Our morphometric data suggest that chicken brains (and bodies) grow significantly faster than quail or budgerigar brains. One could say: chicken time runs fast! To test and further explore this hypothesis, we have started to measure the cell cycle rates of proliferating cells in various brain regions and in various species. The methdology for measuring cell cycle rates involves infusing the embryos with the thymidine analog BrdU, which is incorporated into new DNA during S-phase and can be detected immunohistochemcially. We then sacrifice the embryos at various time points after the BrdU infusion. The longer we wait, the more cells will become BrdU-positive (the brown cells in the image below).
In fact, the rate at wich the percentage of labeled cells increases is directly proportional to the cell cycle time (assuming all cells are cycling). In the illustrated case, which represents chick embryos at stage 26, the estimated cell cycle duration is 10.2 hours. We just ordered some quail eggs and look forward to doing the same experiment in them, at the same stage of development. We expect the quail cells to divide more slowly than the chicken cells.