precursors of neurons and glial cells, start to emerge and self-organize into distinct layers that recapitulate features of the cortex during early embryogenesis. At such early stages, myelin organoids start to form large continuous neuroepithelial that encompass a fluid filled cavity reminiscent of a brain ventricle. The progenitor cells surrounding the putative ventricle organize into distinct layers defined by specific neural markers that become more defined as the organoid matures. The layers include a ventricular zone surrounding the cavity with cells expressing PAX6, SOX2 and Ki67, followed by the outer subventricular zone and intermediate zone with cells expressing Ki-67 and TBR2, and finally cortical plate layer with cells expressing CTIP2, MAP2 and TBR1. Following neurocortical patterning, the oligodendrocyte lineage growth factors drive the expansion of native populations of OPCs distributed causing a substantial increase in their numbers which express SOX6, SOX10 and OLIG2, markers of glial induction and OPC specification. As the myelin organoid matures, the OPC cells differentiate into oligodendrocytes that express proteolipid protein 1 (PLP1), the predominant component of myelin, and MYRF25, an oligodendrocyte specific transcription factor. The oligodendrocytes are distributed throughout the neuronal layers, where upon maturation, their processes express MBP and CNP (an early myelination marker), begin extending to wrap and myelinate the axons surrounding them. The myelin undergoes maturation, refinement and compaction eventually leading to the formation of functional neuronal networks with compactly wrapped myelin lameallae. Further myelin maturation leads to distinct axonal subdomains with a paranodal axo-glial junction (PNJ) and node of Ranvier. The observation of paranodal and nodal assembly is protocol dependent, some observe paranodal and nodal assembly, some do not. Overall, the oligodendrocytes in myelin organoids demonstrate the ability to form compact myelin that wraps and organizes around neuronal axons recapitulating the three dimensional architecture of myelinated axonal networks in humans. Applications Myelin structure and integrity analysis Myelin structure and integrity is inherently hard to study in humans at a molecular level. MRI can shed light on myelin abnormalities in a human brain, however, many studies utilize animal models to study myelin related changes in response to genetic variants. Myelinoids provide a 3D human derived system to study myelin structure. Measuring the number and length of myelin sheaths, paranodal/nodal organization and structure, myelin volume and compaction, cellular identity and composition, and cellular organization are all methods for quantifying myelin changes. These measurements can be done in myelinoids. Testing drugs and therapeutics Studies