 However, he could not prove that the adult-generated cells were neurons rather than glia, since no phenotypic markers were available that could be used in conjunction with thymidine autoradiography.The absence of specific markers for neurons and glia and continued skepticism surrounding the novel concept of adult neurogenesis limited further development of the research.In the mid 1970s and the early 1980s, Michael Kaplan and his colleagues reexamined the initial observations using the electron microscope and added substantial confidence that neurogenesis could occur in the adult brain.