Gyrencephaly

Gyrencephaly is the property of the cerebral cortex whereby its surface is folded into gyri (ridges) and sulci (grooves), producing a highly convoluted outer layer. This contrasts with lissencephaly, in which the cortex is smooth. The degree of folding is commonly quantified by the gyrification index, the ratio of the total cortical surface area to the outer, exposed surface area; a higher index indicates more extensive folding. Humans are among the most gyrencephalic mammals, though there is substantial variation across species.

Gyrification is assessed in vivo by imaging methods such as MRI and in postmortem specimens by histological and 3D reconstruction techniques. The pattern and extent of folding vary by species and by cortical region, with regions involved in higher-order processing often showing more complex folding. While humans have highly developed gyrification, other primates and many mammals also exhibit complex cortical folding to different degrees.

Development of gyrencephaly occurs mainly during fetal life as the cortex expands and folds form under a combination of genetic instructions and mechanical forces. Factors include neuronal proliferation and migration, maturation of cortical layers, and tension along white matter tracts that helps create gyri and sulci. The process continues late in gestation and can vary between species and individuals.

Clinically and evolutionarily, gyrencephaly underpins the functional capacity of complex brains. Abnormal folding patterns are associated with developmental disorders such as lissencephaly (smooth brain) and polymicrogyria (excessively folded cortex), and can relate to epilepsy, cognitive impairment, or motor deficits. Across mammals, the degree of gyrification generally increases with neocortical size and specialization, reflecting evolutionary trade-offs between brain surface area and skull constraints.