title: Integration of the developing cranium and brain: A primate postnatal study
Creators: name: Manson, Amy
Publisher: name: University of Liverpool Repository
Year: 2023

description: Development of the craniofacial region involves many complex interactions between often rapidly growing tissues such as the brain, particularly in highly encephalised primate species. Competition for space among the growing tissues of the head is recognised as being a major developmental constraint. Here, I investigate postnatal morphological changes and interactions of tissues of the head in two primate species – highly encephalised modern humans, and rhesus macaques with a relatively smaller brain and larger face. This thesis focusses on interactions between the growing brain, cranial base and face, with some consideration given to other cranial soft tissues. Particular focus is given to the spatial packing hypothesis and wiring economy principle as they relate to cranial postnatal ontogenetic relations in humans and macaques. The purpose of this research is to further our understanding of postnatal developmental trajectories and interactions, enabling future application of this knowledge to enhance studies of human and primate evolution and interpretation of fossil evidence, as well as potentially improving our understanding of tissue interactions and pathologies in a clinical context.
Chapters 2, 3 and 4 of this thesis utilise several methods of morphological quantification including volumetrics, traditional morphometrics, and geometric morphometric analyses to assess interactions between cranial tissues during postnatal ontogeny. These methods were applied to magnetic resonance imaging (MRI) data for modern humans and rhesus macaques. Diffusion tensor imaging and tractography was also produced from diffusion weighted MRI of both species, allowing measurement of neuronal connection lengths within the brain. In Chapter 5, a novel finite element model of a human neonatal skull was created by segmentation from CT and MRI data allowing simulation of cranial soft tissue growth as a first step towards understanding the complex biological interactions between these tissues and the cranium during ontogeny.
The spatial packing hypothesis proposes that relatively large brains can be accommodated on relatively short basicrania by increased basicranial flexion, and that relatively large faces result in a flattened cranial base. While this has been shown to be applicable across phylogeny, it does not appear to apply during postnatal ontogeny in primates. Brain growth was found to influence basicranial-facial shape; however, the relationship and potential methods of interaction are complex. The face had minimal influence on basicranial shape. Brain shape was not influenced by the wiring economy principle, which proposes minimisation of neuronal wiring lengths to maintain efficiency of the brain network. Brain shape was largely linked to increases in size independent of neuronal wiring lengths. It is hoped that the insights into postnatal interactions between growing cranial components presented here will aid future research into the mechanisms governing brain shape, and the influence of the growing brain on the surrounding cranial form, in primates and other species.