Aggregate formation of erythrocytes in postcapillary venules

Sialylated Glycan Bindings from SARS-CoV-2 Spike Protein to Blood and Endothelial Cells Govern the Severe Morbidities of COVID-19

Consistent with well-established biochemical properties of coronaviruses, sialylated glycan attachments between SARS-CoV-2 spike protein (SP) and host cells are key to the virus's pathology. SARS-CoV-2 SP attaches to and aggregates red blood cells (RBCs), as shown in many pre-clinical and clinical studies, causing pulmonary and extrapulmonary microthrombi and hypoxia in severe COVID-19 patients. SARS-CoV-2 SP attachments to the heavily sialylated surfaces of platelets (which, like RBCs, have no ACE2) and endothelial cells (having minimal ACE2) compound this vascular damage. Notably, experimentally induced RBC aggregation in vivo causes the same key morbidities as for severe COVID-19, including microvascular occlusion, blood clots, hypoxia and myocarditis. Key risk factors for COVID-19 morbidity, including older age, diabetes and obesity, are all characterized by markedly increased propensity to RBC clumping. For mammalian species, the degree of clinical susceptibility to COVID-19 correlates to RBC aggregability with p = 0.033. Notably, of the five human betacoronaviruses, the two common cold strains express an enzyme that releases glycan attachments, while the deadly SARS, SARS-CoV-2 and MERS do not, although viral loads for COVID-19 and the two common cold infections are similar. These biochemical insights also explain the previously puzzling clinical efficacy of certain generics against COVID-19 and may support the development of future therapeutic strategies for COVID-19 and long COVID patients.

Hemorheological alterations following an acute bout of nordic hamstring exercise in active male participants1

BACKGROUND

The Nordic hamstring exercise (NHE) has been proven to be an effective preventive technique for hamstring injuries. Hemorheological parameters (erythrocyte deformability and aggregation) play a critical role in exercise influencing oxygenation. Although previous studies presented hemorheological alterations induced by different types of exercise, changes in red blood cell (RBC) deformability and aggregation following NHE remain unknown. Present study was designed to explore possible alterations in hemorheological and oxidative parameters after an acute bout of NHE.

METHODS

10 healthy, male, active students (mean age 19.9±0.23, BMI: 21.56±0.54) participated to the study. They performed a single session of seven-repetitions of NHE followed by a familiarisation period. Blood samples were obtained before and immediately after the exercise from the antecubital vein. Hemorheological parameters were measured by an ektacytometer.

RESULTS

NHE did not change deformability, hematocrit and oxidative stress but, increased RBC aggregation index (AI, p = 0.011) and decreased RBC aggregation half time (t½, p = 0.009).

CONCLUSIONS

Our results suggest that, increased RBC aggregation following an acute bout of NHE may result in increased plasma skimming and thus ease the flow of blood.

Red blood cell aggregates and their effect on non-Newtonian blood viscosity at low hematocrit in a two-fluid low shear rate microfluidic system

Red blood cells (RBCs) are the most abundant cells in human blood. Remarkably RBCs deform and bridge together to form aggregates under very low shear rates. The theory and mechanics behind aggregation are, however, not yet completely understood. The main objective of this work is to quantify and characterize RBC aggregates in order to enhance the current understanding of the non-Newtonian behaviour of blood in microcirculation. Suspensions of human blood were flowed and observed in vitro in poly-di-methyl-siloxane (PDMS) microchannels to characterize RBC aggregates. These microchannels were fabricated using standard photolithography methods. Experiments were performed using a micro particle image velocimetry (μPIV) system for shear rate measurements, coupled with a high-speed camera for flow visualization. RBC aggregate sizes were quantified in controlled and measurable shear rate environments for 5, 10 and 15% hematocrit. Aggregate sizes were determined using image processing techniques, while apparent viscosity was measured using optical viscometry. For the samples suspended at 5% H, aggregate size was not strongly correlated with shear rate. For the 10% H suspensions, in contrast, lowering the shear rate below 10 s-1 resulted in a significant increase of RBC aggregate sizes. The viscosity was found to increase with decreasing shear rate and increasing hematocrit, exemplifying the established non-Newtonian shear-thinning behaviour of blood. Increase in aggregation size did not translate into a linear increase of the blood viscosity. Temperature was shown to affect blood viscosity as expected, however, no correlation for aggregate size with temperature was observed. Non-Newtonian parameters associated with power law and Carreau models were determined by fitting the experimental data and can be used towards the simple modeling of blood's non-Newtonian behaviour in microcirculation. This work establishes a relationship between RBC aggregate sizes and corresponding shear rates and one between RBC aggregate sizes and apparent blood viscosity at body and room temperatures, in a microfluidic environment for low hematocrit. Effects of hematocrit, shear rate, viscosity and temperature on RBC aggregate sizes have been quantified.

