Abstract
The olfactory ventricle (OV), an anterior extension of the lateral ventricular system into the olfactory bulbs, is generally considered a transient developmental structure that closes during early postnatal life. However, this view largely derives from a limited number of laboratory models, and the evolutionary distribution and cellular organization of the adult OV remain poorly understood. Here, we combined a comparative survey of mammalian neuroanatomy with phylogenetic analyses and cellular characterization of the OV across species. We compiled anatomical data for 154 mammalian species spanning 26 orders and 80 families, revealing widespread persistence of a patent OV into adulthood. Phylogenetic analyses supported OV presence as the ancestral mammalian state and indicated that its absence is phylogenetically structured and consistent with repeated secondary losses. Chiroptera displayed multiple lineage-specific losses, whereas variation within Muridae extended to the intraspecific level, with contrasting OV persistence among Mus musculus and Rattus norvegicus populations. Comparative histological analyses in sheep, Peromyscus maniculatus, and CD1 mice revealed that, despite marked interspecific differences in OV morphology, the persistent ventricular region retains a conserved cellular organization characterized by progenitor- and glial-associated markers, proliferative cells, immature neuronal populations, and a differentiated apical/ciliary interface. Although these observations do not establish local neuronal production or demonstrate that the OV constitutes an autonomous neurogenic niche, their convergence across species indicates that the persistent adult OV is not simply an anatomically residual lumen but a specialized component of mammalian olfactory bulb organization. Together, our findings identify the OV as an ancestral and widely conserved feature of the mammalian brain and provide an evolutionary and cellular framework for investigating the developmental mechanisms controlling its persistence and its potential contribution to adult olfactory neurogenesis and plasticity.