[Late labelling of pigment epithelial and iridic cells in smooth newts]

The late labelling of the actively proliferating cells of iris and pigment epithelium in the regenerating eye was found in the adult smooth newts within 2 to 12 days following the intraperitoneal injection of 3H-thymidine. This late labelling is due to the long-term presence of the labelled precursors of DNA synthesis in the organism and to their reutilization. The index of labelled nuclei upon reutilization in the pigment epithelium is twice and in the iris 3 to 4 times higher than in the experiments with pulse labelling what cannot be accounted for by the label dilution only in the process of proliferation. The intensity of labelling upon reutilization is 10 to 20 times weaker than upon the pulse 3H-thymidine incorporation. This phenomenon is of general biological value and it is to be taken into consideration when determining the parameters of cell cycles and studying the cell migration. It can also be utilized as a technique for determination of the maximum number of labelled cells and the rate of their entrance into S phase of the cell cycle. Mitoses in the iris cells of the regenerating eye appeared on the 4--6th day following the removal of retina and lens, i.e. long before the formation of retinal rudiment. These data do not confirm the hypothesis by Reyer (1971) and Yamada (1977) on the role of retinal factor in the transition of the iris cells from G2-block to mitosis.