The human ear plays a vital role in perceiving sound and maintaining balance. However, the ear’s intricate structure and limited regenerative capacity make the treatment of hearing loss and related conditions a significant challenge. To address these issues, auditory organoids, an innovative area in biology and medical research, have gained considerable attention. Auditory organoids are artificially created three-dimensional (3D) tissue structures that mimic the human inner ear. They are used to study the development and function of auditory cells or to explore new treatment approaches. While remarkable progress has been made in this field, there are still hurdles to overcome for commercialization.

Organoids are artificially cultured miniature organs derived from stem cells. They exhibit structural and functional characteristics similar to actual organs and are used for studying human physiological processes, drug testing, and disease modeling. Auditory organoids, specifically, aim to replicate the sensory and neural cells of the human inner ear, including partial functions of the cochlea and vestibular system.

This technology involves differentiating induced pluripotent stem cells (iPSCs) or embryonic stem cells (ESCs) into 3D structures using specific growth factors. The resulting organoids include hair cells, supporting cells, and neurons (Fig. 1). Among these, hair cells, which convert sound vibrations into electrical signals, are a key focus, as their successful generation within organoids represents a major research milestone.

Auditory organoid research has advanced rapidly in recent years, with leading contributions from institutions in the United States, Japan, and Europe. Hair cells play a crucial role in converting sound vibrations into neural signals but cannot naturally regenerate once damaged [1]. In 2023, researchers at Harvard Medical School successfully cultured hair cell-like structures from human iPSCs within auditory organoids and demonstrated their response to stimulus. This finding holds significant potential for developing treatments for hearing loss [2-7]. Auditory organoids have emerged as critical platforms for drug development. Traditional animal models often fail to replicate the complexity of the human inner ear. By utilizing organoids, researchers can more accurately evaluate drug efficacy and toxicity using human cells. For instance, the effects of certain antibiotics and chemotherapy drugs on hair cells have been analyzed, improving the safety of these treatments Auditory organoids also enable the modeling of genetic disorders that lead to hearing loss. By manipulating specific genes, researchers can replicate genetic hearing disorders and explore gene therapy options. For example, researchers at Indiana University successfully created auditory organoids from iPSCs carrying mutations in the TMPRSS3 gene, a known cause of genetic hearing loss, and identified the mechanisms underlying the condition [8].

LGR5+ hair cell progenitor cells, known for their regenerative potential in the inner ear, are garnering significant interest in auditory cell research. LGR5 is a marker expressed in inner ear progenitor cells with the potential to differentiate into hair cells. While mature hair cells cannot naturally regenerate, LGR5+ cells can differentiate into hair cells under specific conditions. LGR5+ cells serve as a starting point in developing auditory organoids. Researchers have cultivated these cells to generate hair cells and supporting cells within organoids. For example, activating the Wnt signaling pathway promotes the proliferation of LGR5+ cells, which can be used to model inner ear damage or test therapeutic approaches [6,9]. LGR5+ cells offer several advantages, including high regenerative capacity and the ability to model damaged inner ears more precisely (Fig. 2). In contrast, mature hair cells have limited regeneration potential and are less suitable for studying early developmental processes in the inner ear.

The development of inner ear organoids represents a remarkable advancement in regenerative medicine and auditory research. These 3D structures mimic the architecture and function of the inner ear, enabling researchers to study development, disease, and potential therapeutic applications. A key factor in the differentiation of inner ear organoids is the optimization of signaling pathways that guide the formation of specialized inner ear structures, including hair cells, supporting cells, and sensory neurons.

The integration of bioengineering and advanced imaging techniques has further enhanced the differentiation of inner ear organoids. Single-cell RNA sequencing allows detailed characterization of cell populations within the organoids, enabling researchers to fine-tune protocols for higher fidelity and efficiency [2,4]. Photobiomodulation is a technique to improved differentiation and proliferation of neural stem cells [24-26]. This non-invasive technique has been proved to increase the differentiation of inner ear organoid [27,28]. Further analysis to validate this approach is necessary.

Despite the promising advancements, several technical and ethical challenges remain in auditory organoid research. Current auditory organoids can replicate only partial functions of the inner ear. The actual inner ear comprises diverse structures, including the cochlea, vestibular organs, and auditory nerve, which interact intricately. Recreating these interactions in vitro is a significant challenge. Additionally, the maturity of organoids often falls short compared to real organs, with differentiation into specific inner or outer hair cells remaining a key hurdle. Developing technologies to ensure the stable, large-scale production of auditory organoids is essential. While small-scale production in laboratories suffices for research purposes, clinical and commercial applications require mass production and standardized quality control. For instance, auditory organoids derived from iPSCs have shown limited differentiation efficiency into hair cells. Although organoids based on LGR5+ progenitor cells exhibit higher differentiation rates, current methods rely on harvesting these cells from animal models, necessitating the development of cell lines for scalable use. Research involving stem cells continues to raise ethical concerns. While iPSC technology offers an alternative to ESCs, ethical issues surrounding the procurement and use of cellular resources still require careful consideration.

Yet, neural functionality of these auditory organoids, especially integrative sensory – neuron function which can be demonstrated by synaptic connections and neural firings in electrophysiological assays is not established. Several studies showed good patch clamp outcomes which were solitary sensory cell responses. Nevertheless, to ensure the appropriate functionality, it is essential to prove the sensory – neural connections in various auditory organoids. In recent study, coculture outcomes which showed good synaptic connections have demonstrated more refiled differentiation of detailed hair cell morphology and function [29]. In addition, several applications of biomaterials have been shown to advance the neural integration [30] which could increase the functionality of auditory organoid.

Auditory organoids hold immense potential as innovative tools for studying and treating hearing disorders. The following areas show great promise. Creating patient-specific auditory organoids using their own cells could lead to more effective treatments. For example, patient-derived iPSCs can be used to model genetic hearing disorders and design personalized drug or gene-editing therapies. A long-term goal is the transplantation of lab-grown auditory organoids into patients to restore lost hearing function. Research is ongoing to address challenges related to immune rejection and ensuring stable organoid function in vivo. Organoid-based drug testing offers cost-effective and time-saving advantages, providing precise data for developing new treatments for hearing loss. Although auditory organoid technology is still in its infancy, its potential to transform biology and medicine is vast. With continued advancements, this technology could bring new hope to millions of people suffering from hearing loss.