Cell Migration and Aggregation in the Developing Telencephalon: Pulse-Labeling Chick Embryos with Bromodeoxyuridine

Abstract

Previous studies had concluded that the avian telencephalon develops according to an outside-in schedule of neurogenesis, with relatively little migration of young neuroblasts past older cells. These previous studies had, however, been based on the "cumulative labeling" method, which is less accurate than the "pulselabeling" method typically used in mammals. In the present study, we pulse-labeled chick embryos by injecting low doses of the thymidine analog bromodeoxyuridine (BrdU) directly into the circulatory system of 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 with cases in which BrdU was injected on day 7, suggested that our effective pulse duration is Ͻ24 hr. This was confirmed by injecting tritiated thymidine 24 hr after the BrdU and seeing no double-labeled cells. Several deviations from the previously reported pattern of telencephalic neurogenesis were also noted. Most importantly, the cells born on day 6 in the avian Wulst, the likely homolog of mammalian neocortex, end up homogeneously distributed throughout the Wulst, which suggests that many of them are migrating past older cells. Furthermore, the cells born on day 6 in the ventral hyperstriatum and dorsal neostriatum gradually (over the course of 2-3 d) aggregate into distinct multicellular clusters, which suggests that isochronic cells in these regions adhere preferentially to one another. Finally, the data reveal a proliferative subventricular zone similar to that observed in the ganglionic eminences of mammalian embryos.

Bigger brains cycle faster before neurogenesis begins: a comparison of brain development between chickens and bobwhite quail

“…The embryos were sacrificed 0.5, 3, 6, 8-9 or 12 h after the initial application of BrdU. Previous work has shown that, unlike mammals, thymidine analogues are not rapidly metabolized in avian embryos (Striedter & Keefer 2000). Still, to ensure that high levels of unincorporated BrdU remained available within the egg, embryos sacrificed 6, 9 or 12 h after the initial BrdU application received additional doses of BrdU at 3 h intervals.…”

From sauropsids to mammals and back: New approaches to comparative cortical development

“…Remarkably, the clear‐cut inside‐first outside‐last development of the dorsal pallium has no counterpart in the avian embryonic brains (Tsai et al, 1981). With some exceptions (Striedter and Keefer, 2000), neuroblasts lack the ability to advance through older settled neurons. Consequently, early‐born neurons in the adult pallium occupy outer positions of the telencephalon (closer to the meninges), whereas late‐born neurons locate inner pallial zones (closer to the ventricle).…”

“…Consequently, early‐born neurons in the adult pallium occupy outer positions of the telencephalon (closer to the meninges), whereas late‐born neurons locate inner pallial zones (closer to the ventricle). In the developing hyperpallium of chickens, however, a clear pattern cannot be defined because early‐, mid‐, and late‐born neurons intermingle and show an apparently random distribution (Striedter and Keefer, 2000). In contrast, the pallium in quail develops roughly in an outside‐first inside‐last fashion; the cells born at early stages are distributed in both superficial and deep zones of the pallium, whereas those born at later stages are located specifically deeply in the pallium (Nomura et al, 2008).…”

“…An interesting feature of avian pallial development is the existence of DVR isochronic clusters of neurons (Striedter and Keefer, 2000). Accordingly, neurons generated concurrently in the lateral and ventral parts of the pallium tend to aggregate and form groups or clusters of cells in the DVR.…”

“…Avian brains, in contrast to reptiles, have a substantial abventricular mitotic zone in the subpallium and the lateroventral pallium that can be considered a subventricular zone. The presence of an SVZ in the dorsal pallium of birds, however, is still debated (Striedter and Keefer, 2000; Cheung et al, 2010; and see below).…”