Thymidine incorporation by chick pineal glands as studied by pulse-labelling in vitro.

We have studied the pattern of variations in extent of thymidine incorporation during pulse-labelling in cultured chick pineal glands. During the first 24-30 h in culture the extent of pulse-labelling varied in parallel with the cycle in cumulative incorporation. Pulse-labelling was seen during the period of apparent arrest of cumulative incorporation. However, it was probably "masked" by the SD of assays of cumulative incorporation and represented a minor, but distinct, process of thymidine incorporation. The pattern of variation in extent of pulse-labelling was qualitatively consistent under a wide variety of conditions. Control of this pattern appeared to be different from that of the circadian rhythm in cumulative thymidine incorporation.