Influence of red blood cell aggregation on perfusion of an artificial microvascular network

OBJECTIVE

RBCs suspended in plasma form multicellular aggregates under low-flow conditions, increasing apparent blood viscosity at low shear rates. It has previously been unclear, however, if RBC aggregation affects microvascular perfusion. Here, we analyzed the impact of RBC aggregation on perfusion and 'capillary' hematocrit in an AMVN at driving pressures ranging from 5 to 60 cm H₂ O to determine if aggregation could improve tissue oxygenation.

METHODS

RBCs were suspended at 30% hematocrit in either 46.5 g/L dextran 40 (D40, non-aggregating medium) or 35 g/L dextran 70 (D70, aggregating medium) solutions with equal viscosity.

RESULTS

Aggregation was readily observed in the AMVN for RBCs suspended in D70 at driving pressures ≤40 cm H₂ O. The AMVN perfusion rate was the same for RBCs suspended in aggregating and non-aggregating medium, at both 'venular' and 'capillary' level. Estimated 'capillary' hematocrit was higher for D70 suspensions than for D40 suspensions at intermediate driving pressures (5-40 cm H₂ O).

CONCLUSIONS

We conclude that although RBC aggregation did not affect the AMVN perfusion rate independently of the driving pressure, a higher hematocrit in the 'capillaries' of the network for D70 suspensions suggested a better oxygen transport capacity in the presence of RBC aggregation.

A Two-Dimensional Numerical Investigation of Transport of Malaria-Infected Red Blood Cells in Stenotic Microchannels

The malaria-infected red blood cells experience a significant decrease in cell deformability and increase in cell membrane adhesion. Blood hemodynamics in microvessels is significantly affected by the alteration of the mechanical property as well as the aggregation of parasitized red blood cells. In this study, we aim to numerically study the connection between cell-level mechanobiological properties of human red blood cells and related malaria disease state by investigating the transport of multiple red blood cell aggregates passing through microchannels with symmetric stenosis. Effects of stenosis magnitude, aggregation strength, and cell deformability on cell rheology and flow characteristics were studied by a two-dimensional model using the fictitious domain-immersed boundary method. The results indicated that the motion and dissociation of red blood cell aggregates were influenced by these factors and the flow resistance increases with the increase of aggregating strength and cell stiffness. Further, the roughness of the velocity profile was enhanced by cell aggregation, which considerably affected the blood flow characteristics. The study may assist us in understanding cellular-level mechanisms in disease development.

Atomic force microscopy as a tool to evaluate the risk of cardiovascular diseases in patients

The availability of biomarkers to evaluate the risk of cardiovascular diseases is limited. High fibrinogen levels have been identified as a relevant cardiovascular risk factor, but the biological mechanisms remain unclear. Increased aggregation of erythrocytes (red blood cells) has been linked to high plasma fibrinogen concentration. Here, we show, using atomic force microscopy, that the interaction between fibrinogen and erythrocytes is modified in chronic heart failure patients. Ischaemic patients showed increased fibrinogen-erythrocyte binding forces compared with non-ischaemic patients. Cell stiffness in both patient groups was also altered. A 12-month follow-up shows that patients with higher fibrinogen-erythrocyte binding forces initially were subsequently hospitalized more frequently. Our results show that atomic force microscopy can be a promising tool to identify patients with increased risk for cardiovascular diseases.

"Do-it-yourself in vitro vasculature that recapitulates in vivo geometries for investigating endothelial-blood cell interactions"

Investigating biophysical cellular interactions in the circulation currently requires choosing between in vivo models, which are difficult to interpret due in part to the hemodynamic and geometric complexities of the vasculature; or in vitro systems, which suffer from non-physiologic assumptions and/or require specialized microfabrication facilities and expertise. To bridge that gap, we developed an in vitro "do-it-yourself" perfusable vasculature model that recapitulates in vivo geometries, such as aneurysms, stenoses, and bifurcations, and supports endothelial cell culture. These inexpensive, disposable devices can be created rapidly (<2 hours) with high precision and repeatability, using standard off-the-shelf laboratory supplies. Using these "endothelialized" systems, we demonstrate that spatial variation in vascular cell adhesion molecule (VCAM-1) expression correlates with the wall shear stress patterns of vascular geometries. We further observe that the presence of endothelial cells in stenoses reduces platelet adhesion but increases sickle cell disease (SCD) red blood cell (RBC) adhesion in bifurcations. Overall, our method enables researchers from all disciplines to study cellular interactions in physiologically relevant, yet simple-to-make, in vitro vasculature models.

