Aggregate formation of erythrocytes in postcapillary venules

Abstract

The purpose of the present study was to obtain information on erythrocyte aggregate formation in vivo. The movements of erythrocytes in postcapillary venules of the rat spinotrapezius muscle at various flow rates were recorded with a high-speed video camera before and after infusion of dextran 500. To distinguish aggregates, the following criteria were used: 1) a fixed distance (4 m) between the center points of two adjacent cells, 2) lack of visible separation between the adjacent cells, and 3) movement of the adjacent cells in the same direction. Without dextran 500 infusion, 11 and 5% of erythrocytes formed aggregates in low (33.2 Ϯ 28.3 s) and high pseudoshear (144.2 Ϯ 58.3 s) conditions, respectively, based on the above criteria. After dextran 500 infusion, 53% of erythrocytes satisfied the criteria in the low pseudoshear condition (26.5 Ϯ 17.0 s) and 13% of erythrocytes met the criteria in the high pseudoshear condition (240.0 Ϯ 85.9 s), indicating erythrocyte aggregation is strongly associated with shear rate. Approximately 90% of aggregate formation occurred in a short time period (0.15-0.30 s after entering the venule) in a region 15 to 30 m from the entrance. The time delay may reflect rheological entrance conditions in the venule.hemodynamics; in vivo blood rheology; in vivo microscopy RED BLOOD CELL AGGREGATION is a prominent feature in humans and other athletic species but not sedentary species (30). Numerous in vitro studies have shown that aggregation of blood increases as shear rate decreases. Aggregation also depends on hematocrit and the concentration of macromolecules in the plasma or suspending medium (7,10,15,16,19,25,32). However, the circumstances in which aggregation occurs in vivo and its possible importance in circulatory function are not well understood.Previous whole organ studies in the dog and cat, in which red blood cell aggregation normally occurs, have shown that venous vascular resistance in skeletal muscle increases as arterial pressure and blood flow rate decrease and provided evidence that this effect is dependent on red blood cell aggregation (11,23,24,35). From these studies, it was postulated that aggregate formation increased resistance to flow in the venular network. Recently, Bishop and co-workers (9) reported that, in rodents treated with high molecular mass dextran to induce aggregation, velocity profiles of blood flow in venules became blunted at low flow rates and red blood cells formed aggregates, which would explain, in part, the dependence of venous resistance on flow rate.

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

“…Microcirculation has been successfully studied using optical microscopy in various animal models (e.g., rabbit or mouse ear, hamster or mouse dorsal skin-flap window or skin-fold chamber, or open cremaster muscle) [52][53][54][55]59,63,[70][71][72][73] . The use of these models for high-resolution imaging of individual flowing or static cells may be somewhat limited because of significant light scattering from surrounding tissue (e.g., skin or muscles) and/or the relatively deep location of vessels below the skin.…”

“…The monitoring of single RBCs and their small aggregates has usually been performed in two modes: 1) in selected microvessels (small venules or capillaries) with slow flow using a frame rate of 20-30 frames per second (fps), or 2) using a short time-exposure mode (∼0.1-1 ms) by which only single images of fast-moving cells can be captured [56][57][58] . Only a few studies demonstrated relatively high-speed imaging for measuring flow velocity in the range of 750-2500 fps in blood flow, and approximately 500 fps in lymph flow [19,26,59] . Due to motion distortion, these speeds are not quite fast enough to image shape and subcellular structures of individual fast-moving cells.…”

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

“…Vital microscopy, mainly transmission microscopy, has produced images of single cells in selected superficial blood microvessels with reasonable spatial ($0.8-2 mm) and temporal (10 À3 s) resolution [Rosenbaum et al, 2002;Kim et al, 2005;Lipowsky, 2005]. However, to date, images of single cells in fast flow ($1-5 mm/s) are obtained during one short exposure only (i.e., no successive high-speed framing), while continuous imaging (i.e., successive framing) can be realized only in relatively slow flow (<0.1 mm/s).…”

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

“…In a recent study (11), we examined the aggregation process in postcapillary venules, which is the site in the venous network where it first appears. In that study, we showed that aggregation occurs in a localized region of the postcapillary venule, ϳ15-30 m from the vessel entrance.…”