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.

Expression patterns of developmental regulatory genes show comparable divisions in the telencephalon of Xenopus and mouse: insights into the evolution of the forebrain.

In this study, we review data on the existence of comparable divisions and subdivisions in the telencephalon of different groups of tetrapods based on expression of some developmental regulatory genes, having a particular focus in the comparison of the anuran amphibian Xenopus and the mouse. The available data on Xenopus, mouse, chick and turtle indicate that apparently all tetrapod groups possess the same molecularly distinct divisions and subdivisions in the telencephalon. This basic organization was likely present in the telencephalon of stem tetrapods. Each division/subdivision is characterized by expression of a unique combination of developmental regulatory genes, and appears to represent a self-regulated and topologically constant histogenetic brain compartment that gives rise to specific groups of cells. This interpretation has an important consequence for searching homologies, since a basic condition for cell groups in different vertebrates to be considered homologous is that they originate in the same compartment. However, evolution may allow individual cell groups derived from comparable (field homologous) subdivisions to be either similar or dissimilar across the vertebrate groups, giving rise to several possible scenarios of evolution, which include both the evolutionary conservation of similar (homologous) cells or the production of novel cell groups. Finally, available data in the lamprey, a jawless fish, suggest that not all telencephalic subdivisions were present at the origin of vertebrates, raising important questions about their evolution.

Subpallial origin of part of the calbindin-positive neurons of the claustral complex and piriform cortex.

The aim of the present study was to investigate whether part of the calbindin-positive neurons of the claustral complex and piriform cortex originate in the subpallium. To that end, we prepared organotypic cultures of embryonic telencephalic slices, and applied the cell tracker CMTMR to the ventricular/subventricular zone of the lateral or medial ganglionic eminence. Following 48 h of incubation, we observed a number of CMTMR-labeled cells (showing red fluorescence) of subpallial origin in the claustral complex and piriform cortex. To know whether some of these cells of subpallial origin were calbindin-positive, we performed immunofluorescence for calbindin using an Alexa 488-conjugated secondary antiserum (green fluorescence). Our results showed that some of the CMTMR-labeled cells of subpallial origin in the claustral complex and piriform cortex are calbindin-positive (and possibly GABAergic). The subpallial origin of part of these cells was confirmed by observation of double labeled neurons in the claustral complex that expressed both Lhx6 mRNA (a marker of cells derived from the medial ganglionic eminence) and calbindin. Future studies will be required to analyze the existence of a subpopulation of non-GABAergic calbindin cells in the claustral complex and piriform cortex, and to know their origin.