Microglia are surveyors of the healthy brain and involved in the regulation of neuronal activity via synapses. Recently, microglial abnormalities are observed in brains of patients with neuropsychiatric disorders including autism spectrum disorders and schizophrenia, suggesting that microglia are implicated in the neuropathology of these disorders. Cumulative evidence has shown that the abnormalities in the maturation of dendritic spines, including synaptic pruning, are one of the pathogenesis in these disorders. To reveal microglial roles in the synaptic maturation, we initially examined the morphology of microglia, including spatial relationship between microglial processes and synapses, in the developing marmoset brains.
We previously reported that the number of dendritic spines of neurons showed a peak at 3 months postnatal (3M) in the marmoset brains and were gradually decreased thereafter. Immunostaining study with anti-Iba1 antibody revealed that the number of microglia with highly ramified processes were increased during development after birth and showed a peak at 3 months postnatal (3M) in the prefrontal cortex of marmosets. To investigate the interaction between neuronal spines and microglial processes, we visualized neurons in the prefrontal cortex by injecting Alexa Fluor 568 in 3M and 6M of brain samples and immunostained with anti-Iba1 antibody. We confirmed that the several microglial processes mainly contacted with spine head in 3M of brain samples. In contrast, we found that the microglial processes in 6M of brain samples contacted around the whole spine, including spine neck. Since the different pattern of microglial contact with synapses is an important insight in order to reveal the function of microglia in the maturating synapses, further analysis in the different stages of development will be needed.