Forms and measures of adult and developing human corpus callosum: Is there sexual dimorphism?

Abstract

The sexual dimorphism of the human corpus callosum (CC) is currently controversial, possibly because of difficulties in morphometric analysis. We have reinvestigated the issue by using morphometric techniques specially designed to yield objective measurements of CC size and shape. The development of the CC was studied with similar techniques in order to investigate whether its final shape and size might be influenced by axonal elimination, as could be expected from previous animal studies. We have measured the CCs of 32 men and 26 women; 27 male and 19 female CCs were from brain tissue, the others were from magnetic resonance imaging graphs. Women tended to have 1) a smaller cross-sectional callosal area (CCA); 2) a larger fraction of CCA in the posterior fifth of the CC; 3) more slender CCs; and 4) more bulbous splenia. These differences could not be detected by simple inspection but were demonstrated by measurement and statistical analysis. However, CCA was correlated with the other sexually dimorphic parameters, and the sex-related differences in the latter became nonsignificant when variations in CCA were factored out or when male and female populations with similar CCA were compared. In addition, we analyzed CCs of 16 male and 16 female fetuses and of 13 male and 15 female infants and children. This sample ranged in age between 20 weeks of gestation and 14 years but covered in detail the period up to 14 months after birth. CCA increased throughout the latter period but decreased slightly between about 33 weeks of gestation and the beginning of the second postnatal mouth. This decrease coincided with thinning of the CC and a marked increase in bulbosity of the splenium. No sexual dimorphism could be demonstrated until the beginning of the postnatal period. In the age group between birth (at term) and the 14th month, CCA was, as in the adult, larger in males. Unlike in the adults, the CC was longer in males and the bulbosity index was the same in the two sexes. Axonal elimination may play a role in the perinatal pause in CCA growth and in the concomitant changes in callosal shape.

Schizophrenia, neurodevelopment and corpus callosum

“…Finally, the studies exploring regional differences in CC size are particularly difficult to interpret since methods of CC partitioning vary across studies (reviewed in Bishop and Wahlstein 96 ), including in S research, and the size of the CC partitions can be influenced by CC shape. 70 On the whole, the majority of the studies found a reduction of the CC in the schizophrenics, as also noticed in a review 97 of 27 imaging studies published between 1986 and 2000. At first glance, these conclusions seem to support the hypoconnectivity hypothesis.…”

“…The main factors determining callosal size in development appear to be the increase in-axon diameter and myelination. 28,70 Even a considerable loss of callosal axons could be masked by an increase in the diameter of the remaining ones, which is what one would expect if the remaining axons, as suggested above, sprout into vacated territories. The same cautions must be applied to the interpretation of regional differences in CC size.…”

“…Indeed, in children, as in animals, CC size increases progressively, in parallel with increased axonal caliber and myelination. 70 Four imaging studies suggest that myelination of CC axons might be abnormal in S. A decrease in the density of the inferior part of the CC was reported, and was found to correlate with the reality distortion score. 78 Also, decreased fractional anisotropy was found in the splenium of the CC using tensor diffusion imaging (TDI), 89,91 possibly involving axons that interconnect temporal and/or occipital areas.…”

“…They are also the only cortical connections for which rather complete developmental data exist, including the quantitative estimates of axonal elimination in normal development in cat, 16,28 monkey, 30 and by inference in man. 70 The differentiation of the terminal arbor of callosal axons has also been studied in detail and a number of developmental stages have been identified. 33,39 Obviously, a deficit of callosal connections cannot by itself explain the pathophysiology of S. Rather, abnormalities in callosal connections are likely to be an indicator of a more diffuse impairment of corticocortical networks since the developmental rules that apply to callosal connections seem to apply to other cortico-cortical connections as well.…”

Sex Differences in the Human Corpus Callosum: Myth or Reality?

“…Cowell et al (28) and (29) were also excluded because they use the same raw data as Allen et al (4). Clarke et al (25) and Prokop et al (102) were each treated as two separate studies because two different methods were used to examine two different groups of subjects in each study. Holloway et al (66) was treated as three separate studies because three different samples of subjects were examined.…”