The initiation of post-capillary margination of leukocytes: studies in vitro on the influence of erythrocyte concentration and flow velocity.

Leukocyte margination is an important rheological phenomenon, being a prerequisite for leukocyte adhesion and invasion of the tissues in inflammation. In the present study the influence on leukocyte radial distribution of erythrocyte concentration and flow velocity was studied with fluorescence microscopy in a glass capillary model. This consisted of a narrow (10 microns) short stenosis expanding into a 50 microns straight tube. The blood cells were brought to the centre of the stream by the stenosis whereas the radial position of the leukocytes was determined in the straight post-stenotic segment of the glass tube. The analyses showed that a 0 hematocrit 86% (flow velocity 0.2 mm/s) to 99% (flow velocity 1.2 mm/s) of the leukocytes stayed in the centre of the tube after leaving the stenosis. No leukocytes were observed at the capillary wall. At 10% hematocrit and low flow velocity, 0.1 and 0.3 mm/s, 36 and 34%, respectively, of the leukocytes were displaced to a marginal position. At 1.0 mm/s only 10% of the leukocytes were displaced from the central stream. At 40% hematocrit, 45, 47 and 40% of the leukocytes were found in a wall near position at 0.1, 0.4 and 0.7 mm/s flow velocity, respectively. Although the results obtained in the present study are valid only for the special geometry of the in vitro model they clearly indicate the importance of leukocyte-erythrocyte interactions for the initiation of leukocyte margination in post-capillary vessels.