Dynamics of red cell egress from bone marrow after blood letting

To study the dynamics of red cell egress from the marrow, the demand for red cell delivery was enhanced in splenectomized rats by removing 25% of the blood volume, which was then immediately replaced by plasma. Within 15 min, packed cell volume declined by 22% of the initial value. Corrected reticulocyte count followed a biphasic pattern: an initial peak amounting to 26% occurred within 4-6 h followed by a more marked peak after 48 h. Quantitative electron microscopy of the marrow sinus wall indicated significant increases in the numbers of reticulocytes and leucocytes in transit after 5 h. Vesicles containing a flocculant substance were seen in the endothelium. The vesicles could cause segmental destabilization of endothelium, providing suitable areas for cell migration. There was also a significant reduction in the mean length of the overlapping segments of the endothelial cells, suggesting that these cells slid over one another to increase the luminal calibre. Within the rigid confines of bones where the marrow volume is fixed, this increase may lead to the displacement of mature haemopoietic cells into the lumen and whence into the circulation.