Cytoarchitecture of the human cerebral cortex: MR microscopy of excised specimens at 9.4 Tesla

Abstract

Background and purpose: The laminar patterns displayed by MR microscopy (MRM) form one basis for the classification of the cytoarchitectonic areas (Brodmann areas). It is plausible that in the future MRM may depict Brodmann areas directly, and not only by inference from gross anatomic location. Our purpose was to depict the laminar cytoarchitecture of excised, formalin-fixed specimens of human cerebral cortex by use of 9.4-T MR and to correlate MR images with histologic stains of the same sections.

Methods: Formalin-fixed samples of human sensory isocortex (calcarine, Heschl's, and somatosensory cortices), motor isocortex (hand motor area of M1), polar isocortex (frontal pole), allocortex (hippocampal formation), and transitional periallocortex (retrosplenial cortex) were studied by MRM at 9.4 T with intermediate-weighted pulse sequences for a total overnight acquisition time of 14 hours 17 minutes for each specimen. The same samples were then histologically analyzed to confirm the MR identification of the cortical layers. Curves representing the change in MR signal intensity across the cortex were generated to display the signal intensity profiles for each type of cortex.

Results: High-field-strength MR imaging at a spatial resolution of 78 x 78 x 500 micro m resolves the horizontal lamination of isocortex, allocortex, and periallocortex and displays specific intracortical structures such as the external band of Baillarger. The signal intensity profiles demonstrate the greatest hypointensity at the sites of maximum myelin concentration and maximum cell density and show gradations of signal intensity inversely proportional to varying cell density.

Conclusion: MRM at 9.4 T depicts important aspects of the cytoarchitecture of normal formalin-fixed human cortex.

Fig 1.

Homotypical polar isocortex. A, MRM image of the frontal pole. B, Corresponding histologic section with Nissl stain. From superficial to deep, depending on the concentration of the cells, the myelin fiber connections, the corresponding variation in the signal intensity, and the individual layers of the cortex can be identified: I, the molecular layer; II, the external granular layer; III, the external pyramidal layer; IV, the internal granular layer; V, the internal pyramidal layer, which is subdivided into an outer layer, Va, and an inner layer, Vb; and VI, the multiform layer (magnification ×8).

Fig 2.

Heterotypical sensory isocortex. A–C, MRM image (A) of the calcarine cortex with corresponding Nissl (B) and Luxol Fast Blue (C) stains. The thin, prominent, sharply defined intracortical band of low signal intensity corresponds to the highly myelinated plexus designated the external band of Baillarger (line of Gennari) (Layer IVB) (curved arrow in A and C). The prominent granule cells (long arrow in B) seen in layer II on the Nissl stain appear as a gray band (long arrow in A) on the MRM image. Note that the cortex is thicker at the crowns of the gyri and thinnest at the depths of the sulci. The changing relationship of layer IVB to the underlying white matter shows that the variation in cortical thickness results from thinning of the deep layers V and VI (magnification ×5). D–G, Signal intensity profiles oriented perpendicular to the cortical surface, color coded by location on the cortex. Focal “dips” in signal intensity at the line of Gennari (1) and within the subcortical white matter (2) correspond to the presence of heavily myelinated fibers, confirmed by histologic staining (B and C). The lower signal intensity in the subcortical white matter compared with that in the line of Gennari may reflect not only the greater concentration of myelin in the U fibers but also the increased concentration of iron within them. The dip in the middle of the blue graph (E) identifies the sulcus (S) between two adjacent gyri.

Fig 3.

Allocortex of the hippocampal formation. A and B, MRM images of adjacent sections through the midhippocampal formation in the coronal plane. C, Nissl stain specimen corresponding to A. From lateral to medial, the MRM displays: 1, alveus; 2, stratum oriens; 3, stratum pyramidale; 4, stratum radiatum; 5, stratum lacunosum; 6, stratum moleculare; 7, hippocampal sulcus; 8, stratum moleculare; 9, the dentate granule cell layer (straight arrow in A, B, and C); and 10, the polymorphic layer (arrowhead in B and C). Also seen are the caudate nucleus (CN), subiculum (Sub), and the lamellar retinotopic organization of the lateral geniculate body (curved arrow) (magnification ×6.25).

Fig 4.

Signal intensity profiles of the hippocampal allocortex. The profile perpendicular to the cortex (green line, profile in inset a) displays deep dips of low signal intensity at the acellular heavily myelinated alveus (A), the stratum lacunosum (L), the hippocampal sulcus (S), and the unmyelinated densely cellular dentate granule cell layer (D). In the profile along the CA from CA1 to CA4 (curved red line, profile in inset b), signal intensity is inversely related to cellular density. Signal intensity is highest in CA1 (lowest cellular density), decreases to its lowest signal intensity in CA2 (highest cellular density), and then rises progressively toward CA4, as cell density decreases. The blue line fitted to this signal intensity curve approximates an average of the signal intensity in each area. Note that signal intensity in CA4 is lower than that in CA1.

Fig 5.

Transitional cortex in the retrosplenial region of the cingulate gyrus. MRM (A) and Nissl stain (B) show progressive thickening of the cortex and increasing definition of the individual layers from the periallocortical regions (BA 29, 30) into the adjacent isocortex (BA 23). BA 29a-c are characterized by a thin cortex with very prominent myelination (asterisk) in a thick layer I. BA 29d shows funnel-shaped broadening of the cortex toward BA 30, with poor lamination, but beginning definition of layer II. BA 30 shows greater cortical thickness and a well-defined layer II. Isocortical BA 23 shows clear definition of layer IV as well as layer II. Sp indicates splenium (magnification ×6.25).