Capillarization and venularization of hepatic sinusoids in porcine serum-induced rat liver fibrosis: a mechanism to maintain liver blood flow.

The processes of capillarization and venularization of sinusoids after porcine serum-induced rat liver fibrosis were studied by light and electron microscopy. Accompanying the development of fibrosis in the walls of central veins, most of the sinusoidal outlets collapsed, resulting in the formation of hepatic limiting plates around central veins. A few remaining sinusoids underwent capillarization (the development of a basal lamina and the defenestration of the sinusoidal endothelial cell), followed by venularization (the transformation into venules of sinusoids, characterized by the enlargement of the diameters with the lumina being lined with several endothelial cells, which lose fenestrae and develop a basal lamina). These newly formed venules served to maintain blood flow from sinusoids into central veins and thus have been designated the "outlet venules." Diameters of these venules could reach about 25 microns. They were classified into two types: (a) the septal outlet venules, which developed inside the septa; and (b) the angular outlet venules, which drained blood directly from the parenchyma into the fibrotic central veins at the angles between two septa. Associated with venularization, perisinusoidal stellate cells (fat-storing cells or Ito cells) differentiated to myofibroblasts.