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).