Homologous cell type markers highlight subdivisions of the domestic chick hippocampal formation

The hippocampal formation (HF) is a brain region essential for both cognitive and emotional functions in mammals. These complex functions are mediated by neural circuits connecting the subdivisions of the HF. To clarify the evolutionary continuity of the HF, the avian HF has recently attracted increasing attention. However, the lack of consensus regarding its subdivision has hindered detailed comparisons. This study aimed to establish a consensus on the avian HF subdivision. We investigated the molecular anatomy of the chick HF using recently identified homologous cell markers that correspond to those in the mammalian HF. Using these markers, we obtained expression patterns consistent with the subdivisions of the V-shaped complex, dorsal medial region, and dorsal lateral region, suggesting homology with corresponding subdivisions of the mammalian HF. Visualizing these cells within the HF provides a strong foundation for establishing a unified framework for avian HF subdivision.