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.