Structure and function of sinusoidal endothelium of bone marrow.

The endothelium of marrow sinuses is the site of cellular and molecular exchange between the circulation and the hemopoietic compartment. Here, mature cells are delivered into circulation by passing through the cytoplasm of a single endothelial cell. Similarly, stem cells are probably trapped here. The endothelium which is structurally polarized, and shows segmental surface differentiations, can maintain a gradient for a variety of substances between blood and marrow (the marrow-blood barrier). Transendothelial cell migration involves channel formation through fusion-fission reorganization of the membrane. The endothelium is involved in other interactions particularly with perisinal macrophages. Little is known of the surface characteristics of the endothelium, an area that may help the understanding of how the cell can selectively exchange cells and molecules between the marrow and blood.