Avian scale development. II. A study of cell proliferation

The spatial and temporal distribution of DNA synthesis and the cell cycle duration change during scale development in the chick embryo. Previous histological studies indicated that the two epidermal surfaces of the scale, outer and inner, originated from the placode and interplacode regions, respectively. The present autoradiographic study demonstrated that the epidermal placodes did not incorporate tritiated thymidine for 20–35 hours after their appearance, while the interplacode regions incorporate continuously during this period. The synthetic indices (number of labeled nuclei/total number of basal nuclei) of the two surfaces of the scale also differed early in formation. An explanation for this observation became evident, when it was found that the generation time of the cells constituting the outer and inner surfaces of the scale differed during development. The DNA synthetic phase was constant, whereas the G1‐phase varied concurrently with the generation time.