Histogenetic compartments of the mouse centromedial and extended amygdala based on gene expression patterns during development.

The amygdala controls emotional and social behavior and regulates instinctive reflexes such as defense and reproduction by way of descending projections to the hypothalamus and brainstem. The descending amygdalar projections are suggested to show a cortico-striato-pallidal organization similar to that of the basal ganglia (Swanson [2000] Brain Res 886:113-164). To test this model we investigated the embryological origin and molecular properties of the mouse centromedial and extended amygdalar subdivisions, which constitute major sources of descending projections. We analyzed the distribution of key regulatory genes that show restricted expression patterns within the subpallium (Dlx5, Nkx2.1, Lhx6, Lhx7/8, Lhx9, Shh, and Gbx1), as well as genes considered markers for specific subpallial neuronal subpopulations. Our results indicate that most of the centromedial and extended amygdala is formed by cells derived from multiple subpallial subdivisions. Contrary to a previous suggestion, only the central--but not the medial--amygdala derives from the lateral ganglionic eminence and has striatal-like features. The medial amygdala and a large part of the extended amygdala (including the bed nucleus of the stria terminalis) consist of subdivisions or cell groups that derive from subpallial, pallial (ventral pallium), or extratelencephalic progenitor domains. The subpallial part includes derivatives from the medial ganglionic eminence, the anterior peduncular area, and possibly a novel subdivision, called here commissural preoptic area, located at the base of the septum and related to the anterior commissure. Our study provides a molecular and morphological foundation for understanding the complex embryonic origins and adult organization of the centromedial and extended amygdala.

Identification and morphogenesis of the eminentia thalami in the zebrafish.

This study documents early zebrafish brain expression patterns (2-5 days postfertilization) of proliferating neural (PCNA) as well as early-determined (Pax6, Zash-1a, Zash-1b, neurogenin1, neuroD) and differentiating (Hu-proteins) neuronal cells. These patterns are used to outline the spatiotemporal local dynamics of secondary neurogenesis as well as neuronal migration and differentiation in the region of the eminentia thalami. The analysis presented not only allows identification for the first time of the eminentia thalami in the zebrafish model system (because it forms a neurogenin1/neuroD-guided locus of neurogenesis in contrast to adjacent preoptic region and ventral thalamus) but furthermore shows that the entopeduncular complex is a derivative of the embryonic zebrafish eminentia thalami, which has never been reported for a teleost before. An analysis of the relevant literature shows that the mammalian entopeduncular nucleus/avian paleostriatum primitivum/reptilian globus pallidus clearly are part of the basal ganglia (i.e., the pallidum). In amniote embryos, an anterior entopeduncular area is recognized at the base of the medial ganglionic eminence (i.e., the future pallidum; part of alar plate of prosomere 5), separate from the more posterior eminentia thalami (alar prosomere 4). There is a comparable periventricular eminentia thalami in (young and adult) amphibians and teleosts. However, the migrated anterior entopeduncular nucleus of anuran amphibians likely is homologous to part of the pallidum of other vertebrates and has no developmental relationship to the eminentia thalami. In contrast, the migrated teleostean entopeduncular complex does not correspond to a pallidal division but is indeed the adult derivative of the early-recognized eminentia thalami as shown in this study.