The role of the sinus wall in the passage of erythrocytes through the spleen.

The passage of erythrocytes through the walls of splenic sinuses was studied in rats treated with phenylhydrazine. The sinus wall contained no preformed apertures. Instead, erythrocytes and other cells passing through the spleen entered interendo-thelial slits and squeezed through. The presence of phenylhydrazine-induced Heinz bodies within erythrocytes impeded their transmural passage. Normally, few erythro-cytes were found in transit across the walls of sinuses. However, after phenylhydrazine, many of the interendothelial slits in sinuses were occupied by damaged erythrocytes. Virtually all of the interendothelial slits in sinuses appeared capable of passing eryth-rocytes. The slits seldom exceeded 0.2-0.5 in width, even with an erythrocyte in passage. Bands of intraendothelial micro-filaments running alongside the interendo-thelial slits appeared to have a major role in causing the slits to remain narrow. Thus. the sinus wall, reinforced by microfilament-ous bands, significantly controlled the circulation of erythrocytes through the spleen. Normal red cells were pliant enough to squeeze through, but cells containing Iargs rigid inclusions were held in the slits or delayed in their passage. When many of the slits were occupied by cells slow in passage , the outflow track became blocked, and circulation through the spleen was impeded. Therefore, cells pooled in the cords, macrophages increased in number, and the sequences initiated that resulted in splenomegaly. T HE MAMMALIAN SPLEEN selectively removes certain aged, damaged, or abnormal red blood cells from the circulation.' The sequestering mechanisms that are distinctive to the spleen, are believed to depend on its distinctive vasculature and reticulum.2'3 The electron microscope studies of the spleen in hemoglobin H disease,4 after partial splenectomy,5 and in animals treated with phenylhydrazine6 strongly suggest the wall of the splenic sinus as a critical site in the control of erythrocyte passage through red pulp. There are no preformed apertures in the sinus wall.7 Instead, slits between the sinus endothelial cells, which are otherwise closed, are widened while cells pass through thm. Even when widened, the slits remain narrow passages, so that an erythrocyte must be pliant in order to squeeze through them. In this paper, we present electron microscopic findings on Heinz body-bearing erythrocytes crossing the sinus wall in rats treated with phenyl-hydrazine. We demonstrate the extent of the surface of the sinus wall capable of permitting cellular passage, provide additional evidence that the inter