Design of a microfluidic system for red blood cell aggregation investigation

The purpose of this paper is to design a microfluidic apparatus capable of providing controlled flow conditions suitable for red blood cell (RBC) aggregation analysis. The linear velocity engendered from the controlled flow provides constant shear rates used to qualitatively analyze RBC aggregates. The design of the apparatus is based on numerical and experimental work. The numerical work consists of 3D numerical simulations performed using a research computational fluid dynamics (CFD) solver, Nek5000, while the experiments are conducted using a microparticle image velocimetry system. A Newtonian model is tested numerically and experimentally, then blood is tested experimentally under several conditions (hematocrit, shear rate, and fluid suspension) to be compared to the simulation results. We find that using a velocity ratio of 4 between the two Newtonian fluids, the layer corresponding to blood expands to fill 35% of the channel thickness where the constant shear rate is achieved. For blood experiments, the velocity profile in the blood layer is approximately linear, resulting in the desired controlled conditions for the study of RBC aggregation under several flow scenarios.

The effect of red blood cell aggregation on velocity and cell-depleted layer characteristics of blood in a bifurcating microchannel

Red blood cell (RBC) aggregation is a multifaceted phenomenon, and whether it is generally beneficial or deleterious remains unclear. In order to better understand its effect on microvascular blood flow, the phenomenon must be studied in complex geometries, as it is strongly dependent on time, flow, and geometry. The cell-depleted layer (CDL) which forms at the walls of microvessels has been observed to be enhanced by aggregation; however, details of the characteristics of the CDL in complex regions, such as bifurcations, require further investigation. In the present study, a microchannel with a T-junction was used to analyze the influence of aggregation on the flow field and the CDL. Micro-PIV using RBCs as tracers provided high resolution cell velocity data. CDL characteristics were measured from the same data using a newly developed technique based on motion detection. Skewed and sharpened velocity profiles in the daughter branches were observed, contrary to the behavior of a continuous Newtonian fluid. RBC aggregation was observed to increase the skewness, but decrease the sharpening, of the velocity profiles in the daughter branches. The CDL width was found to be significantly greater, with a wider distribution, in the presence of aggregation and the mean width increased proportionally with the reciprocal of the fraction of flow entering the daughter branch. Aggregation also significantly increased the roughness of the interface between the CDL and the RBC core. The present results provide further insight into how RBC aggregation may affect the flow in complex geometries, which is of importance in both understanding its functions invivo, and utilizing it as a tool in microfluidic devices.

Role Ca(2+) in mechanisms of the red blood cells microrheological changes

To assess the physiological role of intracellular Ca(2+) in the changes of microrheological red blood cell (RBC) properties (RBC deformability and aggregation), we employed several types of chemicals that can increase and decrease of the intracellular Ca(2+) concentration. The rise of Ca(2+) influx, stimulated by mechanical loading, A23187, thrombin, prostaglandin F(2α) was accompanied by a moderate red cell deformability lowering and an increase of their aggregation. In contrast, Ca(2+) entry blocking into the red cells by verapamil led to a significant RBC aggregation decrease and deformability rise. Similar microrheological changes were observed in the red blood cells treated with phosphodiesterase inhibitors IBMX, vinpocetine, rolipram, pentoxifylline. When forskolin (10 μM), an AC stimulator was added to RBC suspension, the RBC deformability was increased (p <0.05). Somewhat more significant deformability rise appeared after RBC incubation with dB-AMP. Red cell aggregation was significantly decreased under these conditions (p<0.01). On the whole the total data clearly show that the red cell aggregation and deformation changes were connected with an activation of both intracellular signaling pathways: Ca(2+) regulatory mechanism and Gs-protein/adenylyl-cyclase-cAMP system. And the final red cell microrheological regulatory effect is connected with the crosstalk between these systems.

Erythrocyte aggregation as a cause of slow flow in patients of acute coronary syndromes

BACKGROUND

There are multiple lines of evidence to suggest the role of erythrocyte aggregation (EA) in microcirculatory dysfunction during conditions of very slow flow. Such conditions might develop in the myocardium of patients with acute coronary syndromes (ACS).

