Expression of cLhx6 and cLhx7/8 suggests a pallido-pedunculo-preoptic origin for the lateral and medial parts of the avian bed nucleus of the stria terminalis
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We investigated the origin of the avian bed nucleus of the stria terminalis (BST) and other parts of the avian subpallial amygdala, by studying the expression of the LIM-homeobox chick genes Lhx6 (cLhx6) and Lhx7/8 (cLhx7/8) in the embryonic chicken telencephalon. Our results indicate that these genes are expressed in a subpallial subdomain partially overlapping the expression of Nkx2.1, which includes pallidal, peduncular, commissural preoptic and pallidoseptal subdivisions comparable to those of mammals. The lateral and medial parts of the avian BST express cLhx6 and/or cLhx7/8, suggesting that they derive from the Nkx2.1-expressing subpallial domain. Our results indicate that the avian lateral BST (BSTL) contains two components, a dorsal part rich in cLhx6 and lacking cLhx7/8 expression that may derive from the pallidal subdivision, and a ventral part showing moderate or light expression of cLhx6 and cLhx7/8, which may derive from the peduncular subdivision. Moreover, the medial BST (BSTM1 and BSTM2) shows moderate to strong expression of cLhx6 and very strong expression of cLhx7/8 throughout development, and appears to derive from both the peduncular and the commissural preoptic subdivisions. Based on this, the avian dorsal BSTL appears comparable to the mammalian BSTL, whereas the avian ventral BSTL and at least part of BSTM may be comparable to the anterior and posteromedial parts of the mammalian BSTM. We also identified a ventrolateral portion of BSTM (BSTM3) and other cell corridors expressing cLhx6 and/or cLhx7/8 in chicken and propose their homology with specific parts of the extended amygdala of mammals.