Imagine reaching old age and still being able to engage effortlessly in conversations (Cheslock and De Jesus, 2023). This possibility hinges on preserving the mechanosensory hair cells we are born with, which in mammals are exceptionally long-lived but, with very few exceptions, cannot be replaced once lost (Kirkegaard and Jørgensen, 2000; Savas, 2023). In humans, the loss of these cells results in chronic hearing deficits, making everyday activities such as conversing in noisy settings or talking on the phone overwhelming challenges (Reynard and Thai-Van, 2024). For many, this hearing impairment not only isolates them from social interactions but also aggravates mental health issues such as anxiety and depression (Luppa et al., 2024). Most insights into maintaining mechanosensory function come from studying non-mammalian vertebrates, which can regenerate lost hair cells naturally throughout their lives (Choi et al., 2024; Denans et al., 2019; Pinto-Teixeira et al., 2013; Benkafadar et al., 2024). Emerging research is revealing that multiple mechanisms regulate hair-cell regeneration, even within the same species, offering hope for future breakthroughs in treating hearing loss in humans.

Among the most studied hair-cell-bearing organs are the neuromasts of the lateral line, which enable fishes and amphibians to detect low-frequency mechanical fluctuations in the surrounding water (Valera et al., 2021; Tidswell et al., 2024). These aquatic vertebrates use the lateral line to orient relative to the water flow direction, escape predators and locate prey, and to develop social avoidance reactions. The hair cells also take a more prominent role when vision is limited (Montgomery et al., 2000). Fishes and amphibians also have ears, the constituent hair cells of which control balance and detect high-frequency mechanical signals (Liu and Bagnall, 2023). Importantly, ears and neuromasts must remain functional throughout the life of the animal, despite persistent environmental insult to the hair cells. The lateral line in zebrafish has always offered researchers two key advantages (Barrallo-Gimeno and Llorens, 2022; Plazas and Elgoyhen, 2021; Holmgren and Sheets, 2021; Pickett and Raible, 2019). First, the unparalleled accessibility to high-resolution microscopy, chiefly thanks to its superficial location and the availability of many zebrafish lines expressing various genetically encoded fluorescent makers that label the different cell types (Pinto-Teixeira et al., 2015; Hewitt et al., 2024). Second, hair cells in neuromasts can be quickly and easily eliminated using pharmacological, genetic or physical approaches. The ensuing recovery of the lost cells occurs within a few days, enabling high temporal-resolution recording of the entire regenerative process (Pinto-Teixeira et al., 2013). In recent years, advances in single-cell transcriptomics have added to the above advantages (Lush et al., 2019; Baek et al., 2022; Kozak et al., 2020). Single-cell RNA sequencing (scRNA-seq) and hairpin chain reaction-based fluorescent in situ hybridization (HCR-FISH) have helped to molecularly characterize cellular sub-populations in neuromasts (Shi et al., 2023). Moreover, the easy production of mutant animals gives access to the molecular components that drive regeneration (Parvez et al., 2024).

The initial discovery of resident hair-cell progenitors in neuromasts took place almost two decades ago (López-Schier and Hudspeth, 2006). Every study that followed has agreed that the general mechanism of hair-cell regeneration in the zebrafish lateral line follows a series of steps that start with the birth of new hair cells in pairs from the mitotic division of unipotent hair-cell progenitors (UHCP; Thomas and Raible, 2019; Denans et al., 2019; Mackenzie and Raible, 2012). Although it has been established how supporting cells re-enter mitosis after hair-cell death, the mechanisms that direct UHCP behavior have remained obscure (Kozak et al., 2023). In a recent preprint, Bell and colleagues followed evidence from scRNA-seq experiments showing that the gene foxg1a is expressed in the lateral line during hair-cell regeneration (Bell et al., 2024 preprint). They use a mutant foxg1a allele to find a reduced number of hair cells during neuromast formation. Using nls-Eos transgenic lines expressing a photoconvertible fluorescent protein to disentangle the effect of fox1ga mutants on the behavior of different sub-populations of supporting cells in the neuromast, they conclude that Foxg1a controls cellular proliferation of a population of isl1a-expressing central hair-cell progenitor cells.

However, a nagging issue in the field is that UHCP have not been found in the ear of zebrafish, which also readily regenerate hair cells (Jimenez et al., 2022). There is currently no known molecular marker specific for UHCP, the unambiguous identification of which can only be achieved using live videomicroscopy (Kozak et al., 2023). Therefore, one explanation for not finding UHCP in the ear is that these organs are much more difficult to image live. Another possibility is that UHCP as described in the neuromast simply do not exist in the ear. In another recent preprint, Beaulieu and colleagues show robust evidence that hair cells in the zebrafish ear regenerate through a mechanism that markedly differs from that of the neuromasts, resembling more closely the process of hair-cell regeneration in birds (Beaulieu et al., 2024 preprint; Bhave et al., 1995). Here, after genetic hair-cell ablation, new hair cells arise from direct transdifferentiation of a progenitor pool of cells. Transdifferentiation is the direct identity conversion of one cell into another without an immediate intervening mitosis (Wang et al., 2023). Although traditionally this term has been reserved for identity interconversion between cells belonging to different lineages, we agree with the authors of this preprint that transdifferentiation sensu lato would entail the direct conversion from any postmitotic cell into another. The authors show that a transient wave of supporting cell proliferation occurs after hair-cell ablation, but its function is that of maintaining the precursor pool of cells that would otherwise be depleted by continuous identity conversion to maintain a stable population of hair cells. Beaulieu and colleagues (Beaulieu et al., 2024 preprint) used scRNA-seq data and HCR-FISH to distinguish hair cells by their age and location: cabp1b-positive hair cells are recently differentiated and peripheral, whereas scn5lab-expressing hair cells are located centrally in the ear's crista. A clever combination of a capsaicin-mediated hair cell ablation genetic system and photoconvertible nls-Eos transgenic lines allowed the authors to measure the behavior of different sub-populations of supporting cells. A possible alternative interpretation is that the zebrafish inner ear produces hair cells in pairs, as the neuromast does, after the mitotic division of a resident progenitor. Then, identically to birds, one of the cells immediately takes a mature hair-cell identity, whereas its sibling remains hidden in a dormant immature state until further hair-cell death induces its emergence without an intervening mitosis (Stone and Rubel, 2000). Regardless, this is a very interesting discovery that is in line with what is known in mammalian inner ear cells and, therefore, may be the most clinically relevant for humans.

Together, these preprints highlight the need to combine studies using the zebrafish ears and lateral line to further understand hair-cell regeneration. This field will also benefit from taking advantage of scRNA-seq-derived data, together with newly-developed phiC31 Integrase Genomic Loci Engineered for Transgenesis (pIGLET) to generate diverse transgenic lines with reproducible expression patterns in the ears and neuromasts of zebrafish, and compare how different cellular populations behave during hair-cell death and regeneration (Lalonde et al., 2024; Brown et al., 2023).

Note added in proof

Beaulieu et al. (2024 preprint) has now been accepted as: Beaulieu, M. O., Thomas, E. D. and Raible, D. W. (2024). Transdifferentiation is temporally uncoupled from progenitor pool expansion during hair cell regeneration in the zebrafish inner ear. Development 151, dev202944. doi:10.1242/dev.202944.

Funding

The authors acknowledge funding from the New York University Abu Dhabi.

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Competing interests

H.L.-S. is scientific advisor and paid consultant for Sensorion (France). The company had no role in this study.