**4. Future directions and conclusion**

After SARS-CoV and MERS-CoV, SARS-CoV-2 is the third coronavirus in terms of pathogenicity to jump to humans within two decades. This suggests that similar zoonotic coronavirus spillovers can happen again in the near future. Nevertheless, the events relating to coronavirus pathogenesis and transmission are not completely known yet. There is a lack of efficient *in vitro* systems to accurately model host tissues. As conventional animal models, like mice, are not natural hosts to SARS-CoV-2 infection, there is a surge in the development of alternate pre-clinical models to recapitulate the targeted human organs.

Herein, organoid technology used to model human organ development and various human pathologies in a petri dish has played a significant role in understanding SARS-CoV-2 infection and replication. For drug response studies, drug screening, and repurposing, organoids, especially patient-derived organoids, have become popular. Organoid-based studies are leading to personalizing drugs, formulating regenerative medicine, and establishing gene therapy. In comparison to age-old animal models and cell lines, there has been a noticeable improvement in the reproducibility of results and statistical power of experiments. From all previous data, human organoids of lung, gut, kidney, brain, and blood vessels represent excellent experimental models to study the biology of SARS-CoV-2 [44].

Having said that, researchers working in this field are still trying to identify and troubleshoot the inherent challenges in various aspects of handling organoids, including the maintenance costs, cross-technique artifacts, and interpretation of data [26]. It is now well known that the generation and handling of organoids are way more tedious than two-dimensional cell culture protocols. Moreover, the essential growth factors being more expensive and not explicitly tested for applications in the organoid system, one has to prepare them in-house. With the emergence of various commercial sources for reagents tailored to the organoid culture, there is reason to believe that this problem will be fixed quickly. Moreover, the range of cellular heterogeneity for a particular organoid system needs to be improved. Also, mimicking the native micro-and matrix-environment encountered by cells within organoids remains a challenge. Reverse engineering methodologies are only in their infancy as it comes to developing rigorous protocols for the *in vitro* maturation of organoids that are largely fetal-like in cultures [102]. Advances in stem cell, progenitor cell, and pluripotent stem cell handling and directed differentiation techniques will soon help create more physiologically relevant organoids.

In combination with genome editing techniques for manipulating 3D models, organoid technology will be implemented at a large scale in basic and clinical research in the forthcoming era [14]. Progress with other technologies, such as microRNA switches and potentially CRISPR–Cas9, 3D bioprinting, and 3D organoids, will further advance the fast-developing multi-organ disease modeling COVID-19 and its associated therapeutic build-up. Though organoid technology suffers from multiple lacunae but COVID-19 has shown the feasibility and practicality of the organoid platform, suggesting further investment to create an *in vitro* organ mimicking reliable system for successful and effective discovery of therapeutics.
