Radioautographic evidence for slow astrocyte turnover and modest oligodendrocyte production in the corpus callosum of adult mice infused with 3H-thymidine.

To find out whether glial cells proliferate in the corpus callosum of adult mice, two series of experiments were carried out. The first one made use of 9-month-old "aged" male mice. Some of them were given 3H-thymidine as a 2-hour pulse to examine which cells became labeled and, therefore, had the ability to divide. Others were sacrificed after a continuous infusion of 3H-thymidine for 30 days to examine whether the label would then appear in different cells. In other aged animals, the 30-day infusion was followed by 60 or 180 days without 3H-thymidine to determine whether cells retained or lost their label with time. A second series of experiments was carried out in 4-month old "young adult" male mice to seek confirmation of the main conclusions. Following the 3H-thymidine pulse given to aged mice, only immature glial cells were labeled. After a 30-day infusion, 12.1% astrocytes and 1.1% oligodendrocytes were labeled, so that the net daily addition rate of astrocytes averaged 0.4% and of oligodendrocytes, 0.04%. In young adult mice, the rate after a 7-day infusion averaged 0.9% for astrocytes and 0.08% for oligodendrocytes. However, when the 30-day infusion into aged mice was followed by 60 and 180 days without 3H-thymidine, the labeled astrocytes decreased to 5.3% and 0%, respectively, whereas the number of labeled oligodendrocytes did not change significantly. The interpretation of the results is that the immature cells present in the corpus callosum of mice continue dividing throughout life and their progeny give rise to astrocytes and oligodendrocytes. In the case of astrocytes, the production of new cells occurs in parallel with a loss, so that the astrocyte population turns over. In the case of oligodendrocytes, there is a small production of new, apparently stable cells.