Recent advances in stem cell therapy for neurodegenerative disease: Three dimensional tracing and its emerging use

Table 1 Comparison of major clearing techniques

	Hydrogel-based method	Solvent-based method	Aqueous-based method
Types	CLARITY, MAP, SHIELD, PACT, PARS	3DISCO, iDISCO, BABB	FocusClear, CUBIC, Scale12, SeeDB2
Component	FocusClear/80% glycerol or histodenz	Benzyl/alcohol series	Urea, glycerol or sucrose
Process	Hydrogel monomer infusion → hydrogel-tissue hybridization → Electrophoretic tissue clearing	Dehydration with lipid solvation → Clearing by RI matching	Decolorization by pigment removal → Delipidation using mild detergents → Expansion and RI matching
RI match	1.38–1.48	1.44–1.56	1.38–1.48
Features	Minimizing structural damage and loss of biomolecules	Fast and easy clearing. Permanent preservation of the endogenous fluorescent signal	Biocompatibility, biosafety and preservation of protein function. Penetrating more rapidly and deeply into tissues
Limitations	Expansion of tissue size. Longer incubation	Toxic nature of many solvents, substantial shrinkage of tissue (up to 50%)	Expansion of tissue size. Longer incubation
References	[107,109]	[110,111]	[112,113]

RI: Refractive index; CLARITY: Clear, lipid-exchanged, acrylamide-hybridized rigid, imaging/immunostaining compatible, tissue hYdrogel; MAP: Magnified analysis of proteome; SHIELD: Stabilization to harsh conditions via intramolecular epoxide linkages to prevent degradation; PACT: PAssive clarity technique; PARS: Perfusion-assisted agent release in situ; 3DISCO: 3D imaging of solvent-cleared organs; iDISCO: Immunolabelling-enabled DISCO; BABB: Benzyl alcohol/benzyl benzoate; CUBIC: Clear, unobstructed brain or body imaging cocktails and computational analysis; SeeDB: See deep brain.