The Pathologic Anatomy of the Splenic Red Pulp

Summary

The splenic red pulp is composed of splenic sinuses, which functionally represent the rapid compartment of the splenic circulation, and of the pulp cords which represent its slow compartment. Abnormal cellular elements that enter the pulp cords by way of the terminal arterioles and arterial capillaries may be retained or delayed in their passage through the so-called microcirculation which is interposed between the terminal arterial capillaries and the sinuses. Sequestration of abnormal red cells in the cordal compartment of the spleen is due to decreased plasticity (or increased rigidity) which prevents their easy passage through the walls of sinuses, and perhaps to other membrane abnormalities which make them vulnerable to phagocytic digestion.

Hereditary spherocytosis is the best studied example of massive retention of physiologically abnormal red blood cells in the cords. This also occurs in hereditary elliptocytosis, autoimmune hemolytic anemia with spherocytosis and the sickle-cell hemoglobinopathies. The spherocytes are particularly sensitive to the adverse metabolic conditions in the cordal compartment and undergo early increase in membrane permeability, loss of hemoglobin, and destruction and phagocytosis by the macrophages. Eventually the increased demand for macrophages is met by proliferation of histiocytes or immigration of monocytes, which contributes another factor to the splenomegaly in hereditary spherocytosis. Thus, the primary defect is in the red cells, and macrophages accumulate only in response to an increased phagocytic challenge. In contrast, widening of the pulp cords from proliferation of macrophages may per se be a cause of premature destruction of red blood cells, usually normal ones (secondary hypersplenism). Gaucher’s disease and other histiocytic proliferations may cause such severe widening of the cords that the passage of blood cells from the terminal arterial capillaries into the sinuses is retarded. This results in prolonged exposure of the red blood cells and other formed elements of the blood to an unfavorable metabolic and cellular environment and to their premature destruction. The retardation may be further enhanced by the presence of cordal fibrosis such as one observes in portal hypertension. The unique structure of the microcirculation of the red pulp is not duplicated in any other organ and readily explains why, under certain conditions, the spleen is the only site of destruction of formed elements of the blood. A thorough knowledge of the histologic alterations of the splenic red pulp in various deceased processes greatly contributes to an understanding of why splenectomy is beneficial in some hematologic diseases, of no effect in others, and occasionally even detrimental.

Supported in part by Training Grant CA-05183 from the National Cancer Institute, National Institutes of Health, United States Public Health Service.

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