The intercalated cell masses of the amygdala consist of cell clusters located between the basolateral complex of the amygdala and its surrounding structures including the central nucleus of the amygdala and the external capsule. Although recent studies have revealed that the intercalated cell masses may play an important role in emotional learning and memory, there are only a few reports on its molecular characterization. We examined the expression patterns of transcription factors in the intercalated cell masses in late embryonic stage and postnatal rats, and non-human primates. Dlx5, Foxp2, Pbx3 and Meis2 were expressed in all subdivisions of the intercalated cell masses, while Ebf1, Nkx2.1 and Foxp1 were not. In contrast, Pax6 was only expressed in a small population of the main intercalated islands, but not in the medial or lateral cell clusters. In addition, few Pax6-positive neurons co-expressed Foxp2. Thus the intercalated cell masses do not contain a homogeneous population of neurons, in terms of their molecular constituents. Given that Foxp2, Pbx3 and Meis2 are preferentially expressed in distinct cell populations in the developing striatum, and that the intercalated cell masses of the amygdala appear to be a ventrocaudal expansion of the striatum, the intercalated neurons may share a common origin with some types of neurons located in the dorsal striatum.
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