Adolescence is a period marked by high vulnerability to onset of depression. Neuroimaging studies have revealed considerableatrophy of brain structure in patients with major depressive disorder (MDD). However, the causal structural networks underpinning gray matter atrophies in depressed adolescents remain unclear. This study aimed to examine the initial gray matter alterations in MDD adolescents and investigate their causal relationships of abnormalities within brain structural networks.

First-episode adolescent patients with MDD (n = 80, age = 15.57 ± 1.78) and age- and sex-matched healthy controls (n = 82, age = 16.11 ± 2.76) were included. We analyzed T1-weighted structural images using voxel-based morphometry to identify gray matter alterations in patients and the disease stage-specific abnormalities. Granger causality analysis was then conducted to construct causal structural covariance networks. We also identified potential pathways between the causal source and target.

Compared to controls, MDD patients with shorter illness duration showed gray matter atrophy in localized brain regions such as ventral medial prefrontal cortex (vmPFC), anterior cingulate cortex, and insula. With a prolonged course of MDD, gray matter atrophy extended to widespread brain areas. Causal network results demonstrated that early abnormalities had positive effects on the default mode, frontoparietal networks, and reward circuits. Moreover, vmPFC demonstrated the highest out-degree value, possibly representing the initial source of brain abnormality in adolescent depression.

These findings revealed the progression of gray matter atrophy in adolescent depression and demonstrated the directional influences between initial localized alterations and subsequent deterioration in widespread brain networks.

Hippocampal disruptions represent potential neuropathological biomarkers in depressed adolescents with cognitive dysfunctions. Given heterogeneous outcomes of whole-hippocampus analyses, we investigated subregional abnormalities in depressed adolescents and their associations with symptom severity and cognitive dysfunctions.

MethodsSeventy-nine first-episode depressive patients (ag = 15.54 ± 1.83) and 71 healthy controls (age = 16.18 ± 2.85) were included. All participants underwent T1 and T2 imaging, completed depressive severity assessments, and performed cognitive assessments on memory, emotional recognition, cognitive control, and attention. Freesurfer was used to segment each hippocampus into 12 subfields. Multivariable analyses of variance were performed to identify overall and disease severity-related abnormalities in patients. LASSO regression was also conducted to explore the associations between hippocampal subfields and patients’ cognitive abilities.

Depressed adolescents showed decreases in dentate gyrus, CA1, CA2/3, CA4, fimbria, tail, and molecular layer. Analyses of overall symptom severity, duration, self-harm behavior, and suicidality suggested that severity-related decreases mainly manifested in CA regions and involved surrounding subfields with disease severity increases. LASSO regression indicated that hippocampal subfield abnormalities had the strongest associations with memory impairments, with CA regions and dentate gyrus showing the highest weights.

Hippocampal abnormalities are widespread in depressed adolescents and such abnormalities may spread from CA regions to surrounding areas as the disease progresses. Abnormalities in CA regions and dentate gyrus among these subfields primarily link with memory impairments in patients. These results demonstrate that hippocampal subsections may serve as useful biomarkers of depression progression in adolescents, offering new directions for early clinical intervention.