during cytodifferentiation in embryonic chick tissues: liver, heart and erythrocytes

Tritiated thymidine was administered to four groups of embryos at various stages of development. The first group was killed 45 min after isotope administration. The second group received the isotope three times over a 12 h period, the third group four times over a 24 h period, and the last group eight times over a 48 h period. Sagittal histological sections of the embryos were prepared for radioautography. In the radioautographs the percentage labeled nuclei were scored as an index of cell proliferation. The following observations were made. From stage 10 to 23 (45 min series) there is a progressive increase in the proliferative index of the labeled erythrocytes. This was followed by a precipitous drop as embryonic age increased. At stage 23 of the.12 h series, 97 % of the cells were labeled. The 24 and 48 h series of embryos also exhibit a high labeling index, 97 % and 95 % respectively, by stages 20-24. This indicates that at these stages most of the cells were in the proliferative pool. In correlation with the demonstrated presence of hemoglobin as early as stage 10, it was concluded that cell proliferation and cytodifferentiation in chick primary erythrocytes are not mutually exclusive. A random pattern of proliferative activity existed in the liver of the early embryos; however, at stage 29 the periphery began to show a higher labeling index than was found in the center