The spleen is the largest filter of blood in the body. The major functions of the spleen are to remove aging red blood cells, to participate in innate immunity, and to promote adaptive immune response. The branching arterial system in the spleen is sheathed by lymphoid tissue (white pulp) and ends in a venous sinus system (red pulp). The red pulp, which serves to filter blood and remove senescent erythrocytes, consists of afferent arterial cords (an open blood system lacking an endothelial lining) leading to venous sinuses that are lined by discontinuous endothelium. The white pulp represents the lymphoid region of the spleen and consists of B and T lymphocyte–rich lymphoid sheaths surrounding the arterial system. Despite these common properties, important anatomical differences exist between the spleen of humans and rodents. Thus, caution is required when extrapolating results from one species to another. The goal of this chapter is to discuss the properties of endothelial cells (ECs) that line the various vascular beds of the spleen, and where necessary to point out the differences between the human and rodent architecture.

HISTORICAL BACKGROUND, OR, THE MYSTERII PLENUM ORGANON

For the better part of history, the spleen was attributed with the function of removing agents that would prove harmful to the individual, thus reducing contamination of the healthy blood. In contradistinction to Aristotle, Galen argued that the liver, and not the heart, was the principal organ providing the body with strength and spirit. Galen held that the spleen contained black bile and was responsible for controlling emotions by eliminating sadness and melancholy. Remarkably, this interpretation of splenic function would prevail until the mid-18th century.