4.4. Corpus callosum
The corpus callosum (CC) is the main interhemispheric commissure that arises in the brains of placental mammals (Tomasch J, 1954). The CC is formed in the 20th week of gestation (Tanaka-Arakawa MM et al., 2015), and grows in both anterior and posterior directions, with a more prominent anterior growth (Jovanov-Milosevic N et al., 2009). It mainly grows by fiber addition during gestation and in the early postnatal period, and it undergoes significant development during the first two years of life by progressive increase of layer of myelin sheaths that continues into young adulthood (Deoni SC et al., 2011). CC is the first human brain structure to be called “sexually dimorphic” (Eliot L et al., 2021). Nevertheless, CC sex-related differences are still a matter of debate (Mitchell TN et al., 2003; Schmithorst VJ et al., 2017). Studying the development of CC in different age subgroups revealed a significantly higher ratio of CC to the whole brain in females than in males (Tanaka-Arakawa MM et al., 2015). A slightly larger CC area was reported in female infants when compared to age-matched boys without significant sex differences (Vanucci RV et al., 2017). However, other studies have revealed a significant larger CC area in male infants between birth and the 14th month of age. The anteroposterior CC length was found to be longer in males, but the bulbosity index was similar in both sexes (Clarke S et al., 1989; Schmied A et al., 2020). A greater height of CC in male infants was recognized (Hwang SJ et al., 2004) suggesting a different pattern of spatial arrangement of the CC axons between the sexes, and indicating faster growth of CC inferior-superior regions in males than in females (Oka SJ et al., 1999). Vannucci et al. have revealed a significant expansion in CC mainly in the splenium, genu, and body of CC, n infants, with a greater splenium length-height ratio observed only in female infants (Vanucci RC et al., 2017). Moreover, female infants exhibited well-developed regions of CC except for the isthmus (Makki MI and Hagmann C, 2017). The development of CC in female infants could be due to the influence of maternal factors during brain development (Chang CL et al., 2018) i.e. prenatal testosterone that enhances axonal pruning. Additionally, a significantly larger thickness of splenium in female neonate infants was observed (Hwang SJ et al., 2004) in line with the findings reported on human fetus (Achiron S et al., 2001; de Lacoste MC et al., 1986). In a recent volumetric MRI study employing 3D-T1W imaging, Lewis et al (2022) partitioned the mid-sagittal acquisition of the neonatal CC into 25 equal length segments. The findings revealed a significantly greater exponent in the allometric relationship (log regression) in female neonates compared to their male counterparts. This greater growth in females, surpassing that in males, is considered normal due to the processes of exuberance and pruning involved in the development of the cortex and connectivity (Lewis JD et al., 2022). The differences in size, shape, and thickness of the CC sub-regions may reflect the differences in the functional connectivity between the two hemispheres that later underlie gender differences in verbal abilities, integration of sensory and motor information between the two hemispheres, as well as spatial orientation and attention.