RENEWAL OF DNA IN PURKINJE-CELL NUCLEI OF MOUSE CEREBELLUM - A 9-MONTH FOLLOW-UP AUTORADIOGRAPHIC STUDY

H-3 thymidine was injected into pregnant mice in order to label the DN A in the dividing Purkinje cell (PC) precursors of the embryonic cereb ellum. The retention of H-3-DNA was evaluated in PC nuclei of animals at the age of 25 days, 3, 6 or 9 months by light microscope autoradiog raphy. The number of silver grains decreased in the whole nuclei by 13 .6 % and 19.6 % in animals 6- and 9-month-old, respectively. In the nu cleolar region, the loss of DNA radioactivity was more profound; the s liver grain counts decreased by 22.6 % and 29.1 % in 6-and 9-month-old animals, respectively. No significant differences in the volume and d ry mass concentration were found in the PC nuclei of 25 PD and 9 PM ol d animals. Therefore, the observed changes in grain density counts rep resent the actual measure of H-3-DNA loss, and /or 'spontaneous' renew al of the DNA molecule in PC nuclei, as well as its higher expression in the nucleolar region. Furthermore, it follows from the comparison o f our data with those present in the literature, that DNA synthesized in nerve cell precursors before their withdrawal from the mitotic cycl e is more stable than that synthesized in postmitotic neurones. This s uggests that the repair of DNA in mature neurones might be of an error -prone type.