Entrainment of the rhythm in thymidine incorporation by cultured chick pineal glands.

Young chicks were raised under a standard cycle of illumination and their pineal glands were cultured in organ culture. We have already reported a rhythmic daily cycle in the cumulative incorporation of thymidine into DNA when the glands were incubated under the cycle of illumination to which they were entrained in vivo. Incorporation ceased shortly after the end of the photoperiod and resumed again at, or shortly after, start of the following photoperiod. We have now shown that this cycle was entrained to other lighting schedules and largely unaffected by the time of explanting into culture. The rhythm also persisted in glands cultured in constant darkness. However, when the cycle was maintained by daily renewal of the culture medium it was markedly affected by changes in the time of day at which medium was renewed. Results for glands cultured under constant illumination were inconsistent.