METHODS

EA as a function of shear stress was evaluated by using a cell flow properties analyzer (CFA) in a cohort of 91 ACS patients and in 36 patients with non specific chest pain or heart failure at the time of cardiac catheterization.

RESULTS

The ACS group included 34 patients with acute myocardial infarction and 57 patients with unstable angina. In addition, we examined 36 patients who underwent angiography for non specific chest pain or heart failure. A significant (r=0.44, p<0.0005) correlation was found between the concentration of fibrinogen and the average aggregate size (AAS) only when using conditions of very slow flow and applying relatively low (0.15 dyn/cm(2)) shear stress in the ACS group. This correlation decreased and became insignificant when applying shear stress forces of 1 dyn/cm(2) and more. This correlation was nonsignificant for all the 5 shear stress forces (between 0.15 and 4 dyn/cm(2)) in the samples obtained from the non-ACS group.

CONCLUSION

Erythrocytes that are suspended in autologous plasma obtained from patients with ACS tend to aggregate in conditions of very slow flow. These findings might be detrimental in terms of microcirculatory flow in ACS patients and might open new therapeutic options such as the use of low dose thrombolysis following PCI.

A simulation study on photoacoustic signals from red blood cells

A two dimensional simulation study was performed to investigate the photoacoustic signal properties of non-aggregated and aggregated erythrocytes. Spatial distributions of non-aggregated blood samples were generated by employing a Monte Carlo method and aggregated blood samples were simulated using a hexagonal packing scheme. For the non-aggregating case photoacoustic signals demonstrated a monotonic rise with hematocrit. For the aggregating case it was found that spectral (<20 MHz) intensity increased (11 dB at 15.6 MHz) when the aggregate size increased. This study strongly suggests that the assessment of erythrocyte aggregation level in human blood might be possible by using a photoacoustic spectroscopic method.

Dextran coatings for aggregation control of layer-by-layer assembled polyelectrolyte microcapsules

We propose dextran and dextran polyaldehyde (DPA) coatings for modification of layer-by-layer (LbL) assembled polyelectrolyte microcapsules which provide stability against aggregation in 0.75 M aqueous solutions of mono- and bivalent ions (Na(+), Cl(-), Ca(2+), HPO(4)(2-)). The microcapsules were prepared of three bilayers of poly(4-styrenesulfonate) (PSS) and poly(allylamine) (PAH). Dextran and its derivatives were attached to amino-terminated surface of the microcapsules via three types of chemical bonds of subsequently increasing strength: (1) hydrogen bonds, (2) hydrolyzable covalent cross-links resulting from aldehydes and primary amines coupling, and (3) nonhydrolyzable covalent C-N single bonds of secondary amines. Attachment of the DPA materials via the latter two types of bonds resulted in strengthening the capsules' walls which preserved a fraction of the microcapsules from disintegration upon electrostatic swelling in 0.1 M NaOH. The non-disintegrated fraction of the DPA-coated microcapsules restored their initial size after pH was decreased back to neutral. The microcapsules coated with the original dextran immobilized via hydrogen bonds and the bare microcapsules were fully dissolved under the alkaline conditions. The preserved fraction of the microcapsules was higher for the DPA materials with higher contents of the aldehyde groups and after conversion of the hydrolyzable covalent cross-links to the nonhydrolyzable secondary amines via reduction with NaBH(4). The higher contents of the aldehyde groups and the reduction led to the lower limiting swelling degree of the DPA-coated microcapsules at alkaline pH. The proposed coatings can be used for colloid stabilization of polyelectrolyte microcapsules in aqueous medium, encapsulation of pH-insensitive macromolecules at the postpreparation stage, and pH-triggered release of encapsulated material.

Effect of cell-free layer variation on arteriolar wall shear stress

Relationship between a cell-free layer and wall shear stress (WSS) in small arterioles has been of interest in microcirculatory research. However, influence of temporal variation in the cell-free layer width on the WSS in vivo has not been fully elucidated. In this study, we tested the hypothesis that the layer variation would increase the WSS, and this effect would be enhanced by red blood cell aggregation. The cell-free layer width in arterioles (29.5–67.1 μm ID) in rat cremaster muscles were obtained with a high-speed video camera, and the layer width data were introduced into WSS estimation. Dextran 500 was administrated to elevate the aggregation level of red blood cells to those seen in normal human blood. The variation of the layer was quantified by the variability (coefficient of variation), and its effect on WSS was studied under normal and reduced flow conditions. We found that the dextran-induced red blood cell aggregation significantly elevated the variability (p < 0.01) at low pseudoshear rates of 9.2 ± 0.6 s⁻¹. The WSS estimated without taking account of the variability showed underestimation of its value than that of with consideration of the variability under all flow conditions, and this effect became more pronounced with increasing the variability. The variation of the cell-free layer should, therefore, be considered in the determination of the WSS particularly in the presence of red blood cell aggregation under reduced flow condition.

