Primary neural precursors and intermitotic nuclear migration in the ventricular zone of adult canaries

New neurons continue to be born in the ventricular zone (VZ) of the lateral ventricles in the brain of adult birds. On the basis of serial section reconstruction and electron microscopy, we determined that the VZ of the adult canary brain is composed of three main cell types (A, B, and E). Type A cells were never found in contact with the ventricle and had microtubule-rich processes typical of young migrating neurons. Type B cells were organized as a pseudostratified epithelium, all contacted the ventricle, and most had a characteristic single cilium. Type E cells, also in contact with ventricle, were ultrastructurally similar to the mammalian multiciliated ependymal cells. After six injections of 3H]-thymidine (1 every 12 hr), Types A and B cells were found labeled. Type E cells were never 3H]-thymidine labeled. One to two hours after a single injection of 3H]-thymidine, all labeled cells corresponded to Type B cells. At survivals of 5, 24, and 74 hr after 3H]-thymidine injection, the proportion of labeled Type B cells decreased and that of Type A cells increased, indicating that Type B cells were the primary precursors. Most 3H]-labeled nuclei at 1-2 hr after 3H]-thymidine injection were separated from the ventricular cavity, but most of the mitotic cells were adjacent to the ventricle. This observation and measurements of the distance between labeled nuclei and the ventricular surface at 1, 5, 7, and 11 hr after 3H]-thymidine injection indicate that Type B cell nuclei move toward the ventricle to divide. This work reveals the architecture of the VZ in an adult vertebrate brain, identifies the primary precursor of new neurons, and describes nuclear translocation of these precursors during the cell cycle.