Availability time of tritium-labeled DNA precursors in newt eyes following intraperitoneal injection of 3H-thymidine.

Following intraperitoneal injection of 3H-thymidine into host newts, iris together with a regenerating lens was transplanted from a donor eye into a lentectomized host eye at frequent intervals for 20 hours and then every 1 or 2 days for 14 days. The eyes were fixed 2 hours and 1 or 2 days after implantation and autoradiographs prepared. Following fixation 2 hours after operation, incorporation of 3H-thymidine into DNA, as evidenced by grain counts over nuclei, fell rapidly for 3.5 hours after injection and was no longer apparent after 4.5 hours. However, almost one-half of the implants were lightly labeled when they remained in the host eyes for 1 or 2 days beginning from 1 to 14 days after isotope injection. When these implanted, regenerating lenses were left in the host eyes for longer periods of time, then a light label was found over nuclei in most of the implants remaining in the eye for 3 to 24 days. When 3H-thymidine was injected from 1 to 3 days after extirpation of both lens and neural retina, before DNA synthesis had been initiated in the pigmented retinal epithelium or iris, there were numerous cases of labeled nuclei among depigmenting cells of the pigmented retinal epithelium which was regenerating a new neural retina from 2 to 25 days after isotope injection. Depigmenting cells of the dorsal iris and regenerating lens were similarly labeled. These results provide evidence for the continued availability of small amounts of tritiated DNA-precursor molecules which can be incorporated in DNA of proliferating cells long after the initial injection of 3H-thymidine.