Human Brain Organoid: A Versatile Tool for Modeling Neurodegeneration Diseases and for Drug Screening

Table 1

Advances in generation of organoids.

(a)

Culture medium/supplement/culture strategy	Regions/organoids type	Key features	References
SHH, FGF8	Midbrain simBOs	High efficiency, high homogeneity, easy to specify	[58]
CHIR99021	Midbrain-like MLOs	Robust generation; Homogenous distribution of mDAs, other neuronal subtypes, and functional glial cells, such as astrocytes and oligodendryocytes	[61]
WNT3A and mixed medium with 1 : 1 of fresh and supernatant derived from interfollicular epidermal SCs	Epidermal organoids	Functional with polarity	[120]
RSPO1, WNT3A, WNT7A	Endometrial organoids	Endometrial disease facilitate growth of endometrial disease organoids and the long-term expansion	[121]
WNT and nodal antagonists	Hippal/CB organoids	Original differentiation method; Low efficiency of O2 and nutrient diffusion	[122]
Dkk1 and LeftyA
Floating culture with 40% O2 and 5% CO2 CHIR 99021, BMP4	Hipp/cortex organoids		[10, 13, 24119–121]

(b)

Culture device	Key features	References
1. Spinning bioreactor	High cost and require a high volume of culture medium	[122]
2. Multiple-well culture plates with orbital shakers	Reducing the cost and consumption of the culture medium successful generation of cerebral organoids	[123, 124]
3. Miniaturized multiwell spinning bioreactor	Facilitate the establishment of brain region-specific organoids that mimic the dorsal forebrain, midbrain, and hypothalamus	[14, 95]
4. Collagen hydrogel systems	Consisting of interconnected excitatory and inhibitory neurons with supportive astrocytes and oligodendrocytes fiber for bioengineered organoids; A highly interconnected neuronal network established in organoids at a macroscale tissue format.	[8]
	More importantly, the engineered organoids share structural and functional similarities with the fetal brain, potentially allowing for the study of neuronal plasticity and modeling of disease
5. Carbon fibers (CFs) for midbrain organoids	The porosity, microstructure, or stability CF scaffolds could improve efficiency in iPSC differentiation within organoids relative to the PLGA scaffolds. The midbrain organoids generated in the CF scaffolds could more efficiently enhance terminal differentiation and the survival of midbrain dopaminergic (mDA) neurons.	[59]
6. Brain organoids	The modified hydro-Matrigel with an interpenetrating network (IPN) of alginate has been employed to maintain the mechanical microenvironment for brain organoids, conferring the viable growth environment with the characteristic formation of neuroepithelial buds.	[125, 126]
7. Brain organoids	The platform of “tissue-like” cyborg stretchable mesh nanoelectronics were invented to provide seamless and noninvasive coupling of electrodes to neurons within developing brain organoids, enabling continuous recording of single-cell action without interruption to brain organoid development	[127]

(c)

Coculture of organoids	Key features	References
Co-culture of cancer organoids with other non-tumor cells	Tumor organoids could get other cell types of cells and tissues	[128, 129]
Vascularization of organoids
1. Direct transplantation of the brain organoids into mouse brains		[32, 34, 130]
2. Coculture of brain organoids with epithelial cells followed by transplantation into mouse brains		[131]
3. Genetic operation-based vascularization	Expression of human ETS variant 2 (ETV2) in human cortical organoids (hCOs), led to generation of the functional vascular-like vessels in the vascularized hCOs (vhCOs), improving organization, alleviating hypoxia, and reducing apoptosis	[132]
4. BVO cells infiltrate into brain organoids	High efficiency to generate vascularized human brain organoids	[133]
5. The microfluidic chips-based coculture with epithelial cells		[134]
6. Vascularized spheroid using an injection-molded microfluidic chip	By coculturing the spheroids derived from induced neural stem cells (iNSCs) with perfusable blood vessels, the vascularized spheroid was generated. The vascularized spheroid network significantly improved spheroid differentiation and reduced apoptosis.	[99]

(d)

Differentiation methods
Unguided strategy	Generation of brain organoids with mixed cell lineages of forebrain, midbrain, hindbrain, and retina, enabling the organoids to grow with minimum external interference; High variability and heterogeneity	[11, 31, 95]
Guided strategy	Directed differentiation to generate brain region-specific organoids, such as cerebral cortex, hippocampus, midbrain, and cerebellum	[10, 13, 14, 119, 135, 136]
Fused culture technologies for integration of different regions of the organoids	More closely resembling the complexity of the brain in identity, architecture, and interaction manners enhanced the formation of microcircuits with the local excitatory neurons	[123, 124, 135]
Long-term propagation, storage, and regrowth following the frozen and thaw cycles	CRISPR-Cas9-based knock-in of the mutant KRAS^G12D allele into human colon APC^−/− organoids	[115, 137]
Application of 3D printing technology in	Enabled an engineered organ to maintain the spatial arrangement	[39, 134, 138, 139]
Organoids-on-a-chip based approach to	Could remove the dead cells via connecting with an external pumping	[140]
Generate the tube-shaped epithelial organoids	System, extending tissue lifespan and enabling the colonization of organoid tubes with microorganisms to model the host–microorganism interactions
Generation of microglia cell-containing microglia cerebral organoids	Microglia were naturally developed in cerebral organoids and displayed similar characteristic ramified morphology as in normal fetal brains.	[106, 141]
Generation of microglia-containing hCOs (mhCOs)	Microglia-containing hCOs (mhCOs) were generated via overexpression of the myeloid-specific transcription factor PU.1 in cortical organoids. The mhCOs have become an efficient tool for functional investigation of microglia in neurodevelopmental and neurodegenerative disorders, such as AD	[108]