AbstractWhite matter microstructural development in late childhood and adolescence is driven predominantly by increasing axon density and myelin thickness.Ex vivostudies suggest that the increase in axon diameter drives developmental increases in axon density observed with pubertal onset. In this cross-sectional study, 50 typically developing participants aged 8–18 years were scanned using an ultra-strong gradient magnetic resonance imaging scanner. Microstructural properties, including apparent axon diameter\documentclass[12pt]{minimal}
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}{}$({d}_a)$\end{document}, myelin content, and g-ratio, were estimated in regions of the corpus callosum. We observed age-related differences in\documentclass[12pt]{minimal}
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}{}${d}_a$\end{document}, myelin content, and g-ratio. In early puberty, males had larger\documentclass[12pt]{minimal}
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}{}${d}_a$\end{document}in the splenium and lower myelin content in the genu and body of the corpus callosum, compared with females. Overall, this work provides novel insights into developmental, pubertal, and cognitive correlates of individual differences in apparent axon diameter and myelin content in the developing human brain.