Effects of human epidermal growth factor on the development of bile canaliculi in neonatal rat hepatocytes in primary culture.

We have examined the effects of human epidermal growth factor (hEGF), a growth-promoting factor for hepatocytes, on the cytoskeletal structure and intercellular contacts in neonatal hepatocytes in primary culture. The ultrastructural characteristics of cellular contacts and bile canaliculi (BC) were examined using a newly developed technique of scanning electron microscopy for cultured hepatocytes. The neonatal hepatocytes incubated with hEGF plus insulin for 3 h showed more variety in shape than controls, and their cellular contacts were very loose with many long fibers. At this time, there were no changes in the distribution of microtubules or microfilaments. After treatment with hEGF plus insulin for 21 h, the distribution of microfilaments was altered. Actin filaments no longer surrounded BC, but were observed near the cell periphery; in addition, DNA synthesis was increased to 3.9 times the rate in controls. Treatment with dexamethasone for 3 h caused tight straight cellular contacts, and after 21 h actin filaments appeared around slightly dilated BC, but there was no increase in DNA synthesis. Transmission electron microscopy showed that the transport and secretion of horseradish peroxidase in BC was inhibited after 3-h incubation with hEGF. These results suggested that hEGF first affected the cellular contacts of hepatocytes necessary for the development of cellular polarity, and then affected the distribution of actin filaments, the development of functioning BC being suppressed.