Figure 14-1.

Organization of the Brain, Gray Matter, and White Matter.

A, Transverse section at the level of the thalamus, dog. Gray matter (darker areas) of the cerebral cortex lies beneath the leptomeninges on the external surface of the brain, whereas in the thalamus there is a mixture of gray and white matter. Major white matter areas (light areas) include corona radiata, centrum semiovale, and corpus callosum of the cerebrum, and internal capsule and optic tracts bordering the lateral and ventral surfaces of the thalamus, respectively. B, Gray matter consists primarily of the cell bodies of neurons (arrows) and a network of intermingled thinly myelinated axons, dendrites, and glial cell processes. This network is referred to as the neuropil (N). Other components include oligodendroglia (arrowheads), astrocytes, and microglia. H&E stain. C, White matter primarily consists of well-myelinated axons (arrows) plus oligodendroglia (arrowheads) and astrocytes. The clear spaces surrounding large axons are artifacts formed when the lipid components of myelin lamellae are dissolved away by solvents in the process of embedding tissue in paraffin for sectioning. H&E stain. D, Immunohistochemical (IHC) stain for ionized calcium binding adapter molecule 1 (Iba1). This IHC stain identifies microglia within a section of brain (arrows). Their ramified processes are similarly highlighted (arrowheads). DAB IHC stain. E, IHC stain for Olig2, a transcription factor that is expressed in the nucleus of oligodendrocytes (arrows). DAB IHC stain.

(A, B, and C courtesy Dr. J.F. Zachary, College of Veterinary Medicine, University of Illinois. D and E courtesy Dr. A.D. Miller, College of Veterinary Medicine, Cornell University.)