Diffusion Anisotropy in the Corpus Callosum

Typical appearance of anisotropy in healthy human adult brain. Anisotropy maps obtained with the volume-ratio method are represented by color overlay on a gray-scale sagittal spin-echo T1-weighted image. Note the increased anisotropy of the posterior aspect of the corpus callosum compared with that of the anterior portion.

Typical appearance of anisotropy in healthy human adult brain. The same anisotropy maps from Figure 2 were overlaid on four axial spin-echo T1-weighted images through the corpus callosum.

Histologic findings in cadaver corpora callosa

A, Perivascular fibrous alae on a sagittal celloidin 100-μm-thick section through the corpus callosum (alkaline phosphatase stain, cresyl violet acetate and light green counterstain). A large arteriole (black arrow) is obliquely sectioned, and the extent of the collagenous perivascular ala (white arrows) is seen in an artifactual gap in the tissue.

B, Axial celloidin 100-μm-thick section through the corpus callosum (alkaline phosphatase stain, trichrome counterstain). Collagenous (green) sheets (arrows) are contiguous with and extend laterally away from penetrating callosal arterioles. These alae descend through the corpus callosum in a curtain-like fashion. Note that the axons horizontally oriented in this section.

C, Sagittal paraffin 10-μm-thick section through the corpus callosum (Bielschowsky stain). Axons are stained black, and most are cut in cross-section and appear as black dots (as expected). Two small bundles of obliquely oriented axons (curved arrows) are depicted. Obliquely cut vessels and associated fibrous ala are seen in a tissue cleft (straight arrows).

Schematic representation of diffusion anisotropy in brain white matter. Obliquely oriented cylinders represent axonal fibers and myelin sheaths, and small spheres represent water molecules. When the randomly chosen direction of diffusion is transverse to the fiber direction, the path of diffusion is blocked by hydrophobic myelin sheaths. When the randomly chosen direction of diffusion is parallel to the fiber direction, there are no barriers to free diffusion. Thus, the water molecules demonstrate a preference for diffusion along the direction of the fiber tract (12, 13, 16) (A). Schematic representations illustrate the situations in which axons are more tightly packed (B), the myelin sheaths are thicker (C), obliquely oriented axons are present (D), and the axonal diameter is reduced (E).