**4. Comments and future challenges**

Development of neural organoid techniques has yielded rapid progress in clarifying the mechanisms of human neural development. Organoids display many characteristics of the organs from which they were made, including cellular anatomy and interaction, genetics, and specific tissue functions, advancing our understanding the neuro of biology, developmental science, and regeneration. Some of the limitations and challenges of neural organoids have been addressed, but emerging technologies are still required to be applied in further study. With respect to brain organoids, many points are needed to be improved, such as the maturation of neuronal and glial cells, reliable anatomical organization, long-range axonal projection and synaptic connections, and the precise construction of neuronal circuits. Providing a physiologically relevant microenvironment and the more complex whole-brain organoids to reproduce the developmental events of the human nervous systems may be needed in the future. Retinal organoids serve as an ideal choice for therapeutic transplantation, which still face many challenges as following: low yield, high heterogeneity, degenerative inner cell layers, and cancerogenesis. The next-generation retinal organoids would be anticipated to have an integrated vascular network, mature microglia system, and pigment layer wrapping around as well as the integration of bioengineering technologies. To achieve the goal, several engineering approaches may be useful: (1) engineered biomaterials to investigate cell-cell and cell-matrix interactions; (2) genetic engineering technology to study various aspects of organoids development and performance; (3) organoid-on-a-chip device to create an optimal microenvironment with the purpose of generating organoids with higher physiological relevance. Furthermore, the next generation of organoids probably needs to integrate more bioengineering technologies, aiming to overcome each approach's limitation and provide a superior, synergistic approach for constructing more complex organoids in regenerative and precision medicine.