Computational fluid dynamics of aggregating red blood cells in postcapillary venules

Aggregate formation of red blood cells (RBCs) in a postcapillary venular bifurcation is investigated with three-dimensional computer simulations using the Chimera grid method. Interaction energy between the RBCs is modelled by a depletion interaction theory; RBCs are modelled as rigid oblate ellipsoids. The cell-cell interactions of RBCs are strongly dependent on vessel geometry and shear rates. The experimental data on vessel geometry, pseudoshear rates, and Dextran concentration obtained in our previous in vivo RBC aggregation study in postcapillary venules of the rat spinotrapezius muscle were used to simulate RBC aggregation. The computational results were compared to the experimental results from the in vivo study. The results show that cells have a larger tendency to form an aggregate under reduced flows. Aggregate formation also depends on the angle and location of the cells before they enter the bifurcation region. Comparisons with experimental data are discussed.

Determinants of leukocyte margination in rectangular microchannels

Microfabrication of polydimethylsiloxane (PDMS) devices has provided a new set of tools for studying fluid dynamics of blood at the scale of real microvessels. However, we are only starting to understand the power and limitations of this technology. To determine the applicability of PDMS microchannels for blood flow analysis, we studied white blood cell (WBC) margination in channels of various geometries and blood compositions. We found that WBCs prefer to marginate downstream of sudden expansions, and that red blood cell (RBC) aggregation facilitates the process. In contrast to tubes, WBC margination was restricted to the sidewalls in our low aspect ratio, pseudo-2D rectangular channels and consequently, margination efficiencies of more than 95% were achieved in a variety of channel geometries. In these pseudo-2D channels blood rheology and cell integrity were preserved over a range of flow rates, with the upper range limited by the shear in the vertical direction. We conclude that, with certain limitations, rectangular PDMS microfluidic channels are useful tools for quantitative studies of blood rheology.

Neuraminidase alters red blood cells in sepsis

OBJECTIVE

To investigate the influence of neuraminidase, an enzyme that cleaves sialic acid from the red blood cell (RBC) membrane, on RBC shape and biochemistry in critically ill patients.

DESIGN

Prospective, observational study and in vitro laboratory study.

SETTING

A 31-bed medico-surgical department of intensive care and a university-affiliated cell biology laboratory.

SUBJECTS

Acutely ill patients with and without sepsis and healthy volunteers.

INTERVENTIONS

Blood sampling in volunteers.

MEASUREMENTS AND MAIN RESULTS

Neuraminidase activity was measured using a fluorescent assay. RBC shape was assessed by the second coefficient of dissymmetry of Pearson using a flow cytometry technique at 25 degrees C. Intraerythrocytic 2,3-diphosphoglycerate and lactate contents were also measured. Neuraminidase activity was significantly higher in septic patients compared with nonseptic patients and healthy volunteers (5.42 [4.85-6.00] vs. 4.53 [4.23-5.23] and 1.26 [0.83-1.83] mU/mL; all p < 0.05). Neuraminidase treatment modified the RBC shape in vitro in a dose-response fashion, and most of these alterations were present after 10 hours of incubation. Incubation of RBCs with phosphatidylinositol phospholipase C modified RBC shape and increased sialic acid concentrations in the supernatant, suggesting a leakage of neuraminidase from the RBC membrane. Alterations in shape were associated with increased 2,3-diphosphoglycerate (0.46 +/- 0.25 vs. 0.19 +/- 0.05 mumol/mL; p = 0.006) and lactate content (0.81 +/- 0.07 vs. 0.66 +/- 0.05 mmoL/L; p = 0.002).

CONCLUSIONS

In sepsis, desialylation under the influence of increased neuraminidase activity may contribute to the alterations in RBC rheology. Inhibition of neuraminidase may represent a new therapeutic option to ameliorate RBC rheology and perhaps oxygen delivery to the cells.

