An in vitro autoradiographic study of gliogenesis in the embryonic locust brain.

The origin of glial cells in the embryonic locust brain was studied by in vitro labeling with tritiated thymidine during the second half of embryonic life from the 6th day (E6) to hatching (E12). Autoradiographic analysis revealed a germinal zone surrounding the brain. Until the 9th day of embryonic age (E9) this zone consisted of 26 clusters of mitotically active undifferentiated cells. From E10 to E12 only 14 undifferentiated cell clusters were found and, moreover, the labeling declined considerably in the remaining clusters. Clusters were always located at the level of bundles of fibers and labeled cells were present along these fibrous tracts. As early as E6 glial cells were labeled including the glial cells located in the rind between the neurons and the glial cells surrounding the neuropil. The pattern of glial cell labeling was similar to that of the undifferentiated cell clusters, reaching a peak between E6 and E9 then declining. On the basis of these findings it is concluded that glial cells of the developing brain of Locusta migratoria derive from precursor cells located in a peripheral germinal zone and it is suggested that the cells formed utilize the bundles of fibers as a migratory pathway to reach their destination in the brain. Furthermore, this study indicates that the time of appearance of glial cells coincides with that of neuronal maturation.