Postnatal maturation of the rhesus macaque brain: A longitudinal structural MRI study

The primate brain undergoes significant maturational processes after birth, resulting in substantial growth. The expansion of the brain reflects underlying cellular changes that create a more complex and efficient neural network. Structural magnetic resonance imaging (MRI) can document growth in vivo, which relates to the cellular level processes. Normative imaging studies of postnatal brain development in non-human primates are scarce and were conducted with subjects that were raised in laboratory environments. To enrich our understanding of the postnatal brain maturation, a group of naturalistically-reared rhesus macaques (Macaca mullata) was imaged longitudinally over the first year of life. Subsequent analyses of the MRI demonstrated global and local maturational properties. Brain volume increased to near adult size by one year of age, which was described by a logarithmic growth curve. Male and female brains had similar growth trajectories though males had larger brains from birth. Weight gain significantly predicted variation in the growth curve; more rapid weight gain was associated with greater postnatal growth of the brain regardless of initial brain size. Maturation within the brain was asynchronous and generally progressed from posterior to anterior regions. Apparent myelination and lobar size reached an adult-like state earlier in the occipital, parietal and frontal motor areas than prefrontal and temporal areas. At one year of age, the amygdala and hippocampus were larger in males compared to females; sexual dimorphism in these structures was proportional to the whole brain size variation. Hemispheric asymmetry was also evaluated at one year. The cerebrum showed a slight leftward asymmetry, which was also observed in the hippocampus. On average, the amygdala appeared to be symmetric, but there was substantial individual variation in asymmetry values. This study showed some of the common features of normal rhesus macaque brain development from birth to juvenile stage of maturation. The patterns seen by MRI can be correlated with the extensive neuroanatomical work in this species in order to hypothesize what cellular level changes cause the macroscopic growth. This study stands as the first longitudinal MRI dataset of naturalistically-reared rhesus macaques, which serves as a model of early postnatal brain development in the primate