Increased strength of erythrocyte aggregates in blood of patients with inflammatory bowel disease

BACKGROUND

Increased strength of red blood cell (RBC) aggregates are present during the acute inflammatory response and contribute to erythrocyte aggregation and may lead to microvascular dysfunction. Inflammatory bowel diseases (IBDs) are characterized by damage to the bowel wall. This damage may be at least partially attributed to microvascular ischemia caused by enhanced erythrocyte aggregation. The aim of this study was to evaluate the strength of RBC aggregates in the blood of patients with IBD.

METHODS

The strengths of RBC aggregates were characterized by integrative RBC aggregation parameters, determined by measuring of RBC aggregation as a function of shear stress. The results are represented as the area under the curve (AUC) of aggregate size plotted against shear stress. For each patient, dynamic aggregation and disaggregation of RBC were recorded and analyzed according to the RBC aggregate size distribution at the different shear stresses. Aggregation indices were correlated with disease activity and inflammatory biomarkers.

RESULTS

We examined 53 IBD patients and 63 controls. IBD patients had significantly elevated concentrations of inflammation-sensitive proteins and aggregation parameters. The strength of large aggregates, represented by AUC for large fraction aggregates, among patients (15.2 +/- 18.6) was double that of controls (7 +/- 10.9) (P = 0.006). The strength of large aggregates correlated with disease activity (r = 0.340; P < 0.001) with concentration of fibrinogen (r = 0.575; P < 0.001) and with concentration of high sensitivity C-reactive protein (r = 0.386; P < 0.001).

CONCLUSIONS

The strength of RBC aggregates is increased in patients with IBD and correlates with the intensity of the acute phase response. This could contribute to bowel damage in these diseases.

Morphology of small aggregates of red blood cells

Blood can be considered a two-phase liquid composed of plasma as well as cells and cell aggregates. The degree of cell aggregation is an important determinant of blood rheology: The size and shape of the aggregates affect blood viscosity. The microscopic mechanisms of red blood cell adhesion involve a complex interplay of electrostatic, van der Waals, and a range of specific biochemical inter-membrane interactions. Here we use an effective model of these interactions combined with the membrane elasticity theory to calculate the equilibrium shape of a red blood cell doublet and compare it with the experimentally observed red blood cell aggregates both in vitro and in vivo. Special attention is devoted to the shape of doublets formed by dissimilar cells. A possible effect of doublet shape on pathways of the formation of multicellular aggregates is discussed. Red blood cell rouleau formation is expected to take place at intermediate adhesion strengths where the outer doublet surfaces are either concave or flat, whereas in the strong-adhesion regime where the outer doublet surfaces are convex the cells should form rounded clump-like aggregates.

On the effect of dynamic flow conditions on blood microstructure investigated with optical shearing microscopy and rheometry

Red blood cell (RBC) aggregation affects significantly the flow of blood at low shear rates. Increased RBC aggregation is associated with various pathological conditions; hence an accurate quantification and better understanding of the phenomenon is important. The present study aims to improve understanding of the effect of dynamic flow conditions on aggregate formation; whole blood samples from healthy volunteers, adjusted at 0.45 haematocrit were tested in different flow conditions with a plate-plate optical shearing system, image analysis, and a double-walled Couette rheometric cell. Results are presented in terms of aggregation index Aa, aggregate size index As and number of aggregates, which are shown to vary with shear rate γ and with different shear rate variations with time γ. The aggregation index Aa was observed to increase as the shear rate decreased between 10 and 3 s−1. Above 10 s−1, Aa was found to have a minimum value indicating minimal aggregation while, at approximately 3 s−1, Aa reaches a maximum. The aggregation size index As, the number of aggregates, and the blood viscosity were found to vary considerably when the same sample was examined over the same shear rate range, but for different variations of shear rate with time, γ.

Temporal and spatial variations of cell-free layer width in arterioles

Separation of red blood cells and plasma in microcirculatory vessels produces a cell-free layer at the wall. This layer may be an important determinant of blood viscosity and wall shear stress in arterioles, where most of the hydraulic pressure loss in the circulatory system occurs and flow regulatory mechanisms are prominent. With the use of a newly developed method, the width of the cell-free layer was rapidly and repeatedly determined in arterioles (10- to 50-μm inner diameter) in the rat cremaster muscle at normal arterial pressure. The temporal variation of the cell-free layer width was non-Gaussian, but calculated mean and median values differed by <0.2 μm. The correlation length of the temporal variations downstream (an indication of mixing) was ∼30 μm and was independent of pseudoshear rate (ratio of mean velocity to vessel diameter) and of vessel diameter. The cell-free layer width was significantly different on opposite sides of the vessel and inversely related. Increasing red blood cell aggregability reduced this inverse relation but had no effect on correlation length. In the diameter range studied, the mean width of the cell-free layer increased from 0.8 to 3.1 μm and temporal variations increased from 30% to 70% of the mean width. Increased aggregability did not alter either relationship. In summary, the cell-free layer width in arterioles is diameter dependent and shows substantial non-Gaussian temporal variations. The temporal variations increase as diameter increases and are inversely related on opposite sides of the vessel.

