Cell proliferation and cell death in the cerebral hemispheres of developing chick embryos

Cell proliferation in the cerebral hemispheres of chick embryos is shown to occur in two phases: the early phase extends from day 8 to day 14 incubation, and the late phase from day 14 to day 20. When DNA was labeled with tritiated thymidine, natural loss of label was found; it occurred at two distinct periods, from after day 10 to day 14 and from after day 16 to day 20. DNA which loses label during the first period originated between day 7 and day 10 of incubation and the one which loses label during the second period - from after day 10 until day 13 of incubation. These periods were found to correlate with the levels of activity of previously reported enzymes involved in DNA synthesis and degradation. The most likely explanation of the loss of label is natural cell death occurring during these periods. Our results suggest that such natural cell death in the cerebral hemispheres would be a process intricately coordinated with cell proliferation and maturation as well as with the levels of enzyme activities.