Area, West Texas

Through the collaborative efforts of a multidisciplinary team, various subsurface data types have been integrated to produce a conceptual geologic model for reservoir prediction within the upper Clear Fork and Glorieta formations of the Robertson field area, west Texas. Detailed description of 1434 m of core, 109 thin sections, and 241 line-kilometers of 2-D (two-dimensional) seismic indicates the stratigraphic interval accumulated as a progradational succession of platform-interior subtidal and intertidal facies. Reservoir intervals preferentially occur within subtidal facies having intercrystalline porosity associated with replacement by coarsely crystalline dolomite. Intertidal facies were replaced by a finely crystalline phase of fabric-preserving dolomite and are dominated by an ineffective fenestral pore system. Facies and reservoir distribution is largely controlled by antecedent topography. Structural highs generated during the terminal phase of Ouachita-Marathon compression were the preferred site of intertidal facies deposition. Adjacent structural lows have a higher proportion of reservoir-prone subtidal facies. Analysis of compensated-neutron log porosity indicates that subtidal-prone intervals may be broadly characterized as having less than 11% porosity, whereas intertidal-prone intervals generally exceed 11% porosity. From this observation, a simple computer algorithm allows facies interpretation within wells lacking core. Intertidal-subtidal facies ratio maps generated from this algorithm closely match the distribution of pre-Wolfcampian structural elements and present-day structure. Intertidal facies are more common across the crest of structural highs. Future drilling in the North Jenkins area should pursue structural-flank positions where subtidal facies are volumetrically more abundant. Care should be taken, however, to avoid completions below the composite oil-water contact. Subtidal facies are compartmentalized within individual sequences of a progradational sequence set. Progradational stacking and compactional drape over deeper structures produces discrete flow units with potentially independent oil-water contacts.

Structural changes in rat aortic intima due to transmural pressure.

Huang et al. (1997) propose a new hypothesis and develop a mathematical model to explain rationally the in vitro and in situ measured changes (Tedgui and Lever, 1984; Baldwin and Wilson, 1993) in the hydraulic conductivity of the artery wall of rabbit aorta with transmural pressure. The model leads to the intriguing prediction that this hydraulic conductivity would decrease by one half if the thin intimal layer between the endothelium and the internal elastic lamina volume-compresses approximately fivefold. This paper presents the first measurements of the effect of transmural pressure on intimal layer thickness and shows that the intimal matrix is, indeed, surprisingly compressible. We perfusion-fixed rat thoracic aortas in situ with 2 percent glutaraldehyde solution at 0, 50, 100, or 150 mm Hg lumen pressure and sectioned for light and electron microscopic observations. Electron micrographs show a dramatic, nonlinear decrease in average intimal thickness, i.e., 0.62 +/- 0.26, 0.27 +/- 0.14, 0.15 +/- 0.10, and 0.12 +/- 0.07 (SD) micron for 0, 50, 100, and 150 mm Hg lumen pressure, respectively. The volume strain of the intima is more than 20 times greater than the radial strain of the artery wall due to hoop tension and two orders of magnitude greater than the consolidation of the artery wall as a whole assuming constant medial density (Chuong and Fung, 1984). Moreover, in both light and electron microscopic observations, it is easy to find numerous sites where the endothelium puckers into the fenestral pores at high lumen pressure, as predicted by the theory in Huang et al. (1997). In contrast, the average diameter of a fenestral pore increases only 10 percent as the lumen pressure is increased from 0 to 150 mm Hg. These results indicate that the thin intimal layer comprising less than 1 percent of the wall thickness can have a profound effect on the filtration properties of the wall due to the large change in Darcy permeability of the layer and the large reduction in the entrance area of the flow entering the fenestral pores, though the pores themselves experience only a minor enlargement due to hoop tension.

