Magnesium Supplementation Improves Cortical Stratification and Neuronal Differentiation in Blood–Brain Barrier-Integrated Human Brain Organoids

Background/Objectives: Magnesium (Mg) is essential for neuronal maturation, yet its role in human cortical development remains poorly defined. Here, we investigated the effects of physiological (1 mM) and elevated (5 mM) concentrations of MgSO₄ and magnesium pidolate (MgPid) on human brain organoids co-cultured with an in vitro blood–brain barrier (BBB) model. Methods: Human brain organoids derived from induced pluripotent stem cells were co-cultured with an in vitro BBB system and treated for 4 days with either MgSO₄ or MgPid at physiological and elevated concentrations. Cortical organization was assessed by transmission electron microscopy and immunofluorescence analysis. Western blotting for neurotransmitter receptors and Mg transporters, quantification of intraorganoid Mg²⁺ levels, ELISA-based measurement of GABA and dopamine, and analysis of glutamate were performed. Results: High Mg exposure enhanced cortical stratification and neuronal organization, as shown by the localization of CTIP2 in the outermost layer and TBR2 in the inner layer, together with ultrastructural features consistent with advanced differentiation. Elevated Mg increased intraorganoid Mg²⁺ levels without altering Mg transporter abundance and selectively modulated neurotransmitter receptor expression: NMDA-R levels were reduced by MgPid, whereas GABA_A-R and GABA_B-R were upregulated, particularly in response to MgPid. Levels of glutamate, GABA, and dopamine remained unchanged. Conclusions: These findings identify Mg, especially in the form of MgPid, as a modulator of cortical architecture and inhibitory–excitatory receptor balance in human organoids, supporting its potential relevance for neurodevelopmental regulation and Mg-based therapeutic strategies. These results also support organoids as human-relevant, animal-free tools for neuroscience and neuropharmacological research.