Postnatal interaction of size and shape in the human endocranium and brain structures

The uniqueness of human brain growth and development has been considered promising to understand the advanced cognitive ability of humans. Compared with that of chimpanzees, the human endocranium undergoes several characteristic shape changes immediately after birth, which has been termed “endocranial globularization.” However, how the brain structures and surrounding neurocranium interact with each other during early development in the context of brain–neurocranium integration remains to be investigated. We observed shape changes in the human brain and endocranium during postnatal development using magnetic resonance imaging, and analyzed spatial constraints and interactions among subdivisions of the brain influencing endocranial morphology. Our results suggest that during postnatal development, the relative size changes of supratentorial and infratentorial regions and the cranial base largely constrain brain and endocranium shape. Specifically, a disproportionate increase in the size of the cerebellum relative to the cranial base affects infratentorial spatial packing constraints in neonates, causing inferoposterior expansion of the posterior cranial fossa and coronal reorientation of the petrous bone without lateral opening of the tentorium cerebelli. The dramatic size increase of the cerebellum relative to the cranial base immediately after birth is inferred to be characteristic of human development, which corresponds to previously observed bony shape changes.