Complexity of nuclear and polysomal RNAs in growing Dictyostelium discoideum cells

The proliferative activity of undifferentiated brain cells from either 5- or 7-day-old chick embryos has been investigated by labeling the cells with a 24-hr pulse label of [¹⁴C]- or [³H]-thymidine during the early stages (0 to 8 days) of culture. As soon as the neurons and the glial cells could be distinguished (after 4, 7, or 14 days of culture), the cultures were prepared and submitted to the activated autoradiographic method. In some experiments a continuous labeling was applied up to 2 weeks. During the first 48 hr of culture, and for both embryonic ages studied, nearly all neuronal precursors were able to proliferate. After 4 days in culture for the 7-day-old embryo and 7 days in culture for the 5-day-old embryo most of the neuronal cells stopped dividing. These two culture periods correspond to the stage of the embryonic life when the end of the mitotic activity of neuroblasts occurs in vivo. Thus, the proliferation and development in culture of most neuroblasts was found to parallel the in vivo evolution of these cells. Some neuroblasts, however, continued to multiply in vitro for a longer period of time. The astroblasts precursors were found to multiply actively from the 3rd day on, or immediately from time zero, for the 5- and 7-day-old chick embryos, respectively. These observations seem to indicate that the astroblast precursors are in a latent stage until they have reached Day 7. Thereafter, they proliferate actively during the first week of culture and therefore remain in an embryonic stage during this culture period. This fact corresponds also to the in vivo situation, where the glial cell precursors multiply actively around the same time period.