Contributions of collision rate and collision efficiency to erythrocyte aggregation in postcapillary venules at low flow rates

Red blood cell aggregation at low flow rates increases venous vascular resistance, but the process of aggregate formation in these vessels is not well understood. We previously reported that aggregate formation in postcapillary venules of the rat spinotrapezius muscle mainly occurs in a middle region between 15 and 30 microm downstream from the entrance. In light of the findings in that study, the main purpose of this study was to test two hypotheses by measuring collision frequency along the length of the venules during low flow. We tested the hypothesis that aggregation rarely occurs in the initial 15-microm region of the venule because collision frequency is very low. We found that collision frequency was lower than in other regions, but collision efficiency (the ratio of aggregate formation to collisions) was almost nil in this region, most likely because of entrance effects and time required for aggregation. Radial migration of red blood cells and Dextran 500 had no effect on collision frequency. We also tested the hypothesis that aggregation was reduced in the distal venule region because of the low aggregability of remaining nonaggregated cells. Our findings support this hypothesis, since a simple model based on the ratio of aggregatable to nonaggregatable red blood cells predicts the time course of collision efficiency in this region. Collision efficiency averaged 18% overall but varied from 0 to 52% and was highest in the middle region. We conclude that while collision frequency influences red blood cell aggregate formation in postcapillary venules, collision efficiency is more important.

Advances in small animal mesentery models for in vivo flow cytometry, dynamic microscopy, and drug screening

Using animal mesentery with intravital optical microscopy is a well-established experimental model for studying blood and lymph microcirculation in vivo. Recent advances in cell biology and optical techniques provide the basis for extending this model for new applications, which should generate significantly improved experimental data. This review summarizes the achievements in this specific area, including in vivo label-free blood and lymph photothermal flow cytometry, super-sensitive fluorescence image cytometry, light scattering and speckle flow cytometry, microvessel dynamic microscopy, infrared (IR) angiography, and high-speed imaging of individual cells in fast flow. The capabilities of these techniques, using the rat mesentery model, were demonstrated in various studies; e.g., real-time quantitative detection of circulating and migrating individual blood and cancer cells, studies on vascular dynamics with a focus on lymphatics under normal conditions and under different interventions (e.g. lasers, drugs, nicotine), assessment of lymphatic disturbances from experimental lymphedema, monitoring cell traffic between blood and lymph systems, and high-speed imaging of cell transient deformability in flow. In particular, the obtained results demonstrated that individual cell transportation in living organisms depends on cell type (e.g., normal blood or leukemic cells), the cell’s functional state (e.g., live, apoptotic, or necrotic), and the functional status of the organism. Possible future applications, including in vivo early diagnosis and prevention of disease, monitoring immune response and apoptosis, chemo- and radio-sensitivity tests, and drug screening, are also discussed.

Application of Chimera grid to modelling cell motion and aggregation in a narrow tube

A computational scheme using the Chimera grid method is presented for simulation of three-dimensional motion and aggregation of two red blood cells (RBCs) in a narrow tube. The cells are modelled as rigid ellipsoidal particles; the computational scheme is applicable to deformable fluid-filled particles. Attractive energy between two RBCs is modelled by a depletion interaction theory and used for simulating aggregation of two cells. Through the simulation, we show that the Chimera grid method is applicable to the simulation of three-dimensional motion and aggregation of multiple RBCs in a microvessel and microvascular network.

In vivo high-speed imaging of individual cells in fast blood flow

In vivo, label-free, high-speed (up to 10,000 with the potential for 40,000 frames per second), high-resolution (up to 300 nm) real-time continuous imaging with successive framing of circulating individual erythrocytes, leukocytes, and platelets in fast blood flow is developed. This technique, used in an animal model, reveals the extremely high dynamic deformability of erythrocytes in natural flow. Potential applications of this technique are discussed with focus on time-resolved monitoring of the cell deformation dynamics in the native biological environment, which may have diagnostic value for the early diagnosis of diseases.

