Electron microscopy of the red pulp of human spleen

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Abstract
Terminating arterial vessels, the structure of sinuses and cords, and the passage of cells through the sinus wall in the red pulp of human spleen were studied. All terminating arterial capillaries arterial capillaries observed opened into cords. The distance between terminating arterial capillaries and sinuses varied. Macrophages were commonly present at arterial terminations. Arterial capillary endothelium contained filaments about 80 Å in diameter. Blood cells were frequently present in the capillary lumen or in passage through the capillary wall into cords. Endothelial cells of sinuses contained three distinctive structures: loosely organized cytoplasmic filaments, tightly organized finer filaments, and micropinocytotic vesicles. Many micropinocytotic vesicles about 0.1 μ in diameter were present just beneath the plasma membrane of the lateral and luminal sides of sinus endothelial cells and a few at the basal aspect. Loosely organized filaments about 80 Å in diameter ran parallel to the longitudinal axis of the sinus endothelium. The finer filaments about 30–50 Å in diameter were tightly organized as filamentous bands and present basally. The filaments of the bands appeared inserted upon the plasma membrane. They were also present in the cordal reticular cells and terminating arterial capillaries. Free cells were frequently present in passage through the slits of the sinus wall. There were no preformed or fixed apertures in the sinus wall. The basement membrane and reticular fibers were completely covered by the endothelial cells and/or cordal reticular cells.It is likely that those slits between endothelial cells in the sinus wall not covered by the basement membrane are potential passageways for cells moving from the cords into the sinus. The larger cytoplasmic filaments are likely contractile. The filamentous bands appear to maintain cell shape, stabilize the wall in relation to the basement membrane, and are probably operative in the control of cellular passage through the slits of sinus wall.

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