Cell-intrinsic timers and thyroid hormone regulate the probability of cell-cycle withdrawal and differentiation of oligodendrocyte precursor cells.

During vertebrate development many types of precursor cells divide a limited number of times before they stop dividing and terminally differentiate. It is unclear what causes the cells to stop dividing when they do. We have been studying this problem in the oligodendrocyte cell lineage, which is responsible for myelination in the vertebrate central nervous system. Here we show for the first time that in clonal cultures of oligodendrocyte precursor cells purified from embryonic day 18 (E18) rat optic nerves the first oligodendrocytes develop within 3-4 days, equivalent to the time they first differentiate in the nerve, and that this timely differentiation depends on the presence of thyroid hormone. These findings suggest that a cell-intrinsic, thyroid-hormone-regulated timer determines when the first oligodendrocytes develop. Whereas the first oligodendrocytes develop asynchronously within clones, the vast majority develop after the first week in culture and do so more synchronously within clones. We show that beta1 thyroid hormone receptors in the precursor cells increase in clonal cultures in the absence of thyroid hormone in parallel with the increasing sensitivity of the cells to the cell-cycle-arresting activity of thyroid hormone; moreover, the increase in beta1 receptors, like the timer itself, is accelerated at 33 degrees C compared to 37 degrees C, suggesting that the increase in receptors may be part of the intrinsic timer. Finally, we show that the precursor cells do not divide indefinitely when stimulated to divide extensively in the absence of thyroid hormone but, instead, eventually stop dividing and either die or differentiate.