The hepatic extracellular matrix

Abstract

The unique nature of the hepatic extracellular matrix (ECM) is predicted by the special configuration of the space of Disse. Whereas other epithelial organs have two basement membranes (BM) and a substantial ECM interposed between endothelial and epithelial cells, the liver lobule has no BM and only an attenuated ECM, consisting mostly of fibronectin (FN), some collagen type I, and minor quantities of types III, IV, V, and VI. This configuration, together with the abundant fenestrations and gaps of the sinusoidal endothelial cells, seems ideally suited to facilitate the rapid bidirectional exchange of macromolecules normally taking place between plasma and hepatocytes. During organogenesis, the liver anlage is vascularized by continuous capillaries with BM, but by day 13.5 of development (in the rat) the vessels in the immediate proximity of hepatocytes become fenestrated, lacking specialized junctions and BM, suggesting that the hepatocytes produce signals capable of modulating the endothelial phenotype. In regeneration, hepatocyte proliferation precedes vascular proliferation resulting in the formation of hepatocyte clusters that, temporarily, lack sinusoids. Eventually, vascular proliferation follows and the normal hepatocyte-vascular relationships are restored. During this period laminin synthesis by Ito cells is prominent. As soon as hepatocytes become stable, secretion of the sinusoid phenotype-maintaining factors resumes and laminin synthesis and secretion terminates. The interplay between extracellular matrix and liver cells is essential for normal homeostasis and its modification results in derranged hepatic function.