For AD, vascularised, neuroimmune cerebral organoids—including microglia-competent variants—interrogate the interplay between lipid handling, amyloid/tau processing and neuroinflammation; for PD, nigrostriatal assembloids recover long-range dopaminergic connectivity and enable assays of axonopathy and α-synuclein propagation; for MS, human iPSC myelination co-cultures and emerging microfluidic myelination devices provide quantitative, human-specific myelin formation and repair endpoints. The gene discussed is MAPT; the disease is Alzheimer disease.