Neurons and glial cells in bipolar disorder: A systematic review of postmortem brain studies of cell number and size.

Bipolar disorder (BD) is a complex neurobiological disease. It is likely that both neurons and glial cells are affected in BD, yet how these cell types are changed at the structural and functional level is still largely unknown. In this review we provide an overview of postmortem studies analyzing structural cellular changes in BD, including the density, number and size of neurons and glia. We categorize the results per cell-type and validate outcome measures per brain region. Despite variations by brain region, outcome measure and methodology, several patterns could be identified. Total neuron, total glia, and cell subtypes astrocyte, microglia and oligodendrocyte presence appears unchanged in the BD brain. Interneuron density may be decreased across various cortical areas, yet findings of interneuron subpopulations show discrepancies. This structural review brings to light issues in validation and replication. Future research should therefore prioritize the validation of existing studies in order to increasingly refine the conceptual models of BD.