Centriole ciliation and cell cycle variability during G1 phase of BALB/c 3T3 cells

Although variability in the duration of the cell cycle is thought to reflect growth-regulatory processes that control cell cycle progression, the precise timing of the variable period within the G1 phase of the cell cycle has not been defined. In particular, the timing of cell cycle variability in relation to the cell's commitment (R point) to the initiation of DNA synthesis remains controversial. In order to investigate cell cycle variability, indirect immunofluorescence was used to measure the formation of the primary cilium as a possible marker of G1 events in both stimulated quiescent and exponentially growing cells. The primary cilium, an internal "9 + 0" nonmotile structure formed by one of the interphase centrioles, was first detected in postmitotic BALB/c 3T3 cells 5 hr before the initiation of DNA synthesis, an interval similar to that for the reassembly of the primary cilium in serum-stimulated quiescent fibroblasts. This similarity in the timing of ciliation suggests that serum-stimulated quiescent cells reenter the cell cycle in early G1 and recapitulate much of G1. Moreover, the rate of cilia formation in both postmitotic and serum-stimulated quiescent cells was identical to the rate of DNA synthesis initiation. Thus, cell cycle variability occurs before ciliation in both stimulated quiescent and exponentially growing cells. Furthermore, since ciliation also precedes the R point, variability in the centriole cycle occurs before the R point and thus may reflect processes controlling the cell's commitment to the initiation of DNA synthesis.