An intrinsic timer that controls cell-cycle withdrawal in cultured cardiacmyocytes

Developing cardiac myocytes divide a limited number of times before they st op and terminally differentiate, but the mechanism that stops their divisio n is unknown. To help study the stopping mechanism, we defined conditions u nder which embryonic rat cardiac myocytes cultured in serum-free medium pro liferate and exit the cell cycle on a schedule that closely resembles that seen in vivo. The culture medium contains FGF-1 and FGF-2, which stimulate cell proliferation, and thyroid hormone, which seems to be necessary for st able cell-cycle exit. Time-lapse video recording shows that the cells withi n a clone tend to divide a similar number of times before they stop, wherea s cells in different clones divide a variable number of times before they s top. Cells cultured at 33 degrees C divide more slowly but stop dividing at around the same time as cells cultured at 37 degrees C, having undergone f ewer divisions. Together, these findings suggest that an intrinsic timer he lps control when cardiac myocytes withdraw from the cell cycle and that the timer does not operate by simply counting cell divisions. We provide evide nce that the cyclin-dependent kinase inhibitors p18 and p27 may be part of the timer and that thyroid hormone may help developing cardiac myocytes sta bly withdraw from the cell cycle. (C) 1999 Academic Press.