Highlights d The neonatal mouse utricle contains five transcriptionally unique hair cell subtypes d Islr and Cnmd mark transitional epithelial cells d Transitional epithelial cells contribute to the expanding postnatal utricle d Computational trajectory analysis reveals gene dynamics of the expanding utricle

Highlights d In vivo model to study cochlear hair cell death d Cochlear hair cells die rapidly and synchronously d Single-cell RNA-sequencing reveals two stages of hair cell demise d Major types of dying cochlear hair cells express different sets of genes

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Mammalian inner ear hair cell loss leads to permanent hearing and balance dysfunction. In contrast to the cochlea, vestibular hair cells of the murine utricle have some regenerative capacity. Whether human utricular hair cells regenerate remains unknown. Here we procured live, mature utricles from organ donors and vestibular schwannoma patients, and present a validated single-cell transcriptomic atlas at unprecedented resolution. We describe previously unknown markers of 25 sensory and non-sensory cell types, with genes of hair cell and supporting cell subtypes displaying striking divergence between mice and humans. We further uncovered transcriptomes unique to hair cell precursors, which we validated to be 14-fold more robust in vestibular schwannoma utricles, representing ongoing regeneration in humans. Lastly, trajectory analysis of the supporting cell-hair cell axis revealed 5 distinct patterns of dynamic gene expression and associated pathways, including mTOR signaling and synaptogenesis. Our dataset constitutes a foundational resource, accessible via a web-based interface, serving to advance knowledge of the normal and diseased human inner ears and tools to stimulate human inner ear regeneration.