**2. Applications of organoids**

Researchers traditionally used *in vivo* animal model systems and *in vitro* 2-D cell culture systems through decades for preclinical studies. Although these models have been demonstrated to be good and have provided invaluable insights into disease biology, they suffer from certain drawbacks. Excessive cell line passage introduces molecular and genetic alteration leading to variation from the original tissue phenotype and may not satisfactorily depict the original disease model's complexity and pathogenesis. The animal models may not appropriately reciprocate human disease development, suffers from ethical concerns, are very costly, and are time-consuming [17]. The recently developed 3D organoid technology, on the other hand, has evolved and opened up new options for basic and translational study and has fundamentally transformed *in vitro* disease research. Organoids can be easily manipulated, have the potential to recapitulate tissue complexity and physiology, and can be scaled up for high throughput drug screening because they have unique properties such as selforganization, lineage differentiation, signaling process, and maintenance of cell to cell communication, mimicking organ histology [17]. Human and animal cell-based organoid technology has advanced rapidly over the last decade [1], and various organoids mimicking several organs/tissues like the breasts [22], cerebral cortex [2], stomach, intestine [23], kidney [24], liver [25], lung [26–28], pituitary gland [29], prostate [30], pancreas have been developed.

With the advent of tumor organoid culture, patient-derived tumor organoids (PDTOs) have become popular tools to study molecular tumorigenesis, understand tumor heterogenity, predict drug responses, immunotherapy, and precision cancer therapy. At the moment, several tumor organoid biobanks have been developed catering to a variety of cancer types, including lung [31], breast [32], gut [33], and brain [34], liver [35], colorectal [36], pancreas [37], prostate [38], and ovary [39]. The role of tumor immune microenvironments (TIME) is significant in improving cancer immunotherapies, and PDTOs have started playing a crucial role in modeling the tumor-immune landscape. PDTO-based TIME studies can help evaluate immunotherapies such as checkpoint inhibition and adoptive T-cell treatment [4]. Thus, optimizing the tumor organoid culture method is critical for developing organoidguided customized cancer immunotherapy [40].

Human organoids are suited for genetic modification and customization and bridge the gap between fundamental research and clinical practice. This technique has aided oncology, biological, pharmacological, regenerative, and personalized medicine studies [1]. Despite the initial advances, the technology is still nascent and is expected to be employed in various applications such as developmental and stem cell biology, toxicology, drug discovery, personalized medicine, disease modeling, immune interaction, and regenerative modeling (**Figure 2**). Healthy human organoids from different tissues may be utilized to test comparative therapeutic toxicities in combination with disease-related organoids. Cardiac organoids, liver organoids, kidney organoids, and other organoids are now used to dictate intolerable adverse effects, such as hepatotoxicity, cardiotoxicity, nephrotoxicity, and other tissue toxicity [41]. Patient-specific relevant organotypic models will go a long way in reframing basic findings, testing innovative ideas in 3D, and validating crucial data without sacrificing animal life for science. This aspect is dealt with in other publications [42, 43].
