Quantitative magnetization transfer measured pool-size ratio reflects optic nerve myelin content in ex vivo mice

Abstract

Optic nerves from mice that have undergone retinal ischemia were examined using a newly implemented quantitative magnetization transfer (qMT) technique. Previously published results indicate that the optic nerve from retinal ischemia mice suffered significant axon degeneration without detectable myelin injury at 3 days after reperfusion. At this time point, we acquired ex vivo qMT parameters from both shiverer mice (which have nearly no myelin) and control mice that have undergone retinal ischemia, and these qMT measures were compared with diffusion tensor imaging (DTI) results. Our findings suggests that the qMT estimated ratio of the pool sizes of the macromolecular and free water protons reflected the different myelin contents in the optic nerves between the shiverer and control mice. This pool size ratio was specific to myelin content only and was not significantly affected by the presence of axon injury in mouse optic nerve 3 days after retinal ischemia.

Figure 1

A): the T₂ weighted image of one coronal slice from a control mouse. Optic nerves are located inside the small rectangle. B): the relative anisotropy map of the same slice from this mouse. C): magnified relative anisotropy map inside the small rectangle. D): magnified pool size ratio map. RA maps have the best contrast between the optic nerve and surrounding tissues, therefore they were used to determine the position of the optic nerves and choose ROIs for each mouse.

Figure 2

Immunohistochemical results for A) uninjured optic nerve of a control mouse B) injured optic nerve of a control mouse C) uninjured optic nerve of a shiverer mouse and D) injured optic nerve of a shiverer mouse. A and B show the axonal degeneration (the green color SMI-31 labeling) and no demyelination (the red color MBP labeling) at three days after the retinal ischemia in a control mouse optic nerve; C and D show the axonal degeneration at three days after the retinal ischemia in a shiverer mouse optic nerve; both A comparing to C and B comparing to D show the dysmyelination in the shiverer mouse optic nerve.

Figure 3

Quantitative analysis of immunohistochemistry in normal (white bars) and injured (black bars) optic nerves (N = 4 for each bar). The counts of axons (pNF stained) showed that the axonal density is comparable between shieverer and control mice optic nerves. Both mice showed significant loss of normal axons in injured optic nerves at 3 days after the retinal ischemia.