DNA SYNTHESIS AND MITOTIC ACTIVITY DURING EARLY DEVELOPMENT OF CHICK LENS

Various developmental stages of lens rudiment, starting from the early lens placode, were labeled with thymidine-3H by culturing the blastoderm for 4 hours in a medium containing the labeled precursor. On autoradiograms the frequency of labeled cells was scored for various morphological compartments of the rudiments and for various developmental stages. A comparable study was made on the frequency of mitotic cells. During the stage when the lens placode indicates orientation of nuclei, the cell labeling frequency decreases slightly over the entire area of the placode. From the time of invagination of the placode the central part of the placode starts to decrease in cell-labeling frequency while the peripheral part retains a high frequency. In the lens vesicle stage the internal layer shows a progressive decrease in labeling frequency which leads to complete disappearance of labeling. The external layer of the vesicle has a high level of labeling throughout the stages investigated. The intermediate zone of the lens vesicle shows a progressive decrease and loss of labeling frequency. The data on mitotic frequency indicate a similar decrease, except that the changes start to be evident later than those of cell-labeling frequency. The results imply that DNA replication and mitosis become less frequent and finally cease in the prospective primary and secondary fiber cells as they differentiate into definitive fibers, and that the cells of lens rudiment which are not participating in fiber differentiation retain a high level of cell-labeling and mitotic frequencies. © 1968 Academic Press Inc.