Quantitative evaluation of leukocyte dynamics in retinal microcirculation

Leukocyte rheology may play a key role in microcirculation because leukocytes have unique properties, such as large cell volume, high cytoplasmic rigidity, and low deformability. However, only a few methods are available to study the dynamic behavior of leukocytes in retinal microcirculation. The authors developed a new method to analyze directly movements of leukocytes in the retinal vessels of primates.

Acridine orange, which has been used as a nuclear stain in histochemistry and cytochemistry, was injected intravenously into cynomolgus monkeys for a vital staining of leukocytes. The fundus image was generated with the argon blue laser and a scanning laser ophthalmoscope. The images were recorded on a magnetic tape and evaluated with a personal computer-based image analysis system.

Each leukocyte was recognized as a single fluorescent dot moving in the retinal vessels. It was possible to analyze the spatial and temporal dynamics of individual leukocytes in the capillaries. Some leukocytes passed through the capillaries, plugging transiently under the physiological condition. Leukocytes that stayed in the same position for a few minutes may have stuck to the endothelium as a result of leukocyte-endothelial interactions. In the postcapillary vessels, leukocytes tended to be displaced from the center stream toward the vessel walls. The mean flow velocity of leukocytes in the perifoveal capillary was 0.92 +/- 0.32 mm/sec.

This study clearly demonstrated that rheologic behaviors of leukocytes in the retinal microcirculation can be studied through the vital staining with acridine orange in vivo. The authors' results suggest a potential role of leukocytes in retinal vascular flow disturbances. This study may open the door to the investigation of leukocyte hemodynamics in the retinal microcirculation in vivo.