Immunoelectron microscopy identification of early proliferating cells in rat liver tissue during hyperplasia induced by lead nitrate.

Recent studies have suggested that hepatic stem cells may be involved in at least some forms of liver epithelial growth. To obtain further information on this controversial hypothesis, we treated rats with lead nitrate to induce liver growth and identified the cells undergoing early DNA synthesis by bromodeoxyuridine immunohistochemistry, using both light and electron microscopic detection methods. Eight hours after an intravenous injection of lead nitrate 100 mumol/kg, DNA synthesis was detected in a few scattered hepatocytes and in nonparenchymal cells in portal connective tissue. At the light microscopic level, identification of nonparenchymal cells was limited to bile duct epithelial cells. Other cell types were also labeled, but their identity could not be established. At the ultrastructural level, however, four types of nonparenchymal cells were identified as containing bromodeoxyuridine immunogold particles. These four types included bile duct epithelial cells, fibroblasts, macrophages and nondescript periductular cells. These periductular cells displayed certain ultrastructural features of bile duct cells but did not line a lumen or display microvilli on their apical membrane, nor did they reside within the bile duct basement membrane. Because proliferation of nonparenchymal cells in portal areas preceded that of hepatocytes, it is suggested that the former reaction reflects a direct mitogenic effect of lead nitrate and not an adaptive growth response secondary to parenchymal enlargement. However, whether DNA synthesis in periductular cells or bile duct cells reflects activation of hepatic stem cells cannot be established from the present morphological observations. If so, such a progenitor compartment must be dormant because it does not seem to play a functional role in this and other forms of adult liver epithelial growth.