Liver sinusoids are irregularly dilated capillaries. The sinusoidal lining consists of a sample squamous epithelium. Endothelial cells constitute the major cellular element of the sinusoidal lining and form a discontinuous endothelium. These cells are separated by gaps 0.1-0.5 mm wide. The endothelial cells also show numerous intracellular pores or fenestrations. They perform important filtration function due to the presence of small fenestrations that allow free diffusion of many substances and show huge endocytic capacity. Liver sinusoidal endothelial cells may function as antigen-presenting cells (APC) and are also active in the secretion of cytokines, eicosanoids, endothelin-1, nitric oxide and some extracellular matrix components. They can store and metabolize substances as serum immunoglobulins, too.