One concern with the use of in vitro systems to model AD pathology, however, is that phenotypes of aberrant extracellular protein aggregation are lost in two-dimensional (2D) cultures simply due to the lack of an interstitial compartment. To overcome this, the next logical approach is to use three-dimensional (3D) culture systems. There exist two main types of 3D culture systems: a scaffold-free 3D culture system, exemplified by self-organizing structures such as organoids or spheroids [51–53] and 3D engineering tissue, in which a variety of support materials (scaffolds or gels) provide a structure within which cells are cultivated [54–57]. One published study in this second category embedded genetically modified human neural precursor cells, which overexpressed mutant APP and PSEN1, into a Matrigel scaffold. After twelve weeks in vitro, these cultures generated AD-like phenotypes including amyloid plaque deposition and hyperphosphorylated tau [40,58]. Another group studied AD-relevant phenotypes in scaffolded 3D culture systems following exogenous amyloid β (Aβ) application [59].
