Overview of imaging and histological analysis of non-lesional brain tissue. Brains were scanned and subsequently sampled histologically to assess markers of neuropathology along the tract running from the mediodorsal nucleus of the thalamus to the prefrontal cortex (A, green) and from the lateral geniculate nucleus of the thalamus to the visual cortex (not shown). MRI provided structural images (D and I) and diffusion-weighted data yielding mean diffusivity (MD) and fractional anisotropy (FA) maps. Histological sampling of grey matter structures [example sampled blocks from prefrontal cortex (B) and mediodorsal nucleus of the thalamus (H) shown for illustration; V1 and lateral geniculate nucleus were also sampled] was followed by manual neuronal/glial profile counts on Luxol fast blue/cresyl violet sections (B and H). MRI cortical thickness masking was undertaken on MRI in regions analogous to histological samples (B and D: corresponding regions for prefrontal cortex); values were validated using histological measures (C). The grey matter structures sampled histologically were masked in their entirety in standard structural MRI space and used as the basis for probabilistic tractography of the tracts of interest in control in vivo brains. Paired thalamic and cortical masks were used as start points (seed masks) and end points (target masks), respectively to yield tractography outputs (G: 3D MDT-PFC tract shown in green), which were transformed using affine registration into the post-mortem image diffusion space. The resulting tract regions of interest (ROI) were used to derive average tract diffusion metrics, excluding regions of lesion-tract overlap. Registered tractography outputs guided histological sampling of white matter blocks corresponding to the tract midpoint (E). White matter sections were stained with anti-proteolipid protein stain (E: brown, myelin) and assessed for light transmittance (T) in regions corresponding to the tract (defined in G) to quantify myelin content (1/T). Adjacent white matter sections were stained with Palmgren silver to assess for axonal pathology in the tract using a point counting method (F). Scale bars: B and H = 80 µm; E and F = 25 µm.
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