Measurement of time in oligodendrocyte-type-2 astrocyte (O-2A) progenitors is a cellular process distinct from differentiation or division.

When stimulated by platelet-derived growth factor (PDGF), oligodendrocyte-type-2 astrocyte (O-2A) progenitors derived from perinatal rat optic nerves undergo a limited number of cell divisions before clonally related cells synchronously and symmetrically differentiate into nondividing oligodendrocytes. The duration of this mitotic period is thought to be controlled by a cell-intrinsic biological clock. Thus, in the presence of PDGF, the measurement of time by the biological clock is intimately coupled to the control of division and differentiation. In contrast, O-2A progenitors grown in the presence of PDGF plus basic fibroblast growth factor (bFGF) divide indefinitely in the absence of differentiation and so do not exhibit a limited period of division. We have tested whether growth in PDGF plus bFGF alters the duration of the limited period of division O-2A progenitors exhibit in response to PDGF alone. Accordingly, O-2A progenitors were grown in the presence of PDGF plus bFGF for varying lengths of time, before being switched to conditions that promote timed differentiation (PDGF but not bFGF). Increasing duration of culture in PDGF plus bFGF led to a gradual shortening of the period for which O-2A progenitors were subsequently responsive to PDGF alone, until eventually all cells differentiated without dividing after switching. In contrast, a short exposure to bFGF was not sufficient to cause a similar alteration in the pattern of differentiation. These results indicate that O-2A progenitors prevented from undergoing timed differentiation nevertheless retain the ability to measure elapsed time, implying that the biological clock in this cell type can be uncoupled from differentiation. Furthermore, they demonstrate that the biological clock does not impose an absolute limit on the number of divisions that an O-2A progenitor can undergo. In contrast with existing hypotheses, our observations suggest that the molecular mechanism that controls timed differentiation must consist of at least two components, with the clock itself being in some manner distinct from mechanisms that limit cell division and/or directly regulate differentiation.