 Combining electron microscopy and tritiated thymidine labeling, they showed that labeled cells in the rat dentate gyrus have ultrastructural characteristics of neurons, such as dendrites and synapses.Although they were able to demonstrate this in repeatable studies in primate cortex, most researchers at the time did not consider this to be evidence of significant neurogenesis in adult mammals.In addition, the concept that there may be brain stem cells that could proliferate, migrate, and then differentiate into new neurons had not yet been introduced.