The “Neostriatum” Develops as Part of the Lateral Pallium in Birds

Abstract

Telencephalic organization in birds is so unusual that many homologies between avian and mammalian telencephalic areas remain controversial. Particularly contested is the avian "neostriatum," which has historically been homologized to either mammalian striatum, lateral neocortex, or endopiriform claustrum. Because homologies between these adult structures have been so difficult to resolve, we have begun to examine how telencephalic development diverges between birds and other vertebrates. To this end, biotinylated dextran was injected into the lateral telencephalon of chick embryos at 3 d of incubation, and the distribution of labeled cells was examined up to 14 d later. The data show that a definite boundary to cellular migration develops just ventral to the neostriatum between 5 and 8 d of incubation. Labeled polyclones within the neostriatum stretch from the ventricular zone to the brain surface and exhibit an increasingly rostrocaudal orientation as development proceeds. Individual polyclones contribute cells to several of the distinct auditory, visual, somatosensory, and olfactory regions within the neostriatum. A comparative analysis suggests that the avian neostriatum develops from a precursor region that in other vertebrates gives rise to olfactory cortex and, when present, to other components of the piriform lobe, such as the endopiriform claustrum and basolateral amygdala. Conclusions about lateral pallial homologies between birds and mammals remain uncertain, however, primarily because so little is known about the development of the lateral pallium in mammals. This lacuna might be filled by applying to mammals the novel fate-mapping method described in the present paper.

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

“…In the long-survival experiments, the tongue-like protrusion of day 6 cells just ventral to the LMD (Figs. 3-5) supports the hypothesis that the LMD is a lineage restriction boundary that separates pallium from subpallium (Striedter et al, 1998). The finding that day 6 cells are distributed homogeneously throughout the Wulst, but clustered in HV, supports the hypothesis that these two structures form separate developmental compartments with distinct modes of cell migration and aggregation.…”

“…Recent comparative embryological studies have shown that the avian and mammalian telencephalons resemble each other in early development before following divergent trajectories (Striedter, 1997;Smith Fernandez et al, 1998;Striedter et al, 1998;Puelles et al, 1999Puelles et al, , 2000. Collectively, these studies suggest that the avian Wulst is homologous to most, if not all, of the mammalian neocortex (Medina and Reiner, 2000) and that the avian neostriatum (Neo) and ventral hyperstriatum (HV) are most readily compared with the mammalian ventrolateral pallium, including the piriform cortex, basolateral amygdala, and ventral claustrum (Puelles et al, 1999).…”

Thoughts on the development, structure and evolution of the mammalian and avian telencephalic pallium

“…At least two lines of evidence allow us to discount this possibility. First, experiments studying polyclones labelled by iontophoresis of biotinylated dextranamine into the early chicken telencephalic wall showed distinctly that neostriatum (ventral DVR) cell populations originate within the pallium, separately from subpallial populations (Striedter et al 1998). Moreover, fate-map analysis with quail grafts at late neural plate stages in the chick forebrain (stages 7^8) showed that the prospective subpallium is neatly separated from the prospective ventral DVR in the pallium, independently of the existence of a migration of subpallial inhibitory interneurons into the pallium, entirely comparable with the mammalian one (Cobos et al 2001; see preliminary data in Rubenstein et al 1998).…”

Pallial origin of basal forebrain cholinergic neurons in the nucleus basalis of Meynert and horizontal limb of the diagonal band nucleus

“…Thus, the vertebrate cortex from the pallial telencephalon (pallium) derives from the proliferation of local neurons (Molyneaux et al, 2007;Cobos et al, 2001b) and from the interneuron migration of ventral subpallial structures (Wichterle et al, 1999;Cobos et al, 2001a;Tuorto et al, 2003;Wonders and Anderson, 2006). Although clonal experiments in avian embryos have demonstrated a countercurrent migration of cells from the pallium to the subpallium (Striedter et al, 1998;Cobos et al, 2001a), this has yet to be analyzed in detail.…”