Hepatic stellate cells: unique characteristics in cell biology and phenotype.

Hepatic stellate cells (HSCs), a mesenchymal cell type in hepatic parenchyma, have unique features with respect to their cellular origin, morphology, and function. Normal, quiescent HSCs function as major vitamin A-storing cells containing over 80% of total vitamin A in the body to maintain vitamin A homeostasis. HSCs are located between parenchymal cell plates and sinusoidal endothelial cells, and extend well-developed, long processes surrounding sinusoids in vivo as pericytes. However, HSCs are known to be 'activated' or 'transdifferentiated' to myofibroblast-like phenotype lacking cytoplasmic lipid droplets and long processes in pathological conditions such as liver fibrosis and cirrhosis, as well as merely during cell culture after isolation. HSCs are the predominant cell type producing extracellular matrix (ECM) components as well as ECM degrading metalloproteases in hepatic parenchyma, indicating that they play a pivotal role in ECM remodeling in both normal and pathological conditions. Recent findings have suggested that HSCs have a neural crest origin from their gene expression pattern similar to neural cell type and/or smooth muscle cells and myofibroblasts. The morphology and function of HSCs are regulated by ECM components as well as by cytokines and growth factors in vivo and in vitro. Liver regeneration after partial hepatectomy might be an invaluable model to clarify the HSC function in elaborate organization of liver tissue by cell-cell and cell-ECM interaction and by growth factor and cytokine regulation.