Neurons of the SCN maintain circadian gene expression for long periods of time. Other cells, however, were thought to rely on the SCN to maintain their oscillations, as the amplitude of gene expression oscillations in fibroblast cultures lessens rapidly with time in culture. But Schibler's group shows that the examination of mass cultures masks the continued ability of each cell to maintain strong oscillations.
By looking at individual fibroblasts, the authors were able to see persistent oscillations. Even after mitosis, daughter cells continued the rhythm of the mother, although with a slight phase shift, probably due to the transient suspension of transcription. Mitosis itself is also gated by the oscillations, although the cellular advantage to this gating is unclear.
Mathematical modeling suggested that global culture oscillations are diluted over time by slight differences in the periodicities of each cell's clock. Resynchronization was achieved in cultured cells by activation of a wide variety of signaling pathways, which reset all cells to a common point. In animals, peripheral clocks are synchronized by feeding cycles, which depend on sleep–wake cycles. As the SCN controls sleep cycles, it automatically synchronizes the oscillations in peripheral cell types.