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Could Sex Differences in White Matter be Explained by g ratio?

Paus T, Toro R - Front Neuroanat (2009)

Bottom Line: We review the original Rushton (1951) model where a g ratio of approximately 0.6 represents an optimal relationship between the axon and fibre diameters vis-à-vis conduction velocity, and point out evidence indicating slightly higher g ratio in large-diameter fibres.We estimate that fibres with a diameter larger than 9.6 mum will have a relatively thinner myelin sheath, and brains with increasingly larger proportions of such large-diameter fibres will have progressively lower concentration of myelin.We conclude by pointing out possible implications of "suboptimal" g ratio for the emergence of "disconnection" disorders, such as schizophrenia, in late adolescence.

View Article: PubMed Central - PubMed

Affiliation: Brain and Body Centre, University of Nottingham Nottingham, UK.

ABSTRACT
Recent studies with magnetic resonance imaging suggest that age-related changes in white matter during male adolescence may indicate an increase in g ratio wherein the radial growth of an axon outpaces a corresponding increase in myelin thickness. We review the original Rushton (1951) model where a g ratio of approximately 0.6 represents an optimal relationship between the axon and fibre diameters vis-à-vis conduction velocity, and point out evidence indicating slightly higher g ratio in large-diameter fibres. We estimate that fibres with a diameter larger than 9.6 mum will have a relatively thinner myelin sheath, and brains with increasingly larger proportions of such large-diameter fibres will have progressively lower concentration of myelin. We conclude by pointing out possible implications of "suboptimal" g ratio for the emergence of "disconnection" disorders, such as schizophrenia, in late adolescence.

No MeSH data available.


Related in: MedlinePlus

The relationship between age and mean values of magnetization-transfer ratio (MTR) in white matter (WM) in male and female adolescents in the (A) frontal, (B) parietal, (C) temporal and (D) occipital lobes. The lines represent the regression equation with the 95% confidence intervals. Reprinted from Perrin et al. (2009).
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Figure 2: The relationship between age and mean values of magnetization-transfer ratio (MTR) in white matter (WM) in male and female adolescents in the (A) frontal, (B) parietal, (C) temporal and (D) occipital lobes. The lines represent the regression equation with the 95% confidence intervals. Reprinted from Perrin et al. (2009).

Mentions: The total volume of WM is determined by the number of axons, their calibre and the thickness of myelin sheath produced by the oligodendrocytes. Given the known elimination of axons in the early post-natal period (e.g. LaMantia and Rakic, 1990, 1994), age-related increases in the volume of WM during brain development in childhood and adolescence can be accounted for by increases in axonal calibre and/or thickness of the myelin sheath. Recent data from our laboratory provide indirect evidence in favour of age-related changes in axonal calibre during male adolescence. First, we have obtained values of magnetization-transfer ratio (MTR), which provides an indirect index of myelination (Kucharczyk et al., 1994; Schmierer et al., 2004). As can be seen in Figure 2, MTR values in lobar WM were about the same in boys and girls at the age of 12 years after which they also diverged, increasing slightly in girls (in the frontal lobe) while decreasing in boys (in the parietal and occipital lobes); the sex by age interaction was significant in all lobes indicating the opposite effect of age on MTR in boys and girls throughout the brain (Perrin et al., 2009). Second, we have examined WM density in the cortico-spinal tract (CST) at the level of the internal capsule and observed a similar divergence, namely age-related increases and decreases in WM density in girls and boys, respectively (Perrin et al., 2009; Figure 3).


Could Sex Differences in White Matter be Explained by g ratio?

Paus T, Toro R - Front Neuroanat (2009)

The relationship between age and mean values of magnetization-transfer ratio (MTR) in white matter (WM) in male and female adolescents in the (A) frontal, (B) parietal, (C) temporal and (D) occipital lobes. The lines represent the regression equation with the 95% confidence intervals. Reprinted from Perrin et al. (2009).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2742663&req=5

Figure 2: The relationship between age and mean values of magnetization-transfer ratio (MTR) in white matter (WM) in male and female adolescents in the (A) frontal, (B) parietal, (C) temporal and (D) occipital lobes. The lines represent the regression equation with the 95% confidence intervals. Reprinted from Perrin et al. (2009).
Mentions: The total volume of WM is determined by the number of axons, their calibre and the thickness of myelin sheath produced by the oligodendrocytes. Given the known elimination of axons in the early post-natal period (e.g. LaMantia and Rakic, 1990, 1994), age-related increases in the volume of WM during brain development in childhood and adolescence can be accounted for by increases in axonal calibre and/or thickness of the myelin sheath. Recent data from our laboratory provide indirect evidence in favour of age-related changes in axonal calibre during male adolescence. First, we have obtained values of magnetization-transfer ratio (MTR), which provides an indirect index of myelination (Kucharczyk et al., 1994; Schmierer et al., 2004). As can be seen in Figure 2, MTR values in lobar WM were about the same in boys and girls at the age of 12 years after which they also diverged, increasing slightly in girls (in the frontal lobe) while decreasing in boys (in the parietal and occipital lobes); the sex by age interaction was significant in all lobes indicating the opposite effect of age on MTR in boys and girls throughout the brain (Perrin et al., 2009). Second, we have examined WM density in the cortico-spinal tract (CST) at the level of the internal capsule and observed a similar divergence, namely age-related increases and decreases in WM density in girls and boys, respectively (Perrin et al., 2009; Figure 3).

Bottom Line: We review the original Rushton (1951) model where a g ratio of approximately 0.6 represents an optimal relationship between the axon and fibre diameters vis-à-vis conduction velocity, and point out evidence indicating slightly higher g ratio in large-diameter fibres.We estimate that fibres with a diameter larger than 9.6 mum will have a relatively thinner myelin sheath, and brains with increasingly larger proportions of such large-diameter fibres will have progressively lower concentration of myelin.We conclude by pointing out possible implications of "suboptimal" g ratio for the emergence of "disconnection" disorders, such as schizophrenia, in late adolescence.

View Article: PubMed Central - PubMed

Affiliation: Brain and Body Centre, University of Nottingham Nottingham, UK.

ABSTRACT
Recent studies with magnetic resonance imaging suggest that age-related changes in white matter during male adolescence may indicate an increase in g ratio wherein the radial growth of an axon outpaces a corresponding increase in myelin thickness. We review the original Rushton (1951) model where a g ratio of approximately 0.6 represents an optimal relationship between the axon and fibre diameters vis-à-vis conduction velocity, and point out evidence indicating slightly higher g ratio in large-diameter fibres. We estimate that fibres with a diameter larger than 9.6 mum will have a relatively thinner myelin sheath, and brains with increasingly larger proportions of such large-diameter fibres will have progressively lower concentration of myelin. We conclude by pointing out possible implications of "suboptimal" g ratio for the emergence of "disconnection" disorders, such as schizophrenia, in late adolescence.

No MeSH data available.


Related in: MedlinePlus