5.2. Thalamocortical tracts
The thalamocortical tracts (TCT) are the major subcortical projections into sensory areas and the associative brain regions such as the frontal cortex (Krsnik Z et al., 2017). These tracts undergo initial growth, pathfinding, and target selection under the control of a variety of signaling molecules and complex cellular and molecular interactions (Molnar Z et al., 2012). Thus, the microstructural changes of TCT are observable during the early development of the human brain (Aeby A., 2009). The thalamus-salience and the thalamus-sensorimotor networks have been found in neonates, but obviously the thalamus-default mode networks and the thalamus-medial visual were not evident until 1 year of age (Alcauter S et al., 2014). These findings highlight the critical role of the thalamus-relayed sensory information for proper functioning during early life. Nevertheless, little is known about the sex effects on the thalamocortical tracts in neonates and infants. Although, a DTI study reported that neonate girls have asymmetric TCT pathway (higher diffusion indices on the right compared to the left), no TCT asymmetry can be found in neonate boys (Saadani-Makki F et al., 2021). When both genders were combined, the right asymmetric TCT was still significant with higher diffusion indices in the right compared to the left, providing insight into faster WM maturation in the left hemisphere compared to the right around 38 weeks of GA (Saadani-Makki F et al., 2021).On the other hand, analyses of diffusion MR tractography failed to identify any TCT hemispheric asymmetry during brain development (Goodyer IM et al., 2017), which might be due to incomplete myelin development of the fetus’ brain. A recent resting-state functional MRI (rs-fMRI) investigation on a large cohort of neonates and infants explored the early sex difference in functional connectivity and the biological correlation with language development, and executive function. Noteworthy sex differences in the longitudinal trajectories of functional connectivity change were identified in temporal areas from birth to 2 years of age, specifically in the right fusiform gyrus in neonates, the right inferior-temporal gyrus in 1-year-olds, and the right superior temporal gyrus in 2- year-olds. These sex differences in functional connectivity were found to be associated with behavioral scores collected throughout infancy and early childhood. The observed variations in sex differences underscore the substantial variability in early life, mirroring the rapid structural brain growth and functional reorganization that correlates with genes on the X-chromosome (Fenke SJ et al., 2023).