Early life stress impairs hippocampal subfield myelination.

The hippocampus is an archicortical structure that is highly sensitive to experience and is made up of individual subfields. These subfields, crucial for learning and memory, rapidly develop and are vulnerable to early stress, yet the mechanisms are unknown. Here, we analyse data from 520 neonates born between 23 and 42 weeks' gestation to assess how early extrauterine exposure-related stress influences subfield maturation. Subfields are segmented automatically by training a U-net model on infant data using HippUnfold, a novel tool for subfield segmentation. Results indicate that subfield volumes are resilient to early stress, while myelination shows greater vulnerability and variation, which may contribute to long-term outcomes. Notably, subfields are not uniformly impacted by stress, with CA1 and CA2 showing the largest effects. Developmental context, including time spent in and ex utero, primarily influences hippocampal subfield myelination.