Unraveling the Relationship Between Nr4a2 and Fezf1 Expression in the Developing Feline Telencephalon Within the Genoarchitectonic Context of Claustral and Amygdalar Development

The developmental origin and molecular specification of the lateral pallium remain incompletely understood, particularly with respect to the emergence of the claustrum and its relationship to neighboring pallial territories. Here, we provide a developmental and genoarchitectonic analysis of the feline lateral pallium and its derivatives, focusing on the expression of Nr4a2 and Fezf1. Our results demonstrate that the lateral pallium acquires a distinct molecular identity before the onset of Nr4a2 expression. Beginning at E26/27, a Satb2-positive and Fezf2-low domain can be identified below the ventral rim of the cortical plate, preceding the appearance of a restricted Nr4a2-positive claustral focus that subsequently expands to encompass both claustral and endopiriform populations, indicating that Nr4a2 is a marker of claustral differentiation rather than of lateropallial identity. The spatial organization of the developing claustral complex resembles that described in rodents and is consistent with the cortical ring theory. In contrast to the restricted developmental profile of Nr4a2, Fezf1 exhibited a broad and dynamic expression pattern. Early Fezf1 expression was widely distributed among pallial progenitor populations, suggesting a role in neurogenesis rather than in regional specification, whereas later expression became restricted to selected populations of the piriform cortex and the amygdala. Comparisons with Otp and Sim1 expression further suggest contributions from both telencephalic and hypothalamic sources to Fezf1-positive amygdalar populations. Our findings do not support a specific association between Fezf1 expression and the lateropallial mantle; instead, Fezf1 appears to participate in pallial neurogenesis and amygdalar differentiation. Collectively, our findings refine the current understanding of feline lateropallial development, clarify the significance of Nr4a2 and Fezf1 expression, and provide a comparative framework for interpreting claustral and amygdalar regionalization across mammals.