In vitro models of liver tissue that mimics the in vivo arrangement of different cell types offer the possibility of capturing complex signaling environment found in vivo. In the liver, hepatocytes and endothelial cells are closely associated in their native environment separated by the extracellular matrix (the space of Disse). This unique configuration was mimicked by embedding primary hepatocytes in collagen gel and overlaying endothelial cells on top of the gel. We found that albumin and fibrinogen secretion was doubled in the co-cultures compared to hepatocytes cultured alone during first week after seeding. However during second week, albumin and fibrinogen secretion profile became similar in co-culture and monoculture indicating the importance of endothelial cells derived factors towards early recovery of hepatocytes after isolation. Endothelial cell-conditioned medium reproduced the effect of endothelial cell co-culture, suggesting a major contribution of soluble factors secreted by endothelial cells. Conditioned medium also increased mRNA levels of various acute phase proteins such as albumin, fibrinogen, transferrin, and á-macroglobulin. The effect of endothelial cell-conditioned medium did not occur beyond 4 days of culture if hepatocytes were not embedded in collagen. This underscores the importance of synergism between the architecture of the culture and soluble factor secreted by endothelial cells. Currently work is in progress to identify the soluble factor responsible for enhanced function.