Glomerular endothelial changes in cyclosporine A-treated rats: Scanning and transmission electron microscopic studies

This study focused on the glomerular structural changes observed after cyclosporine A (CsA)-induced nephrotoxicity. Renal structural changes were examined in rats treated with oral CsA, given as a daily dose of 50 mg/kg for periods of up to 49 days. By means of scanning electron microscopy and morphometry, we first demonstrated that the most conspicuous and reproducible ultrastructural changes could be detected in the endothelial cells of the glomerular capillaries. These changes included a reduction in the fenestral pore size and partial disappearance of endothelial fenestration, the appearance of microvilli-like projections on the endothelial surface, and flattening and widening of the cytoplasmic folds. We believe that the ultrastructural changes observed in this study are partially responsible for the alterations in renal function seen in the cyclosporine A-treated model, and that these alterations are caused by CsA-induced vasospasm.

Facies and Reservoir Characterization of an Upper Smackover Interval, East Barnett Field, Conecuh County, Alabama: ABSTRACT

Facies and Reservoir Characterization of an Upper Smackover Interval, East Barnett Field, Conecuh County, Alabama: ABSTRACT

Endothelial fenestral diaphragms: a quick-freeze, deep-etch study.

The route by which water, solutes, and macromolecules traverse the endothelial cell has long been a subject of study for both physiologists and cell biologists. Recent physiologic studies describe a slit-shaped pore (5.1-5.7-nm wide) as the communicating channel, although no channel of such dimensions has been visible in electron microscopic preparations. That this channel should be found within the fenestral diaphragm has long been suggested. In this report, by the aid of a new technique in tissue processing, we are able to demonstrate a possible morphologic correlate within the fenestral diaphragm of fenestrated capillaries. Quick-freezing and deep-etching of whole tissue blocks allows the sublimation of water from the endothelial pores, thus leaving the channels through the diaphragms empty and readily replicated with a platinum-carbon shadow. The structure of the diaphragm was revealed thus to be composed of radial fibrils of 7 nm in diameter, interweaving in a central mesh, and creating by their geometric distribution, wedge-shaped channels around the periphery of the pore. The average channel had a maximum arc length of 5.46 nm. Fenestrated endothelia from various tissues, including endocrine and exocrine pancreas, adrenal cortex, and kidney peritubular capillaries, displayed the same diaphragmatic structure, whereas continuous capillaries in muscle had no such diaphragm. Photographic augmentation of electron micrographs of etched replicas displayed marked enhancement at n = 8, confirming an octagonal symmetry of the fenestral diaphragm. Finally, cationic ferritin, clearly visible as a marker after etching, heavily bound to the flowerlike structure within the fenestral pore. We conclude that the fenestral diaphragm contains the structure responsible for fenestrated capillary permeability and that the communicating channel has the shape of a wedge.

Depositional Environments and Their Relation to Porosity in Upper Smackover Formation (Jurassic), Paup Spur Field, Miller County, Arkansas: ABSTRACT

Cores from 13 upper Smackover wells in the Paup Spur field, Miller County, Arkansas, and adjacent areas have been studied to define depositional facies from sedimentary structures and carbonate textures. A depositional model is developed from these data. Petrographic studies of thin sections show the relation of diagenetic events to facies, and help unfold the history of porosity development. The vertical succession of facies in the upper Smackover Formation may be interpreted as an upward-shoaling sequence. From the base, burrowed and oncolite-bearing pelmicrite is overlain by burrowed and bimodally cross-bedded oosparite and pelsparite. These in turn are overlain by algal biolithite and pelsparite. Shale and anhydrite of the Buckner Formation overlie the upper Smackover. Contacts between all facies are gradational. A depositional model based on this vertical sequence consists of low-energy peloidal carbonate mud deposited seaward of a moderate to high-energy oolite and pelletal-shoal complex. Bimodal cross-bedding indicates tidal influence on the shoal. Landward of the shoal complex, algal mats and peloidal mud were deposited in a low-energy intertidal environment. Farther landward, sabkha evaporites of the Buckner Formation were deposited. Porosity is highest in the cross-bedded oosparite and pelsparite facies (greater than 25%), where molds of oolites and pellets are the principal pore types. In the algal biolithite facies, porosity reaches 15% and is principally moldic, interparticle, and fenestral pore space. Some interparticle porosity in the algal facies may result from dissolution of anhydrite cement. End_of_Article - Last_Page 495------------