Comparative morphology of the corpus callosum across the adult lifespan in chimpanzees (Pan troglodytes) and humans

Lifespan trajectories of the morphology and tractography of the corpus callosum: A 5.0 T MRI study

The corpus callosum (CC) is the largest white matter fiber bundle connecting the two hemispheres, facilitating interhemispheric integration and hemispheric specialization. Neuroimaging studies have identified the CC as a marker for aging and various neuropsychiatric disorders. However, studies focusing on high-resolution imaging and detailed lifespan characterizations of CC morphology and connectivity are still limited, highlighting the need for further investigation.Utilizing the high-resolution brain imaging capabilities of 5.0 T ultra-high-field MRI, we collected lifespan data from 266 healthy adults aged 18-89. We segmented and measured the midsagittal area, circularity, thickness, and tractography of the CC using both linear regression and nonlinear fitting models. Our analysis revealed that, despite regional variations, these measures generally exhibited a brief initial increase, likely reflecting developmental maturation, followed by a rapid decline associated with aging-related degeneration. Coupling analysis further indicated that the positive correlation between CC morphology and tractography becomes stronger with increasing age, suggesting age-related structural-functional coupling. External validation and correlation with cognitive-behavioral tests showed that CC subregions with significant age-related changes predominantly involve areas connecting the frontal and parietal networks, particularly those associated with executive function and attentional control. These findings provide new insights into the lifespan evolution of CC morphology and tractography, as well as their degeneration associated with cognitive processing and sensory-motor integration.

Genomic, molecular, and cellular divergence of the human brain

While many core biological processes are conserved across species, the human brain has evolved with unique capacities. Current understanding of the neurobiological mechanisms that endow human traits as well as associated vulnerabilities remains limited. However, emerging data have illuminated species divergence in DNA elements and genome organization, in molecular, morphological, and functional features of conserved neural cell types, as well as temporal differences in brain development. Here, we summarize recent data on unique features of the human brain and their complex implications for the study and treatment of brain diseases. We also consider key outstanding questions in the field and discuss the technologies and foundational knowledge that will be required to accelerate understanding of human neurobiology.

Myelin characteristics of the corpus callosum in capuchin monkeys (Sapajus [Cebus] apella) across the lifespan

The midsagittal area of the corpus callosum (CC) is frequently studied in relation to brain development, connectivity, and function. Here we quantify myelin characteristics from electron microscopy to understand more fully differential patterns of white matter development occurring within the CC. We subdivided midsagittal regions of the CC into: I-rostrum and genu, II-rostral body, III-anterior midbody, IV-posterior midbody, and V-isthmus and splenium. The sample represented capuchin monkeys ranging in age from 2 weeks to 35 years (Sapajus [Cebus] apella, n = 8). Measurements of myelin thickness, myelin fraction, and g-ratio were obtained in a systematic random fashion. We hypothesized there would be a period of rapid myelin growth within the CC in early development. Using a locally weighted regression analysis (LOESS), we found regional differences in myelin characteristics, with posterior regions showing more rapid increases in myelin thickness and sharper decreases in g-ratio in early development. The most anterior region showed the most sustained growth in myelin thickness. For all regions over the lifespan, myelin fraction increased, plateaued, and decreased. These results suggest differential patterns of nonlinear myelin growth occur early in development and well into adulthood in the CC of capuchin monkeys.

Heritability in corpus callosum morphology and its association with tool use skill in chimpanzees (Pan troglodytes): Reproducibility in two genetically isolated populations

The corpus callosum (CC) is the major white matter tract connecting the left and right cerebral hemispheres. It has been hypothesized that individual variation in CC morphology is negatively associated with forebrain volume (FBV) and this accounts for variation in behavioral and brain asymmetries as well as sex differences. To test this hypothesis, CC surface area and thickness as well as FBV was quantified in 221 chimpanzees with known pedigrees. CC surface area, thickness and FBV were significantly heritable and phenotypically associated with each other; however, no significant genetic association was found between FBV, CC surface area and thickness. The CC surface area and thickness measures were also found to be significantly heritable in both chimpanzee cohorts as were phenotypic associations with variation in asymmetries in tool use skill, suggesting that these findings are reproducible. Finally, significant phenotypic and genetic associations were found between hand use skill and region-specific variation in CC surface area and thickness. These findings suggest that common genes may underlie individual differences in chimpanzee tool use skill and interhemispheric connectivity as manifest by variation in surface area and thickness within the anterior region of the CC.