Regulation of liver cell polarity by retinoic acid in vitro: bile canaliculi formation

The mechanisms which regulate liver cell polarity and the formation of the bile canaliculi are unknown. Retinoic acid may, like dexamethasone, regulate cell polarity through its effects on the development of the bile canaliculus. We examined these effects on hepatocytes in primary culture by observing the formation of bile canaliculi and the pericanalicular sheath. The hypothesis is that retinoic acid induces organizational changes in cellular polarity in hepatocytes in vitro. To test this hypothesis, we added trans-retinoic acid 10⁻⁵ M to monolayers of hepatocyte cultures derived from preweanling rats. After 48 h the organization of the cytokeratin intermediate filament cytoskeleton was visualized by immunofluorescence on partially extracted preparations, by unembedded whole mount electron microscopy and gel electrophoresis on detergent extracted preparations. Quantitative measurements of the bile canaliculi were performed. Under the influence of retinoic acid, the number and size of bile canaliculi formed were significantly increased and formed a more complex architecture such as branching. The bile canaliculi were functional as demonstrated by the fluorescein diacetate dye uptake and excretion. In conclusion, retinoic acid enhanced cell polarity as indicated by the increase in the size and complexity of the bile canaliculi and the pericanalicular sheaths.