Hippocampal subfield macrostructure and myelination demonstrate distinct maturational signatures

The hippocampus is essential for learning and memory and undergoes prolonged development throughout childhood and adolescence. Evidence concerning hippocampal maturation is based primarily on volumetric measures. However, macrostructural growth and myelination processes may represent distinct biological processes. Using Hippunfold, we quantified hippocampal subfield volumes and myelin-sensitive tissue concentrations as proxies for tissue maturation using T1w/T2w ratios in 175 community-recruited participants aged 9 months to 21 years. Hippocampal subfield volumes were best described by quadratic developmental trajectories, whereas T1w/T2w ratios increased linearly with age across all subfields. Males exhibited larger hippocampal volumes, and females demonstrated higher and steeper age-related increases in T1w/T2w ratios. Sex-related hippocampal subfield differences were observed despite largely comparable cognitive and behavioural profiles. Hippocampal subfield volumes and myelin follow distinct developmental pathways, revealing sex-specific patterns of development that are not fully captured by morphology alone.