Membrane dips over nuclei correlate with DNA synthesis in spreading hepatocytes

Spreading of hepatocytes on different supports was examined using scanning electron microscopy. Positively charged Primaria plates gave a uniform morp hology in 2 h. The spreading was rapid and the surface of the cells showed early prominent dips. The hepatocytes had one or two of these structures co rresponding with nuclearity of the cells. The nuclear origin of the dips wa s confirmed after 6 h. The indentations contained solid structures the numb er, size, and shape of which were identical to the nucleoli seen by light m icroscopy. The spreading on the other supports was less uniform. Nuclear di ps appeared more slowly and were less marked initially in their depths. The nuclear dipping was independent of cell density and took place under condi tions under which the cells undergo phenotypic changes during culture. Indi vidual phenotypic changes occur at different times and rates so that the in itial signal for their onset cannot be determined with any certainty. Howev er, the appearance of the dips was accompanied by DNA synthesis in the norm ally quiescent cells. The process stopped when the dipping was completed. T he unavoidable change in nuclear morphology in spread cells may explain why maintenance of a spherical shape circumvents inappropriate DNA synthesis a nd maintains hepatocyte differentiation in vitro. (C) 2001 Academic Press.