Further studies are needed to clarify the functional involvement of this area, and whether it is a derivative of the adjacent ventral pallium or an independent pallial domain. It is independent of the vomeronasal/olfactory amygdaloid nuclei described in anurans, expresses markers such as Lhx9 present in the mammalian pallial amygdala, and lacks Otp-expressing cells, detected in the adjacent medial amygdala. In the present study carried out in Xenopus laevis, after a detailed analysis of the orientation of the amygdalar radial glia, we propose an additional amygdala derived from the pallial region. In anamniotes, and particularly in amphibian anurans, the amygdaloid complex was described as a region with pallial and subpallial components similar to those described in amniotes. However, the pallial conception is currently being reanalyzed and one of these new proposals is to consider the mouse pallial amygdala as a radial histogenetic domain independent from the rest of the pallial subdomains. In the last two decades, the tetrapartite pallial paradigm defined the pallial portion of the amygdala as a derivative of the lateroventral pallium. It is constituted by a set of nuclei presenting a great cellular heterogeneity and embryonic origin diversity (pallial, subpallial and even extra-telencephalic). The amygdaloid complex plays a crucial role in socio-emotional conduct, learning, survival, and reproductive behaviors. In this sense, the structural model proposal divides the mammalian cortex into discrete categories (cortical types) based on their laminar structure, which can be used to test hypotheses about organization, cortical connections, and even their similar developmental origin. An additional proposal of hexapartite pallial organization suggests that the ventrocaudal pallium gives rise to the posterior pole of the pallial amygdala (reviewed in Medina et al. Although its identification in other vertebrates is a challenge due to differences in shape, size, or cellular composition, homologous territories have been identified in many vertebrates, especially affecting the ventropallial derivatives and the origin of the pallial amygdala (reviewed in ). These data open new venues for understanding the organization and evolution of the pallium. We also identified a distinct ventrocaudal pallial sector comparable to the avian arcopallium and to part of the mammalian pallial amygdala. The ventral pallium, expressing Lhx9, but not Emx1, gives rise to the dorsal ventricular ridge and appears comparable to the avian nidopallium.

Thus, the neocortex homolog cannot be found in the classical reptilian dorsal cortex, but perhaps in a small Emx1-expressing/Lhx9-negative area at the front of the telencephalon, resembling the avian hyperpallium. The latter is rich in Lhx9 expression, being excluded as a candidate of dorsal or lateral pallia, and may belong to a distinct dorsolateral pallium, which extends from rostral to caudal levels.

The lizard medial pallium, expressing all genes, includes the medial and dorsomedial cortices, and the majority of the dorsal cortex, except the region of the lateral cortical superposition. The comparative expression patterns of these genes, critical for pallial development, are better understood when using a recently proposed six-part model of pallial divisions. We cloned and analyzed the expression of the genes Emx1, Lhx2, Lhx9 and Tbr1 in the embryonic telencephalon of the lacertid lizard Psammodromus algirus. However, the scarcity of gene expression data in reptiles, crucial for understanding evolution, makes it difficult to identify homologues of pallial divisions in different amniotes. The comparison of gene expression patterns in the embryonic brain of mouse and chicken is being essential for understanding pallial organization.

Royal reptile dragon quest tact