Cell migration and aggregation in the developing telencephalon: Pulse-labeling chick embryos with bromodeoxyuridine

Previous studies had concluded that the avian telencephalon develops accord ing to an outside-in schedule of neurogenesis, with relatively little migra tion of young neuroblasts past older cells. These previous studies had, how ever, been based on the "cumulative labeling" method, which is less accurat e than the "pulse-labeling" method typically used in mammals. In the presen t study, we pulse-labeled chick embryos by injecting low doses of the thymi dine analog bromodeoxyuridine (BrdU) directly into the circulatory system o f chick embryos at 6 d of incubation. The brains of these embryos were then examined for anti-BrdU-labeled cells at postinjection survival times from 30 min to 10 d. Comparisons across different survival times, as well as wit h cases in which BrdU was injected on day 7, suggested that our effective p ulse duration is <24 hr. This was confirmed by injecting tritiated thymidin e 24 hr after the BrdU and seeing no double-labeled cells. Several deviatio ns from the previously reported pattern of telencephalic neurogenesis were also noted. Most importantly, the cells born on day 6 in the avian Wulst, t he likely homolog of mammalian neocortex, end up homogeneously distributed throughout the Wulst, which suggests that many of them are migrating past o lder cells. Furthermore, the cells born on day 6 in the ventral hyperstriat um and dorsal neostriatum gradually (over the course of 2-3 d) aggregate in to distinct multicellular clusters, which suggests that isochronic cells in these regions adhere preferentially to one another. Finally, the data reve al a proliferative subventricular zone similar to that observed in the gang lionic eminences of mammalian embryos.