**5. Concluding remarks**

For its short existence, the brain organoid technology made respectable progress and drew lots of interest. It became a rather efficient tool and platform to improve the scientific knowledge of the brain evolution and development and to address clinical problems in a general or personalized approach for brain-related pathologies [13, 45, 48, 49, 108].

Howbeit, there is still large room for improvement and development of this technology since many of its issues wait for practical and efficient solutions and applications.

The functional vascularization of the organoids is probably the most penalizing challenge. When it gets solved, this will open the door for bigger and better replicas of the brain with wider applications as the vascularization network is not only a nutrition/metabolite carrier but also an active part of brain development [109].

Most of the current brain organoids are relatively simple mimics of the much more complex brain. For example, at the moment we cannot recapitulate the overwhelming complexity of such a minute brain region as hypothalamus, which is of comparable size to the biggest brain organoids. So far, it is known that tens of signaling molecules dynamically drive the patterning of hypothalamus primordium during early embryogenesis, many of them distributed in a gradient manner with respectable precision [110]. However, the existing hypothalamic brain organoid protocols utilize just two of them in extrinsically uniform concentration. The generated hypothalamic organoids are coarsely differentiated with limited heterogeneity and cell diversity. At later stages, it seems that the control of hypothalamus patterning is also rather complex and poorly known and hard to be recapitulated. Probably better knowledge and experimenting are needed to get an improved recapitulation of this brain region. A promising move in this direction is recent single-cell analysis studies that hold the promise to improve our understanding of the hypothalamus patterning mechanisms [111]. The situation is not very different for the other brain regions than the hypothalamus.

Wider adoption of engineering techniques can help for better and more natural, dynamic control of the microenvironment [90]. They can also improve reproducibility, automate and reduce the costs per organoid.

Another approach to increase the brain organoid complexity is through the fusion of separately grown region-specific ones, thus forming complex structures named assembloids (**Figure 3C**) [112]. Howbeit, this approach is rather young and so far had found limited success and application.

Although the present of the brain organoid technology still seems challenging, looking at the astonishing advances in the embryology and microtechnologies, there is hope that its future could be encouraging with improved products and applications.