In vivo photothermal flow cytometry: imaging and detection of individual cells in blood and lymph flow

Flow cytometry is a well-established, powerful technique for studying cells in artificial flow in vitro. This review covers a new potential application of this technique for studying normal and abnormal cells in their native condition in blood or lymph flow in vivo. Specifically, the capabilities of the label-free photothermal (PT) technique for detecting and imaging cells in the microvessel network of rat mesentery are analyzed from the point of view of overcoming the problems of flow cytometry in vivo. These problems include, among others, the influences of light scattering and absorption in vessel walls and surrounding tissues, instability of cell velocity, and cells numbers and positions in a vessel's cross-section. The potential applications of this new approach in cell biochemistry and medicine are discussed, including molecular imaging; studying the metabolism and pathogenesis of many diseases at a cellular level; and monitoring and quantifying metastatic and apoptotic cells, and/or their responses to therapeutic interventions (e.g., drug or radiation), in natural biological environments.

A computer-based method for determination of the cell-free layer width in microcirculation

OBJECTIVES

The cell-free layer between the erythrocyte column and the vessel wall is an important determinant of hydrodynamic resistance in microcirculatory vessels. The authors report a method for continuous measurement of the width of this layer.

METHODS

The light intensity of a linear array of pixels perpendicular to the vessel axis is continuously determined from a video image of a microcirculatory vessel. A threshold level based on Otsu's method is used to establish the interface between the cell-free layer and the erythrocyte column. To test the method, video images at 750-4500 frames/s were obtained from venules and arterioles in rat spinotrapezius muscle at normal and reduced arterial pressures before and after induction of erythrocyte aggregation with Dextran 500. The current measurements were compared to manual measurements of the same images.

RESULTS

Values obtained by the manual and the new methods were in agreement within the 95% confidence limit by the Bland-Altman analysis and within 90-95% range by the correlation coefficient (R2). The more frequent measurements reveal substantial, rapid variations in cell-free layer width and changes in mean values with alteration of arterial pressure and red cell aggregability.

CONCLUSIONS

A new, computer-based technique has been developed that provides measurements of rapid, time-dependent variations in the width of the cell-free layer in the microcirculation.

Effect of erythrocyte aggregation at normal human levels on functional capillary density in rat spinotrapezius muscle

Previous studies have shown that functional capillary density (FCD) is substantially reduced by erythrocyte aggregation. However, only supranormal levels of aggregability were studied. To investigate the effect of erythrocyte aggregability at the level seen in healthy humans, the FCD of selected capillary fields in rat spinotrapezius muscle was determined with high-speed video microscopy under normal (nonaggregating) conditions and after induction of erythrocyte aggregation with Dextran 500 (200 mg/kg). To examine shear rate dependence, the effect was studied both at normal and reduced arterial pressures (50 and 25 mmHg), the latter achieved by short periods of hemorrhage. In a separate study, volume flow was determined in arterioles (52.1 ± 3.7 μm) under the same conditions. Before Dextran 500 infusion, FCD fell to 91% and 76% of control values, respectively, when arterial pressure was reduced to 50 and 25 mmHg. After Dextran 500 infusion, FCD was 96% at normal arterial pressure and fell to 79% and 37% of normal control values at 50 and 25 mmHg. All FCD values were significantly lower after dextran infusion. FCD reduction after lowering arterial pressure or dextran infusion appeared to be due to plasma skimming rather than capillary plugging. Reduction of FCD by dextran at reduced pressure was compensated by increased red blood cell flux in capillaries with red blood cell flow. We conclude that the level of aggregability seen in healthy humans is an important determinant of FCD only at reduced arterial pressure.

Plasma dependent reduction in red blood cell aggregation after dextran sulfate low-density lipoprotein apheresis--implications for rheological studies

Red blood cell (RBC) aggregation is increased in familial hypercholesterolemia, and is reduced significantly after low density lipoprotein (LDL) apheresis. The purpose of the present study was to clarify whether this reduction depends on changes in plasma composition, RBC membrane properties, or both. RBC aggregation was determined in a computerized cell flow-properties analyzer, before and after LDL apheresis. We compared RBC aggregation in autologous plasma with aggregation in a plasma-free standard solution (0.5% of dextran 500 kDa) to define the separate contributions of plasma and cellular properties to the observed RBC aggregation. RBC aggregation in autologous plasma was reduced by 35.5% after LDL apheresis (P=0.01) but was not significantly affected when measured in dextran 500. This suggests that LDL apheresis attenuated RBC aggregation by altering plasma composition rather than RBC membrane properties. These results are relevant to the understanding of hemorheological changes which follow therapeutic apheresis in hypercholesterolemic patients.