3.4. Gyrification
The sulcal and gyral patterns start to form during the third trimester
of pregnancy (White T et al., 2010), and continue to develop in the
first 2 years of life in parallel with the increase in brain size (Li G
et al. 2013). As the volume of the cortical hemispheres increases by
88% during infancy (Knickmeyer RC et al., 2008), the thickness of the
cerebral cortex increases by 31% (Lyall AE et al., 2015), 42% in
sulcal depth (Meng Y et al., 2014), and expands by about 80% in surface
area (Li G et al., 2013; Lyall AE et al., 2015),, particularly during
the first postnatal year. In neonates, inter-individual variability of
cortical folding was investigated, and significant sex differences were
detected in the superior temporal gyrus, inferior frontal gyrus, and
cingulate cortex (Duan D et al., 2019). Male infants showed greater GM
volumes in the fusiform gyrus, superior, middle, and inferior temporal
gyrus, Heschl’s gyrus, left medial frontal gyrus, left superior frontal
gyrus, as well as left precentral and postcentral gyri (Knickmeyer RC et
al., 2014). However, females demonstrated higher GM volumes in bilateral
superior and middle frontal gyri, precentral gyri, inferior and middle
temporal gyri, and medial frontal gyri (superior), as well as the left
superior temporal gyrus. These possibly explain the enhanced social
interactive capacities and visual preference, along with better
self-regulation in newborn girls than boys (Knickmeyer RC et al., 2014).
It is worth mentioning that some differences in gyrification present
only in children and adults are primarily located in regions associated
with social cognition (Mutlu AK et al., 2018). Furthermore, at birth,
males’ cortical thickness of the left fusiform gyrus was remarkably
greater than that of females (Lyall AE et al., 2015). Kim et al (2016)
observed higher cortical shape complexity in the middle cingulate and
parieto-occipital sulcal regions in male infants, but a more complex
Broca’s area in females (Kim SH et al., 2016). In the other hand, the
authors reported no sex differences in the right anterior superior
temporal gyrus after birth until 2 years of age (Kim SH et al., 2016).
Moreover, males displayed significantly faster expansion rates of the
surface areas of many gyri in the left hemisphere compared to females
i.e., precentral gyrus, superior frontal gyrus, middle frontal gyrus,
olfactory gyrus, insula, superior parietal gyrus, and the right
calcarine gyrus (Lyall AE et al., 2015). Variation in cortical
gyrification emerges as a potentially useful marker for normal
development, as structural abnormalities in cortical folding were
evident in certain neuropsychological disorders such as ASD and
schizophrenia (Yang DY et al., 2016). An investigation using T2W fetal
MRI aimed to assess the timing of sulcal emergence and its temporal
variability. The study identified a trend indicating that females
exhibited a significantly earlier sulcal emergence than males in the
right superior-temporal-sulcus. In addition, a larger temporal
variability of sulcal emergence timing was found in male fetuses in the
right intraparietal sulcus, while the right precentral sulcus showed
lower variability in males. These findings are attributed to the
influence of various cellular and molecular processes, including sex
hormones and sex-biased gene expression (Yun HJ et al., 2022).