In atherosclerotic lesions, smooth muscle cells (SMC) change from a contractile to a synthetic phenotype. The in vivo and in vitro phenotypic transformations of SMC have been confirmed by transmission electron microscopy (TEM), but the relationship between this change and the cell cycle is still unknown. We demonstrated the structural modulation of rabbit arterial SMC in primary culture by TEM and immunocytochemistry and simultaneously studied changes in two-dimensional histograms of the relative DNA and RNA contents by flow cytometry. During the first day of primary culture, the cells exhibited the contractile phenotype and were composed of a population in the G0 phase characterized by low contents of DNA and RNA. On the second day of culture, some of the cells (18.2%) had started but not completed the transition into the synthetic phenotype and a cell population in the G1A phase with an RNA content above the G0 level appeared in almost the same proportion. This cell population could be categorized as an "intermediate" type. Moreover, after 3 days when about three-quarters of the cells had undergone structural transition, the same proportion of cells had entered into the cycling phase, while some cells still remained in the G0 and G1A phases. Thus, cell cycle analysis by flow cytometry corresponded well with the observations obtained by TEM and immunocytochemistry. These results show that flow cytometry can rapidly and relatively conveniently monitor the process of phenotypic modulation in SMC and is a useful method for the analysis of such transitions.
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