Red cell egress from bone marrow in state of transfusion plethora.

Marrow sinuses have been shown to undergo rhythmic dilatations and this has been suggested as a mechanism controlling cell egress from the marrow. Within the rigid frame of bone where the volume of the marrow is fixed, dilatation of sinuses can only lead to the displacement of cells into the lumen. The absence of tight junction between sinus endothelial cells is thought to permit the overlapping segments of these cells to slide over one another. To test this hypothesis, the length of overlapping segments between endothelial cells was measured before and after the induction of transfusion plethora and this was correlated with the magnitude of reticulocyte traffic across the sinus wall. Within one hour after the induction of plethora, marrow sinuses were dilated, packed with red cells, and the mean length of overlapping segments was reduced significantly from 0,75 micrometer to 0.41 micrometer. This was associated with a decline in the magnitude of reticulocyte transit across the wall (0.73 vs 0.13/mm). These observations are consistent with the theory that the overlapping segments of sinus endothelial cells can slide over each other to change the luminal caliber. Transfusion plethora results in a sustained maximal dilatation of sinuses, thus preventing their rhythmic changes necessary for cell delivery into the circulation.